Whitney, M. R., Ting Tse, Y., and C. A. Sidor. 2019. Histological evidence of trauma in tusks of southern African dicynodonts. Palaeontologia Africana 53: 75-80. PDF.
Abstract - Dicynodonts were a clade of globally-distributed therapsids known for their abundance in the fossil record and for surviving the Permo-Triassic mass extinction. The group had distinctive dental adaptations including a beak and, in many species, paired maxillary tusks. The function of these tusks has long been of interest, yet remains poorly understood.We report here on two instances of unusual morphology in tusk dentine from specimens of: 1) Lystrosaurus from the Karoo Basin of South Africa and, 2) an unidentified dicynodontoid from the Luangwa Basin of Zambia. In both, the cross-sectional shape of the tusk root is lobed and infolded, which histological features suggest is a result of abnormal dentine deposition. We infer that this abnormal morphology is likely the consequence of trauma given its reparative nature and structural similarities to trauma-related morphologies reported in the tusks of modern elephants. This study demonstrates that histological sampling of dicynodont tusks can shed light on the biology of this important clade of therapsids.
Showing posts with label histology. Show all posts
Showing posts with label histology. Show all posts
Bone Histology of Phytosaur, Aetosaur, and Other Archosauriform Osteoderms
Just in time for Christmas...
Scheyer, T. M., Desojo, J. B., and I. A. Cerda. 2013. Bone histology of phytosaur, aetosaur, and other archosauriform osteoderms (Eureptilia, Archosauromorpha). Anatomical Record (early view) DOI: 10.1002/ar.22849
Abstract - As in other archosauriforms, phytosaurs and aetosaurs are characterized by the presence of well-developed osteoderms. Here we provide a comparative study on the microstructure of phytosaur (five taxa) and aetosaur (thirteen taxa) osteoderms. For outgroup comparison, we sampled osteoderms of the sister taxon to Aetosauria, Revueltosaurus callenderi, and the doswelliid Jaxtasuchus salomoni. Phytosaur, aetosaur, and Jaxtasuchus osteoderms are composed of a diploe structure, whereas the Revueltosaurus osteoderm microanatomy is more compact. The external cortex of phytosaurs, Revueltosaurus and Jaxtasuchus osteoderms is mainly composed of parallel-fibered bone. In aetosaurs, the external cortex mainly consists of lamellar bone, with lines of resorption within the primary bone indicating successive cycles of bone erosion and deposition. The basal cortex in all the specimens is composed of parallel-fibered bone, with the cancellous internal core being more strongly developed in aetosaurs than in phytosaurs. Woven or fibro-lamellar bone was recorded in both phytosaurian and aetosaurian taxa, as well as in Jaxtasuchus. Structural fibers, which at least partly suggest metaplastic origin, were only recorded in the internal core of two phytosaurs and in the
basal cortex of one aetosaur. Osteoderm thickness and cancellous to compact bone ratios appear to be subject to ontogenetic change. Minimum growth mark counts in osteoderms sampled indicate that some aetosaurs and phytosaurs lived for at least two decades. Bone microstructures are more uniform in phytosaur osteoderms and show a higher level of disparity among aetosaur osteoderms, and at least in the latter, histological features are potentially apomorphic for species/genus level.
Scheyer, T. M., Desojo, J. B., and I. A. Cerda. 2013. Bone histology of phytosaur, aetosaur, and other archosauriform osteoderms (Eureptilia, Archosauromorpha). Anatomical Record (early view) DOI: 10.1002/ar.22849
Abstract - As in other archosauriforms, phytosaurs and aetosaurs are characterized by the presence of well-developed osteoderms. Here we provide a comparative study on the microstructure of phytosaur (five taxa) and aetosaur (thirteen taxa) osteoderms. For outgroup comparison, we sampled osteoderms of the sister taxon to Aetosauria, Revueltosaurus callenderi, and the doswelliid Jaxtasuchus salomoni. Phytosaur, aetosaur, and Jaxtasuchus osteoderms are composed of a diploe structure, whereas the Revueltosaurus osteoderm microanatomy is more compact. The external cortex of phytosaurs, Revueltosaurus and Jaxtasuchus osteoderms is mainly composed of parallel-fibered bone. In aetosaurs, the external cortex mainly consists of lamellar bone, with lines of resorption within the primary bone indicating successive cycles of bone erosion and deposition. The basal cortex in all the specimens is composed of parallel-fibered bone, with the cancellous internal core being more strongly developed in aetosaurs than in phytosaurs. Woven or fibro-lamellar bone was recorded in both phytosaurian and aetosaurian taxa, as well as in Jaxtasuchus. Structural fibers, which at least partly suggest metaplastic origin, were only recorded in the internal core of two phytosaurs and in the
basal cortex of one aetosaur. Osteoderm thickness and cancellous to compact bone ratios appear to be subject to ontogenetic change. Minimum growth mark counts in osteoderms sampled indicate that some aetosaurs and phytosaurs lived for at least two decades. Bone microstructures are more uniform in phytosaur osteoderms and show a higher level of disparity among aetosaur osteoderms, and at least in the latter, histological features are potentially apomorphic for species/genus level.
Aging, Maturation and Growth of Sauropodomorph Dinosaurs
Griebeler, E. M., Klein, N., and P. M. Sander. 2013. Aging, Maturation and Growth of Sauropodomorph Dinosaurs as Deduced from Growth Curves Using Long Bone Histological Data: An Assessment of Methodological Constraints and Solutions. PLoS ONE 8(6): e67012. doi:10.1371/journal.pone.0067012
Abstract - Information on aging, maturation, and growth is important for understanding life histories of organisms. In extinct dinosaurs, such information can be derived from the histological growth record preserved in the mid-shaft cortex of long bones. Here, we construct growth models to estimate ages at death, ages at sexual maturity, ages at which individuals were fully-grown, and maximum growth rates from the growth record preserved in long bones of six sauropod dinosaur individuals (one indeterminate mamenchisaurid, two Apatosaurus sp., two indeterminate diplodocids, and one Camarasaurus sp.) and one basal sauropodomorph dinosaur individual (Plateosaurus engelhardti). Using these estimates, we establish allometries between body mass and each of these traits and compare these to extant taxa. Growth models considered for each dinosaur individual were the von Bertalanffy model, the Gompertz model, and the logistic model (LGM), all of which have inherently fixed inflection points, and the Chapman-Richards model in which the point is not fixed. We use the arithmetic mean of the age at the inflection point and of the age at which 90% of asymptotic mass is reached to assess respectively the age at sexual maturity or the age at onset of reproduction, because unambiguous indicators of maturity in Sauropodomorpha are lacking. According to an AIC-based model selection process, the LGM was the best model for our sauropodomorph sample. Allometries established are consistent with literature data on other Sauropodomorpha. All Sauropodomorpha reached full size within a time span similar to scaled-up modern mammalian megaherbivores and had similar maximum growth rates to scaled-up modern megaherbivores and ratites, but growth rates of Sauropodomorpha were lower than of an average mammal. Sauropodomorph ages at death probably were lower than that of average scaled-up ratites and megaherbivores. Sauropodomorpha were older at maturation than scaled-up ratites and average mammals, but younger than scaled-up megaherbivores.
Abstract - Information on aging, maturation, and growth is important for understanding life histories of organisms. In extinct dinosaurs, such information can be derived from the histological growth record preserved in the mid-shaft cortex of long bones. Here, we construct growth models to estimate ages at death, ages at sexual maturity, ages at which individuals were fully-grown, and maximum growth rates from the growth record preserved in long bones of six sauropod dinosaur individuals (one indeterminate mamenchisaurid, two Apatosaurus sp., two indeterminate diplodocids, and one Camarasaurus sp.) and one basal sauropodomorph dinosaur individual (Plateosaurus engelhardti). Using these estimates, we establish allometries between body mass and each of these traits and compare these to extant taxa. Growth models considered for each dinosaur individual were the von Bertalanffy model, the Gompertz model, and the logistic model (LGM), all of which have inherently fixed inflection points, and the Chapman-Richards model in which the point is not fixed. We use the arithmetic mean of the age at the inflection point and of the age at which 90% of asymptotic mass is reached to assess respectively the age at sexual maturity or the age at onset of reproduction, because unambiguous indicators of maturity in Sauropodomorpha are lacking. According to an AIC-based model selection process, the LGM was the best model for our sauropodomorph sample. Allometries established are consistent with literature data on other Sauropodomorpha. All Sauropodomorpha reached full size within a time span similar to scaled-up modern mammalian megaherbivores and had similar maximum growth rates to scaled-up modern megaherbivores and ratites, but growth rates of Sauropodomorpha were lower than of an average mammal. Sauropodomorph ages at death probably were lower than that of average scaled-up ratites and megaherbivores. Sauropodomorpha were older at maturation than scaled-up ratites and average mammals, but younger than scaled-up megaherbivores.
Two New Aetosaur Papers Including a New Taxon, Stenomyti huangae, from the Chinle Formation of Colorado
Taborda, J. R. A., Cerda, I. A., and J. B. Desojo. 2013. Growth curve of Aetosauroides scagliai Casamiquela 1960 (Pseudosuchia: Aetosauria) inferred from osteoderm histology. From Nesbitt, S. J., Desojo, J. B., and R. B. Irmis (eds.), Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin Geological Society Special Publications 379. doi:10.1144/SP379.19
Abstract - Recent palaeohistological studies on paramedian osteoderms of aetosaurs revealed the presence of growth lines (lines of arrested growth or LAGs) and a minimal or nonexistent secondary remodelling in the bone matrix of these elements. This feature allows the age of individuals to be estimated through growth line count. In the present contribution we study the growth curve of the South American aetosaur Aetosauroides scagliai. We estimated the age (obtained from LAG counting) and body size (body length and body mass were used as
proxies) of different aetosaur specimens in order to reconstruct the growth curve of the South American species. The data obtained for Aetosauroides scagliai were compared with that of other aetosaurs, such as Neoaetosauroides engaeus, Aetosaurus ferratus, Aetobarbakinoides brasiliensis, Typothorax coccinarum andParatypothorax sp. Our results indicate that, if body length is considered as proxy, all studied aetosaur specimens have a similar or almost identical growth rate. However, important variations arose among aetosaur taxa if body mass is considered as proxy, which would be related to a body morphology ranging from slender (e.g. Aetobarbakinoides brasiliensis) to very wide (Typothorax coccinarum) morphotypes. In comparison with extant pseudosuchians (i.e. crocodylians), Aetosauroides scagliai possesses a relatively lower growth rate.
Small, B. J., and J. W. Martz. 2013. A new aetosaur from the Upper Triassic Chinle Formation of the Eagle Basin, Colorado, USA.: From Nesbitt, S. J., Desojo, J. B., and R. B. Irmis (eds.), Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin Geological Society Special Publications 379 doi:10.1144/SP379.18
Abstract - A small aetosaur skull and skeleton and referred material from the Chinle Formation, Eagle Basin of Colorado, USA, is described as a new taxon, Stenomyti huangae gen. et sp. nov, distinguished from other aetosaurs by the following autapomorphies: three premaxillary teeth; four palpebrals; pronounced midline ridge on frontals and parietals; paired ridges flanking midline ridge on parietal and frontal; exclusion of quadratojugal from ventral margin of skull by contact between jugal and quadrate; exclusion of postorbital from infratemporal fenestra; infratemporal fenestra a horizontally oriented oval that embays the posterior edge of the jugal; retroarticular process longer than distance between articular glenoid and posterior edge of external mandibular fenestra; oval to irregularly shaped ventral osteoderms that do not contact each other. Paramedian and lateral osteoderms of S. huangae are nearly identical to those of Aetosaurus ferratus, and other shared cranial characters suggest that
these taxa are closely related and lie outside the clade Typothoracisinae +
Abstract - Recent palaeohistological studies on paramedian osteoderms of aetosaurs revealed the presence of growth lines (lines of arrested growth or LAGs) and a minimal or nonexistent secondary remodelling in the bone matrix of these elements. This feature allows the age of individuals to be estimated through growth line count. In the present contribution we study the growth curve of the South American aetosaur Aetosauroides scagliai. We estimated the age (obtained from LAG counting) and body size (body length and body mass were used as
proxies) of different aetosaur specimens in order to reconstruct the growth curve of the South American species. The data obtained for Aetosauroides scagliai were compared with that of other aetosaurs, such as Neoaetosauroides engaeus, Aetosaurus ferratus, Aetobarbakinoides brasiliensis, Typothorax coccinarum andParatypothorax sp. Our results indicate that, if body length is considered as proxy, all studied aetosaur specimens have a similar or almost identical growth rate. However, important variations arose among aetosaur taxa if body mass is considered as proxy, which would be related to a body morphology ranging from slender (e.g. Aetobarbakinoides brasiliensis) to very wide (Typothorax coccinarum) morphotypes. In comparison with extant pseudosuchians (i.e. crocodylians), Aetosauroides scagliai possesses a relatively lower growth rate.
Small, B. J., and J. W. Martz. 2013. A new aetosaur from the Upper Triassic Chinle Formation of the Eagle Basin, Colorado, USA.: From Nesbitt, S. J., Desojo, J. B., and R. B. Irmis (eds.), Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin Geological Society Special Publications 379 doi:10.1144/SP379.18
Abstract - A small aetosaur skull and skeleton and referred material from the Chinle Formation, Eagle Basin of Colorado, USA, is described as a new taxon, Stenomyti huangae gen. et sp. nov, distinguished from other aetosaurs by the following autapomorphies: three premaxillary teeth; four palpebrals; pronounced midline ridge on frontals and parietals; paired ridges flanking midline ridge on parietal and frontal; exclusion of quadratojugal from ventral margin of skull by contact between jugal and quadrate; exclusion of postorbital from infratemporal fenestra; infratemporal fenestra a horizontally oriented oval that embays the posterior edge of the jugal; retroarticular process longer than distance between articular glenoid and posterior edge of external mandibular fenestra; oval to irregularly shaped ventral osteoderms that do not contact each other. Paramedian and lateral osteoderms of S. huangae are nearly identical to those of Aetosaurus ferratus, and other shared cranial characters suggest that
these taxa are closely related and lie outside the clade Typothoracisinae +
Osteoderm Microstructure of “Rauisuchian” Archosaurs from South America
Cerda, I. A., J. B. Desojo, T. M. Scheyer and C. L. Schultz. In Press. Osteoderm microstructure of “rauisuchian” archosaurs from South America. Geobios (accepted manuscript) doi: http://dx.doi.org/10.1016/j.
Abstract - In this contribution we analyze and discuss the microanatomy and histology of postcranial osteoderms of a number of “rauisuchians” from different localities of South America (Argentina and Brazil). The studied sample includes osteoderms of Fasolasuchus tenax, Prestosuchus chiniquensis, Saurosuchus galilei and an undetermined rauisuchian from Brazil. The bone microanatomy of the osteoderms is variable: whereas some specimens have a rather compact structure, others show a diploe architecture with a central cancellous core bordered by two compact cortices. Both external and basal cortices are mainly composed of poorly vascularized, fine and coarse parallel fibred bone and networks of interwoven and mineralized fiber bundles. The internal region of the non-remodeled specimens consists of a well-vascularized core in which the intrinsic fibers exhibit important variations (even in the same specimen), ranging from coarse, parallel-fibred to woven-fibred bone tissues. Lines of arrested growth (LAGs) are well recorded in both basal and external cortices. Differences in the bone microstructure (compact vs. diploe) could be related to the age, sex and reproductive status of the sampled individuals. Hence, age estimation based on the count of LAGs in rauisuchian osteoderms appears to be reliable only in the early stages of ontogeny. The bone microstructure suggests that rauisuchian osteoderms were originated through a mechanism that involves both intramembranous and metaplastic ossifications.
Abstract - In this contribution we analyze and discuss the microanatomy and histology of postcranial osteoderms of a number of “rauisuchians” from different localities of South America (Argentina and Brazil). The studied sample includes osteoderms of Fasolasuchus tenax, Prestosuchus chiniquensis, Saurosuchus galilei and an undetermined rauisuchian from Brazil. The bone microanatomy of the osteoderms is variable: whereas some specimens have a rather compact structure, others show a diploe architecture with a central cancellous core bordered by two compact cortices. Both external and basal cortices are mainly composed of poorly vascularized, fine and coarse parallel fibred bone and networks of interwoven and mineralized fiber bundles. The internal region of the non-remodeled specimens consists of a well-vascularized core in which the intrinsic fibers exhibit important variations (even in the same specimen), ranging from coarse, parallel-fibred to woven-fibred bone tissues. Lines of arrested growth (LAGs) are well recorded in both basal and external cortices. Differences in the bone microstructure (compact vs. diploe) could be related to the age, sex and reproductive status of the sampled individuals. Hence, age estimation based on the count of LAGs in rauisuchian osteoderms appears to be reliable only in the early stages of ontogeny. The bone microstructure suggests that rauisuchian osteoderms were originated through a mechanism that involves both intramembranous and metaplastic ossifications.
Osteohistology of Mussasaurus patagonicus
Cerda, I. A., Pol, D. and A. Chinsamy. 2013. Osteohistological insight into the early stages of growth in Mussaurus patagonicus (Dinosauria, Sauropodomorpha)Historical Biology DOI:10.1080/08912963.2012.
Abstract - Here, we describe the bone histology of juvenile specimens of the basal sauropodomorph Mussaurus patagonicus and interpret its significance in terms of the early growth dynamics of this taxon. Thin sections from three juvenile specimens (femur length, 111–120 mm) of Mussaurus were analysed. The sampled bones consist of multiple postcranial elements collected from the Late Triassic Laguna Colorada Formation (El Tranquilo Group, Patagonia). The cortical bone is composed of fibrolamellar bone tissue. Vascularisation is commonly laminar or plexiform in the long bones. Growth marks are absent in all the examined samples. The ‘epiphyses’ of long bones are all formed by well-developed hypertrophied calcified cartilage. The predominance of woven-fibred bone matrix in cortical bones indicates a fast growth rate in the individuals examined. Moreover, given the existence of growth marks in adult specimens of Mussaurus, as in other sauropodomorphs, and assuming that the first lines of arrested growth was formed during the first year of life, the absence of growth marks in all the bones suggest that the specimens died before reaching their first year of life. Compared with the African taxon Massospondylus carinatus (another basal sauropodomorph for which the bone histology has been previously studied), it appears that Mussaurus had a higher early growth rate than Massospondylus.
Abstract - Here, we describe the bone histology of juvenile specimens of the basal sauropodomorph Mussaurus patagonicus and interpret its significance in terms of the early growth dynamics of this taxon. Thin sections from three juvenile specimens (femur length, 111–120 mm) of Mussaurus were analysed. The sampled bones consist of multiple postcranial elements collected from the Late Triassic Laguna Colorada Formation (El Tranquilo Group, Patagonia). The cortical bone is composed of fibrolamellar bone tissue. Vascularisation is commonly laminar or plexiform in the long bones. Growth marks are absent in all the examined samples. The ‘epiphyses’ of long bones are all formed by well-developed hypertrophied calcified cartilage. The predominance of woven-fibred bone matrix in cortical bones indicates a fast growth rate in the individuals examined. Moreover, given the existence of growth marks in adult specimens of Mussaurus, as in other sauropodomorphs, and assuming that the first lines of arrested growth was formed during the first year of life, the absence of growth marks in all the bones suggest that the specimens died before reaching their first year of life. Compared with the African taxon Massospondylus carinatus (another basal sauropodomorph for which the bone histology has been previously studied), it appears that Mussaurus had a higher early growth rate than Massospondylus.
Paleohistological Estimation of Bone Growth Rate in Extinct Archosaurs
Cubo, J., LeRoy, N., Martinez-Maza, C., and L. Montes. 2012. Paleohistological estimation of bone growth rate in extinct archosaurs. Paleobiology 38:335-339. doi: http://dx.doi.org/10.1666/08093.1
Abstract - The
clade Archosauria contains two very different sister groups in terms of
diversity (number of species) and disparity (phenotypic variation): Crurotarsi
(taxa more closely related to crocodiles than to birds) and Ornithodira
(pterosaurs and dinosaurs including birds). The extant species of Crurotarsi
may constitute a biased sample of past biodiversity regarding growth patterns
and metabolic rates. Bone histological characters can be conserved over
hundreds of millions of years in the fossil record and potentially contain
information about individual age at death, age at sexual maturity, bone growth
rates, and basal metabolic rates of extinct vertebrates. Using a sample of
extant amniotes, we have constructed a paleobiological model to estimate bone
growth rate from bone histological traits. Cross-validation tests show that
this model is reliable. We then used it to estimate bone growth rates in a
sample of extinct archosaurs including Crurotarsi and Ornithodira. After
testing for phylogenetic signal, optimization of femoral growth rates through
squared change parsimony onto a time-calibrated tree of amniotes shows two
divergent evolutionary trends: whereas bone growth rates increase from the last
common ancestor of Ornithodira to extant birds, they decrease from the last
common ancestor of Crurotarsi to extant crocodiles. However, we conclude, on
the basis of recent evidence for unidirectional airflow in the lungs of
alligators, that crocodiles may have retained the capacity of growing at high
rates.
Aesop's Fable - The Triassic version - the Dinosaur and the Crocodile
You can read it here:
http://www.economist.com/node/21540979
Triassic palaeontology in the Economist.
http://www.economist.com/node/21540979
Triassic palaeontology in the Economist.
Osteohistology of Some Triassic Archosauromorphs
Botha-Brink, J., and R. M. H. Smith. 2011. Osteohistology of the Triassic archosauromorphs Prolacerta, Proterosuchus, Euparkeria, and Erythrosuchus from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 31:1238-1254. DOI:10.1080/02724634.2011.621797
Abstract - The South African non-archosauriform archosauromorph Prolacerta and the archosauriforms Proterosuchus, Erythrosuchus, and Euparkeria were important constituents of the Early to early Middle Triassic Karoo ecosystem following the end-Permian mass extinction. We present new data on the osteohistology of these stem archosaurs and provide insight into their paleobiology. Bone tissues of the Early Triassic Prolacerta contain a poorly defined fibro-lamellar complex, with parallel-fibered bone in some regions, whereas the contemporaneous Proterosuchus exhibits rapidly forming uninterrupted fibro-lamellar bone early in its ontogeny, which becomes slow forming lamellar-zonal bone with increasing age. The early Middle Triassic Erythrosuchus deposited highly vascularized, uninterrupted fibro-lamellar bone throughout ontogeny, whereas the growth of the contemporaneous Euparkeria was relatively slow and cyclical. When our data are combined with those of previous studies, preliminary results reveal that Early and Middle Triassic non-crown group archosauromorphs generally exhibit faster growth rates than many of those of the Late Triassic. Early rapid growth and rapid attainment of sexual maturity are consistent with life history expectations for taxa living in the unpredictable conditions following the end-Permian mass extinction. Further research with larger sample sizes will be required to determine the nature of the environmental pressures on these basal archosaurs.
Long Bone Microstructure of Middle Triassic Pachypleurosaurids
Missed this article from May 2011 in the special issue of Compte Rendus Paleovol in honor of Dr. Armand de Ricqlès.
Hugi, J., Scheyer, T. M., Sander, P. M., Klein, N., and M. R. Sánchez-Villagra. 2011. Long bone microstructure gives new insights into the life of pachypleurosaurids from the Middle Triassic of Monte San Giorgio, Switzerland/Italy. Compte Rendus Palevol 10:413-426. doi:10.1016/j.crpv.2011.03.009
Abstract - The long bone microstructure of four pachypleurosaurid taxa from Monte San Giorgio (Switzerland/Italy) was studied. Pachypleurosaurids are secondarily aquatic reptiles that lived during the Middle Triassic in varying marine environments of the Tethys. All four pachypleurosaurids show high compactness values in their long bones based on a thick cortex and a calcified cartilaginous core, which remains in the medullary region throughout the ontogeny. Parts or even the entire embryonic bone layer composed of a mixture of woven fibered bone tissue and parallel-fibered bone tissue is preserved in both pachypleurosaurid genera. The rest of the cortex consists of lamellar-zonal bone tissue type. Differences in the microstructure of the bones between the pachypleurosaurids are reflected in the occurrence of remodelling processes, which, if present, affect the innermost growth marks of the cortex or the calcified cartilaginous core. Further variation is present in the spacing pattern of the growth cycles, as well as in the degree of vascularisation of the lamellar-zonal bone tissue type. Our data on the microstructure of the long bones support previous studies on morphology and facies distribution, which indicated different habitats and adaptation to a secondary aquatic lifestyle for each pachypleurosaurid taxon. Life history data furthermore reflect different longevities and ages at sexual maturity. The bone histological data of the stratigraphically youngest and oldest pachypleurosaurid species might indicate possible climate-dependant reproductive seasons similar to Recent lacertilian squamates.
Paleohistology of Rauisuchid Osteoderms
Yes, I am still in Argentina. I'd planned on posting a little more, especially since I visited Ischigualasto National Park and spent some hours prospecting in the Ischigualasto Formation (it did not disappoint) but I've been up to my eyeballs in aetosaur specimens and trying to keep myself fed (meals take a long time here), so forgive me. Meanwhile here is a paper I've been waiting on for awhile now regarding osteoderm histology of rauisuchids with comparisons to some other archosaur groups as well.
Scheyer, T. M. and J. B. Desojo. 2011. Palaeohistology and external microanatomy of rauisuchian osteoderms (Archosauria: Pseudosuchia). Palaeontology (advance online publication) DOI: 10.1111/j.1475-4983.2011.01098.xhttp://onlinelibrary.wiley.com/doi/10.1111/j.1475-4983.2011.01098.x/abstract
Abstract - The presence of postcranial dermal armour is plesiomorphic for Archosauria. Here, we survey the external microanatomy and histology of postcranial osteoderms (i.e. dorsal paramedian and caudal osteoderms) of rauisuchians, a widely distributed assemblage of extinct predatory pseudosuchians from the Triassic. The osteoderms of eight rauisuchian taxa were found to be rather compact bones, which usually lack significant bone remodelling or large areas of cancellous bone. The presence of highly vascularized woven or fibrolamellar bone tissue deposited in the core areas indicates higher growth rates during earlier life stages, whereas a more compact parallel-fibred bone matrix indicates reduced growth rates in later development. This pattern of change corroborates earlier studies on long bone histology. With the exception of a bone tissue found in the sample of Batrachotomus kupferzellensis, which might be the result of metaplastic ossification, the general mode of skeletogenesis is comparable with intramembraneous ossification. The lack of cancellous bone tissue and remodelling processes associated with bone ornamentation, as well as the predominantly intramembraneous mode of ossification, indicates that rauisuchian osteoderm formation differs profoundly from that of the osteoderms of the only extant pseudosuchian lineage, the crocodylians.
Scheyer, T. M. and J. B. Desojo. 2011. Palaeohistology and external microanatomy of rauisuchian osteoderms (Archosauria: Pseudosuchia). Palaeontology (advance online publication) DOI: 10.1111/j.1475-4983.2011.01098.xhttp://onlinelibrary.wiley.com/doi/10.1111/j.1475-4983.2011.01098.x/abstract
Abstract - The presence of postcranial dermal armour is plesiomorphic for Archosauria. Here, we survey the external microanatomy and histology of postcranial osteoderms (i.e. dorsal paramedian and caudal osteoderms) of rauisuchians, a widely distributed assemblage of extinct predatory pseudosuchians from the Triassic. The osteoderms of eight rauisuchian taxa were found to be rather compact bones, which usually lack significant bone remodelling or large areas of cancellous bone. The presence of highly vascularized woven or fibrolamellar bone tissue deposited in the core areas indicates higher growth rates during earlier life stages, whereas a more compact parallel-fibred bone matrix indicates reduced growth rates in later development. This pattern of change corroborates earlier studies on long bone histology. With the exception of a bone tissue found in the sample of Batrachotomus kupferzellensis, which might be the result of metaplastic ossification, the general mode of skeletogenesis is comparable with intramembraneous ossification. The lack of cancellous bone tissue and remodelling processes associated with bone ornamentation, as well as the predominantly intramembraneous mode of ossification, indicates that rauisuchian osteoderm formation differs profoundly from that of the osteoderms of the only extant pseudosuchian lineage, the crocodylians.
Crocodylian-like Trunk Bracing System in Armored Basal Tetrapods
Buchwitz, M., Witzmann, F., Voigt, S. and Golubev, V. 2011. Osteoderm microstructure indicates the presence of a crocodylian-like trunk bracing system in a group of armoured basal tetrapods. Acta Zoologica (Stockholm) 00:1–21. DOI: 10.1111/j.1463-6395.2011.00502.x
Abstract - The microstructure of dorsal osteoderms referred to the chroniosuchid taxa Chroniosuchus, Chroniosaurus, Madygenerpeton and cf. Uralerpeton is compared to existing data on the bystrowianid chroniosuchian Bystrowiella and further tetrapods. Chroniosuchid osteoderms are marked by thin internal and relatively thick external cortices that consist of lowly vascularised parallel-fibred bone. They are structured by growth marks and, in case of Madygenerpeton, by lines of arrested growth. The cancellous middle region is marked by a high degree of remodelling and a primary bone matrix of parallel-fibred bone that may include domains of interwoven structural fibres. Whereas the convergence of Bystrowiella and chroniosuchid osteoderms is not confirmed by our observations, the internal cortex of the latter displays a significant peculiarity: It contains distinct bundles of shallowly dipping Sharpey’s fibres with a cranio- or caudoventral orientation. We interpret this feature as indicative for the attachment of epaxial muscles which spanned several vertebral segments between the medioventral surface of the osteoderms and the transversal processes of the thoracic vertebrae. This finding endorses the hypothesis that the chroniosuchid osteoderm series was part of a crocodylian-like trunk bracing system that supported terrestrial locomotion. According to the measured range of osteoderm bone compactness, some chroniosuchian species may have had a more aquatic lifestyle than others.
Abstract - The microstructure of dorsal osteoderms referred to the chroniosuchid taxa Chroniosuchus, Chroniosaurus, Madygenerpeton and cf. Uralerpeton is compared to existing data on the bystrowianid chroniosuchian Bystrowiella and further tetrapods. Chroniosuchid osteoderms are marked by thin internal and relatively thick external cortices that consist of lowly vascularised parallel-fibred bone. They are structured by growth marks and, in case of Madygenerpeton, by lines of arrested growth. The cancellous middle region is marked by a high degree of remodelling and a primary bone matrix of parallel-fibred bone that may include domains of interwoven structural fibres. Whereas the convergence of Bystrowiella and chroniosuchid osteoderms is not confirmed by our observations, the internal cortex of the latter displays a significant peculiarity: It contains distinct bundles of shallowly dipping Sharpey’s fibres with a cranio- or caudoventral orientation. We interpret this feature as indicative for the attachment of epaxial muscles which spanned several vertebral segments between the medioventral surface of the osteoderms and the transversal processes of the thoracic vertebrae. This finding endorses the hypothesis that the chroniosuchid osteoderm series was part of a crocodylian-like trunk bracing system that supported terrestrial locomotion. According to the measured range of osteoderm bone compactness, some chroniosuchian species may have had a more aquatic lifestyle than others.
Paleobiological Implications of Basal Ichthyosaur from Histological Data
Kolb, C., Sánchez-Villagra, M. R., and T. M. Scheyer. 2011. The palaeohistology of the basal ichthyosaur Mixosaurus Baur, 1887 (Ichthyopterygia, Mixosauridae) from the Middle Triassic: Palaeobiological implications. Comptes Rendus Palevol doi:10.1016/j.crpv.2010.10.008
Abstract - Here, we provide the first bone histological examination of an ontogenetic series of the basal ichthyosaur Mixosaurus encompassing postnatal to large adult specimens. Growth marks are present in sampled humeri, a femur, a fibula, as well as in other skeletal elements (gastral ribs). Ontogenetic changes are traceable throughout stylo- and zeugopodial development, but interior remodelling and resorption deleted part of the internal growth record in the primary cortex. Mixosaurus humeri started as flat structures consisting of a core of endochondral woven bone and residual calcified cartilage, whereas growth continued by deposition of periosteal fibrolamellar and parallel-fibred bone. Unlike the fast-growing post-Triassic ichthyosaurs that lack growth marks, microstructural and life history data are now becoming available for a basal ichthyosaur. The high growth rate of Mixosaurus may indicate that higher metabolic rates characterised small, non-thunniform ichthyosaurs, as had been suggested already for post-Triassic, cruising forms.
Abstract - Here, we provide the first bone histological examination of an ontogenetic series of the basal ichthyosaur Mixosaurus encompassing postnatal to large adult specimens. Growth marks are present in sampled humeri, a femur, a fibula, as well as in other skeletal elements (gastral ribs). Ontogenetic changes are traceable throughout stylo- and zeugopodial development, but interior remodelling and resorption deleted part of the internal growth record in the primary cortex. Mixosaurus humeri started as flat structures consisting of a core of endochondral woven bone and residual calcified cartilage, whereas growth continued by deposition of periosteal fibrolamellar and parallel-fibred bone. Unlike the fast-growing post-Triassic ichthyosaurs that lack growth marks, microstructural and life history data are now becoming available for a basal ichthyosaur. The high growth rate of Mixosaurus may indicate that higher metabolic rates characterised small, non-thunniform ichthyosaurs, as had been suggested already for post-Triassic, cruising forms.
An Upclose Look at the Microanatomy of Aetosaur Osteoderms
Aetosaurs are characterized by their elaborate bony carapaces composed of numerous osteoderms. In fact aetosaur taxonomy is almost based solely on the morphology (especially the surface ornamentation) of osteoderms. Despite this detailed studies of the microstructure of aetosaur oseoderms are lacking. In 2008 I published a paper with Michelle Stocker and Randall Irmis that provided the first histological data for aetosaur osteoderms, but we were mostly looking at providing an estimated age at time of death for the holotype of Sierritasuchus macalpini to determine the ontogenetic stage of the specimen.
This new study focuses on aetosaurine osteoderms from Argentina and Brazil, including specimens assigned to Aetosauroides scagliai. One of the very cool things these authors did was not only to look a parasaggital sections of the rectangular osteoderms, they also looked at transverse sections. Some of the key findings are as follows:
- Aetosaur osteoderms lend themselves well to this type of study as secondary remodeling is minimal.
-Unlike all other sampled archosaurs, aetosaur osteoderm ossification was not metaplastic in nature (i.e. pre-existing, fully developed tissue is ossified), instead the osteoderms seemingly underwent intermembraneous ossification where new tissue displaces preformed tissue rather than incorporating it. This is currently unique among archosaurs.
- Cyclic growth lines (Lines of arrested growth of LAG's) are well developed. Based on this the specimens sampled belonged to a range of subadult animals between two and nine years of age at time of death (minimum ages).
- The center of ossification in aetosaur osteoderms is at the level of the raised dorsal eminence.
- Aetosaur plates probably grew by adding peripheral layers. Interestingly most faster growth occurred along the medial and lateral margins. This accounts for the assymetrical placement of the dorsal eminence that is characteristic of aetosaurines.
- Well-developed Sharpey's fibers along the medial and lateral margins of the osteoderms suggest strong lateral and medial attachments along a row of osteoderms. In contrast the attachments with anterior of posterior plates were poor, presumably allowing for flexion and movement in the carapace.
-Finally, the ornamentation of the osteoderms is formed by local resorption and partitial redeposition of the cortical bone. Acceleration of growth in particular areas enhances the degree of sculpture through time and the pattern is established early and then maintained through future growth. This is seemingly why the ornamentation in juvenile specimens does not differ significantly from that of adults. This is extremely significant if you are using this patterning to diagnose taxa.
Overall an important study and excellent paper.
Cerda, I. A., and J. B. Desojo. 2010: Dermal armour histology of aetosaurs (Archosauria: Pseudosuchia), from the Upper Triassic of Argentina and Brazil. Lethaia, DOI: 10.1111/j.1502-3931.2010.00252.x.
Abstract - One of the most striking features documented in aetosaurs is the presence of an extensive bony armour composed of several osteoderms. Here, we analyse the bone microstructure of these elements in some South American Aetosaurinae aetosaurs, including Aetosauroides scagliai. In general terms, Aetosaurinae osteoderms are compact structures characterized by the presence of three tissue types: a basal cortex of poorly vascularized parallel-fibred bone tissue, a core of highly vascularized fibro-lamellar bone, and an external cortex of rather avascular lamellar bone tissue. Sharpey’s fibres are more visible at the internal core, toward the lateral margins and aligned parallel to the major axis of the dermal plate. No evidence of metaplastic origin is reported in the osteoderms, and we hypothesize an intramembranous ossification for these elements. The bone tissue distribution reveals that the development of the osteoderm in Aetosaurinae starts in a position located medial to the plate midpoint, and the main sites of active osteogenesis occur towards the lateral and medial edges of the plate. The osteoderm ornamentation is originated and maintained by a process of resorption and redeposition of the external cortex, which also includes preferential bone deposition in some particular sites. Given that no secondary reconstruction occurs in the osteoderms, growth marks are well preserved and they provide very important information regarding the relative age and growth pattern of Aetosaurinae aetosaurs.
This new study focuses on aetosaurine osteoderms from Argentina and Brazil, including specimens assigned to Aetosauroides scagliai. One of the very cool things these authors did was not only to look a parasaggital sections of the rectangular osteoderms, they also looked at transverse sections. Some of the key findings are as follows:
- Aetosaur osteoderms lend themselves well to this type of study as secondary remodeling is minimal.
-Unlike all other sampled archosaurs, aetosaur osteoderm ossification was not metaplastic in nature (i.e. pre-existing, fully developed tissue is ossified), instead the osteoderms seemingly underwent intermembraneous ossification where new tissue displaces preformed tissue rather than incorporating it. This is currently unique among archosaurs.
- Cyclic growth lines (Lines of arrested growth of LAG's) are well developed. Based on this the specimens sampled belonged to a range of subadult animals between two and nine years of age at time of death (minimum ages).
- The center of ossification in aetosaur osteoderms is at the level of the raised dorsal eminence.
- Aetosaur plates probably grew by adding peripheral layers. Interestingly most faster growth occurred along the medial and lateral margins. This accounts for the assymetrical placement of the dorsal eminence that is characteristic of aetosaurines.
- Well-developed Sharpey's fibers along the medial and lateral margins of the osteoderms suggest strong lateral and medial attachments along a row of osteoderms. In contrast the attachments with anterior of posterior plates were poor, presumably allowing for flexion and movement in the carapace.
-Finally, the ornamentation of the osteoderms is formed by local resorption and partitial redeposition of the cortical bone. Acceleration of growth in particular areas enhances the degree of sculpture through time and the pattern is established early and then maintained through future growth. This is seemingly why the ornamentation in juvenile specimens does not differ significantly from that of adults. This is extremely significant if you are using this patterning to diagnose taxa.
Overall an important study and excellent paper.
Cerda, I. A., and J. B. Desojo. 2010: Dermal armour histology of aetosaurs (Archosauria: Pseudosuchia), from the Upper Triassic of Argentina and Brazil. Lethaia, DOI: 10.1111/j.1502-3931.2010.00252.x.
Abstract - One of the most striking features documented in aetosaurs is the presence of an extensive bony armour composed of several osteoderms. Here, we analyse the bone microstructure of these elements in some South American Aetosaurinae aetosaurs, including Aetosauroides scagliai. In general terms, Aetosaurinae osteoderms are compact structures characterized by the presence of three tissue types: a basal cortex of poorly vascularized parallel-fibred bone tissue, a core of highly vascularized fibro-lamellar bone, and an external cortex of rather avascular lamellar bone tissue. Sharpey’s fibres are more visible at the internal core, toward the lateral margins and aligned parallel to the major axis of the dermal plate. No evidence of metaplastic origin is reported in the osteoderms, and we hypothesize an intramembranous ossification for these elements. The bone tissue distribution reveals that the development of the osteoderm in Aetosaurinae starts in a position located medial to the plate midpoint, and the main sites of active osteogenesis occur towards the lateral and medial edges of the plate. The osteoderm ornamentation is originated and maintained by a process of resorption and redeposition of the external cortex, which also includes preferential bone deposition in some particular sites. Given that no secondary reconstruction occurs in the osteoderms, growth marks are well preserved and they provide very important information regarding the relative age and growth pattern of Aetosaurinae aetosaurs.
Temnospondyl Paleoenvironmental Adaptations: Evidence from Bone Histology
Sanchez, S., Germain, D., De Ricqles, A., Abourachid, A., Goussard, F. and Tafforeau, P. 2010. Limb-bone histology of temnospondyls: implications for understanding the diversification of palaeoecologies and patterns of locomotion of Permo-Triassic tetrapods. Journal of Evolutionary Biology, early online. doi:10.1111/j.1420-9101.2010.02081.x
Abstract - The locomotion of early tetrapods has long been a subject of great interest in the evolutionary history of vertebrates. However, we still do not have a precise understanding of the evolutionary radiation of their locomotory strategies. We present here the first palaeohistological study based on theoretical biomechanical considerations among a highly diversified group of early tetrapods, the temnospondyls. Based on the quantification of microanatomical and histological parameters in the humerus and femur of nine genera, this multivariate analysis provides new insights concerning the adaptations of temnospondyls to their palaeoenvironments during the Early Permian, and clearly after the Permo-Triassic crisis. This study therefore presents a methodology that, if based on a bigger sample, could contribute towards a characterization of the behaviour of species during great evolutionary events.
Abstract - The locomotion of early tetrapods has long been a subject of great interest in the evolutionary history of vertebrates. However, we still do not have a precise understanding of the evolutionary radiation of their locomotory strategies. We present here the first palaeohistological study based on theoretical biomechanical considerations among a highly diversified group of early tetrapods, the temnospondyls. Based on the quantification of microanatomical and histological parameters in the humerus and femur of nine genera, this multivariate analysis provides new insights concerning the adaptations of temnospondyls to their palaeoenvironments during the Early Permian, and clearly after the Permo-Triassic crisis. This study therefore presents a methodology that, if based on a bigger sample, could contribute towards a characterization of the behaviour of species during great evolutionary events.
Tracking Charles L. Camp in the Blue Hills of Arizona
As my longtime readers know one of my favorite aspects of paleontological research is redocumenting historic localities. It is important to exactly relocate these sites in order to place them in our modern stratigraphic framework, thus the fossils collected from these sites add more data to our local biostratigraphy. But I also just get the thrill of following in our predecessors footsteps, seeing the terrain as they saw it. It simply makes their data more relevant and in a way much more easier to understand when you take a walk in their shoes (or boots).
Last week it was off to the Blue Hills northeast of St. Johns, Arizona. In 1923 and 1924 Charles Camp of the UCMP made significant vertebrate fossil collections from these localities, including the type specimen of the phytosaur Machaeroprosopus zunii. The stratigraphic position of this specimen was in doubt because Camp had misinterpreted underlying Chinle Formation strata as belonging to the Moenkopi Formation. Jeff Martz and I had been interested in the stratigraphic position of this specimen for awhile so with old field notes and photos in hand, and with one of the main local landowners showing us the best way to access the badlands, we were on our way.
We were successful in relocating the M. zunii quarry almost immediately. The two photos directly below are one taken by Camp in 1923 of the M. zunii excavation (courtesy of the UCMP) and me at the same spot in 2010.
We were successful in relocating the M. zunii quarry almost immediately. The two photos directly below are one taken by Camp in 1923 of the M. zunii excavation (courtesy of the UCMP) and me at the same spot in 2010.
More difficult to find was Camp's "meal pots" locality, a greenish mudstone and fine sandstone horizon that produced numerous microvertebrates including plates of the diminuative aetosaur Acaenasuchus geoffreyi and some of the oldest recovered elements of the pseudosuchian Revueltosaurus. Camp's field notes were a little ambiguous regarding this site, but we were able to finally relocate it (see photo below).
Why Were Dicynodonts so Successful Before and After the End-Permian Extinction?
Botha-Brink, J., and K. Angielczyk. 2010. Do extraordinarily high growth rates in Permo-Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction? Zoological Journal of the Linnean Society, early online, doi: 10.1111/j.1096-3642.2009.00601.x
Abstract - Dicynodonts were the most diverse and abundant herbivorous therapsids of the Permo-Triassic. They include Lystrosaurus, one of the few taxa known to survive the end-Permian extinction and the most abundant tetrapod during the Early Triassic postextinction recovery. Explanations for the success of Lystrosaurus and other dicynodonts remain controversial. This study presents an assessment of dicynodont growth patterns using bone histology, with special focus on taxa associated with the end-Permian extinction event. Bone histological analysis reveals a high cortical thickness throughout the clade, perhaps reflecting a phylogenetic constraint. Growth rings are absent early in ontogeny, and combined with high vascular density, indicate rapid, sustained growth up to the subadult stage. Extraordinarily enlarged vascular channels are present in the midcortex of many dicynodonts, including adults, and may have facilitated a more efficient assimilation of nutrients and rapid bone growth compared to other therapsids. Both increased channel density and enlarged vascular channels evolved at or near the base of major radiations of dicynodonts, implying that the changes in growth and life history they represent may have been key to the success of dicynodonts. Furthermore, this exceptionally rapid growth to adulthood may have contributed to the survival of Lystrosaurus during the end-Permian extinction and its dominance during the postextinction recovery period.
Abstract - Dicynodonts were the most diverse and abundant herbivorous therapsids of the Permo-Triassic. They include Lystrosaurus, one of the few taxa known to survive the end-Permian extinction and the most abundant tetrapod during the Early Triassic postextinction recovery. Explanations for the success of Lystrosaurus and other dicynodonts remain controversial. This study presents an assessment of dicynodont growth patterns using bone histology, with special focus on taxa associated with the end-Permian extinction event. Bone histological analysis reveals a high cortical thickness throughout the clade, perhaps reflecting a phylogenetic constraint. Growth rings are absent early in ontogeny, and combined with high vascular density, indicate rapid, sustained growth up to the subadult stage. Extraordinarily enlarged vascular channels are present in the midcortex of many dicynodonts, including adults, and may have facilitated a more efficient assimilation of nutrients and rapid bone growth compared to other therapsids. Both increased channel density and enlarged vascular channels evolved at or near the base of major radiations of dicynodonts, implying that the changes in growth and life history they represent may have been key to the success of dicynodonts. Furthermore, this exceptionally rapid growth to adulthood may have contributed to the survival of Lystrosaurus during the end-Permian extinction and its dominance during the postextinction recovery period.
Limb Bone Histology of the Upper Triassic Dicynodont Placerias hesternus
Green, J. L., Schweitzer, M. H., and E.-T. Lamm. 2010. Limb Bone Histology and Growth in Placerias hesternus (Therapsida: Anomodontia) from the Upper Triassic of North America. Palaeontology 53:347-364. doi: 10.1111/j.1475-4983.2010.00944.x
Abstract- Patterns of bone deposition are reported and deduced from mid-shaft sections of 21 limb bones of the dicynodont Placerias hesternus from the Placerias Quarry(Upper Triassic), Arizona, USA. All sampled elements of P. hesternus have a large medullary cavity completely filled with bony trabeculae surrounded by dense cortical bone. Dense Haversian bone extends from the perimedullary region to at least the mid-cortex in all sampled bones. Primary bone in the outer cortex of limb elements of P. hesternus is generally zonal fibrolamellar with a peripheral layer of parallelfibred bone. These data suggest periodic rapid osteogenesis followed by slower growth. Among dicynodonts, this strategy is most similar to growth previously reported in other Triassic (Lystrosaurus, Wadiasaurus) and some Permian taxa (Oudenodon, Tropidostoma). An external fundamental system(EFS), suggesting complete or near complete cessation of appositional growth, is present in the largest tibia. This is the first report of EFS in dicynodonts and may represent the attainment of maximum size in P. hesternus. Slow-growing peripheral bone was observed in elements of varying size in our sample and may support a differential growth pattern between P. hesternus individuals from this locality. A complete growth series of P. hesternus, analysis of Placerias specimens from other localities, and further sampling of other Upper Triassic dicynodonts are needed to better understand a more complete picture of the growth and remodelling patterns that we have initially investigated.
I've only been able to briefly scan this, but it looks like a cool study. Two presented hypotheses that caught my eye are that the bone make-up in Placerias suggests an amphibious or aquatic lifestyle, and that the cessation of growth in many of the specimens may been due to a response to changing environmental conditions.
Abstract- Patterns of bone deposition are reported and deduced from mid-shaft sections of 21 limb bones of the dicynodont Placerias hesternus from the Placerias Quarry(Upper Triassic), Arizona, USA. All sampled elements of P. hesternus have a large medullary cavity completely filled with bony trabeculae surrounded by dense cortical bone. Dense Haversian bone extends from the perimedullary region to at least the mid-cortex in all sampled bones. Primary bone in the outer cortex of limb elements of P. hesternus is generally zonal fibrolamellar with a peripheral layer of parallelfibred bone. These data suggest periodic rapid osteogenesis followed by slower growth. Among dicynodonts, this strategy is most similar to growth previously reported in other Triassic (Lystrosaurus, Wadiasaurus) and some Permian taxa (Oudenodon, Tropidostoma). An external fundamental system(EFS), suggesting complete or near complete cessation of appositional growth, is present in the largest tibia. This is the first report of EFS in dicynodonts and may represent the attainment of maximum size in P. hesternus. Slow-growing peripheral bone was observed in elements of varying size in our sample and may support a differential growth pattern between P. hesternus individuals from this locality. A complete growth series of P. hesternus, analysis of Placerias specimens from other localities, and further sampling of other Upper Triassic dicynodonts are needed to better understand a more complete picture of the growth and remodelling patterns that we have initially investigated.
I've only been able to briefly scan this, but it looks like a cool study. Two presented hypotheses that caught my eye are that the bone make-up in Placerias suggests an amphibious or aquatic lifestyle, and that the cessation of growth in many of the specimens may been due to a response to changing environmental conditions.
Bone Histology of the Late Triassic Dinosauriform Silesaurus opolensis
Fostowicz-Frelik, L., and T. Sulej. 2009. Bone histology of Silesaurus opolensis Dzik, 2003 from the Late Triassic of Poland. Lethaia. 10.1111/j.1502-3931.2009.00179.x
Abstract- The phylogenetic relationships of Silesaurus opolensis have been the subject of intense debate since its discovery. Silesaurus possesses some features characteristic of ornithischian dinosaurs, such as the presence of a beak at the front of the lower jaw, yet it lacks a number of important femoral and dental synapomorphies of Dinosauria. The microstructure of the long bones (femur, tibia and metatarsal) and ribs of this species reveals a relatively intensive rate of growth, comparable with that seen in small dinosaurs and the gracile crocodylomorph Terrestrisuchus. Cortical bone formed mainly by periosteal tissue with fibro-lamellar matrix (in older specimens parallel fibred) shows very little secondary remodelling and only in one specimen (large tibia ZPAL Ab III ⁄ 1885) few lines of arrested growth are present in the outermost cortex. The vascularization is relatively dense, mainly longitudinal and ceases towards the periphery, forming almost avascular parallel fibred bone at the bone surface. This indicates maturation and significant decrease in the growth ratio in mature specimens of S. opolensis. The delicate trabeculae exhibit cores formed by the primary cancellous tissue lined with lamellar endosteal bone. The rather intense growth of S. opolensis implies a relatively high metabolic rate. Moreover, evidence from the fibro-lamellar tissue, predominant in the cortex, suggests that this kind of rapid bone deposition could be more typical of Archosauria than previously assumed, a prerequisite for the evolution of the very fast growth rates observed in large ornithischians, sauropods and large theropods.
Abstract- The phylogenetic relationships of Silesaurus opolensis have been the subject of intense debate since its discovery. Silesaurus possesses some features characteristic of ornithischian dinosaurs, such as the presence of a beak at the front of the lower jaw, yet it lacks a number of important femoral and dental synapomorphies of Dinosauria. The microstructure of the long bones (femur, tibia and metatarsal) and ribs of this species reveals a relatively intensive rate of growth, comparable with that seen in small dinosaurs and the gracile crocodylomorph Terrestrisuchus. Cortical bone formed mainly by periosteal tissue with fibro-lamellar matrix (in older specimens parallel fibred) shows very little secondary remodelling and only in one specimen (large tibia ZPAL Ab III ⁄ 1885) few lines of arrested growth are present in the outermost cortex. The vascularization is relatively dense, mainly longitudinal and ceases towards the periphery, forming almost avascular parallel fibred bone at the bone surface. This indicates maturation and significant decrease in the growth ratio in mature specimens of S. opolensis. The delicate trabeculae exhibit cores formed by the primary cancellous tissue lined with lamellar endosteal bone. The rather intense growth of S. opolensis implies a relatively high metabolic rate. Moreover, evidence from the fibro-lamellar tissue, predominant in the cortex, suggests that this kind of rapid bone deposition could be more typical of Archosauria than previously assumed, a prerequisite for the evolution of the very fast growth rates observed in large ornithischians, sauropods and large theropods.
Introducing Sierritasuchus macalpini
The new issue of the Journal of Vertebrate Paleontology [28(3)] contains a new paper by me (and coauthors Michelle Stocker and Randy Irmis) describing a new taxon of aetosaur. If you have never heard of aetosaurs they are heavily armored, probably omnivorous, crocodile-line archosaurs that are extremely common fossils in Late Triassic terrestrial deposits. Aetosaurs are characterized by their armor, which consists of row after row of rectangular armor plates (osteoderms). Interestingly the dorsal (upper) surface of these plates is ornamented and this armor pattern is diagnostic of taxa. Even more informative is the morphology of what are termed lateral plates, or plates protecting the flanks of the animal. All aetosaurs can roughly be divided into three groups (clades) based on lateral plate morphology (Parker, 2007).
This is the fourth (and last) paper in a series which reanalyzes the genus Desmatosuchus (see also Parker, 2005, 2007, 2008). In 2000 I was studying the collections at the University of Michigan Museum of Paleontology (UMMP) when I came across the partial skeleton of small aetosaur that had been assigned to the genus Desmatosuchus. This specimen immediately caught my attention because although it represented a Desmatosuchine, it was quite different from Desmatosuchus (the holotype was on display upstairs). However, this specimen was was a bit of an enigma because of its incompleteness, crushing (it was also overprepared), and the fact that it appeared to be a juvenile. I gave a presentation on this specimen back in 2001 at the Western Association of Vertebrate Paleontologist’s meeting where I concluded that it either represented a new taxon or was possibly a juvenile form of Longosuchus (another Desmatosuchine) (Parker, 2001). Despite moving on to other projects, this fossil was always in the back of my mind and in 2004 enlisted Michelle Stocker to work with me on the description. The specimen was distinct enough from both Desmatosuchus and Longosuchus to warrant erection as a new taxon; however, we were still bothered by the small size of the specimen because very little is known regarding ontogentic changes to aetosaur armor morphology. Then two things happened; 1) we discovered juvenile Typothorax (another aetosaur) material at Petrified Forest National Park that clearly showed that little if any change occurred through ontogeny; 2) Bill Mueller at Texas Tech University turned us on to a lateral plate from a larger individual that was clearly referable to the same taxon, and possessed the same characters as the UMMP specimen. Still, we needed to be sure so we recruited Randy Irmis to conduct histological work to determine an ontogentic stage for the material. Randy was able to conclude that although the specimen was not fully grown, it was not exactly a very young juvenile either. Thus we felt confident enough to complete the study and erect a new taxon.
This is the first published study that attempts ontogentic stage determination in an aetosaur using the histology of osteoderms, and we hope that this will become a very important tool for future studies. For those who are wondering, the name Sierritasuchus is from Sierrita de la Cruz Creek near where the specimen was found. The species name S. macalpini honors the late Archie MacAlpin who collected the specimen in 1939. MacAlpin was a student of Ermine Cowles Case who published much on the Late Triassic of Texas. MacAlpin was later a geology professor at the University of Notre Dame. Sierritasuchus is currently only known from two specimens from the Tecovas Formation (Dockum Group) of Texas. It differs from both Desmatosuchus and Longosuchus (the two best known desmatosuchines) by various characters of the osteoderms and vertebrae. The picture above shows the majority of the holotype material (UMMP V60817). The reconstruction for this post was generously completed by Jeff Martz.
REFERENCES
Parker, W.G. 2001. An enigmatic aetosaur specimen from the Upper Triassic Dockum Formation of Texas. Western Association of Vertebrate Paleontologists with Mesa Southwest Museum and Southwest Paleontological Society Abstracts 2001:23.
Parker, W.G. 2005. A new species of the Late Triassic aetosaur Desmatosuchus (Archosauria: Pseudosuchia). Compte Rendus Palevol 4:327-340.
Parker, W.G. 2007. Reassessment of the aetosaur “Desmatosuchus” chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). Journal of Systematic Palaeontology 5:41-68.
Parker, W.G. 2008. Description of new material of the aetosaur Desmatosuchus spurensis (Archosauria: Suchia) from the Chinle Formation of Arizona and a revision of the genus Desmatosuchus. PaleoBios 28:1-40.
This is the fourth (and last) paper in a series which reanalyzes the genus Desmatosuchus (see also Parker, 2005, 2007, 2008). In 2000 I was studying the collections at the University of Michigan Museum of Paleontology (UMMP) when I came across the partial skeleton of small aetosaur that had been assigned to the genus Desmatosuchus. This specimen immediately caught my attention because although it represented a Desmatosuchine, it was quite different from Desmatosuchus (the holotype was on display upstairs). However, this specimen was was a bit of an enigma because of its incompleteness, crushing (it was also overprepared), and the fact that it appeared to be a juvenile. I gave a presentation on this specimen back in 2001 at the Western Association of Vertebrate Paleontologist’s meeting where I concluded that it either represented a new taxon or was possibly a juvenile form of Longosuchus (another Desmatosuchine) (Parker, 2001). Despite moving on to other projects, this fossil was always in the back of my mind and in 2004 enlisted Michelle Stocker to work with me on the description. The specimen was distinct enough from both Desmatosuchus and Longosuchus to warrant erection as a new taxon; however, we were still bothered by the small size of the specimen because very little is known regarding ontogentic changes to aetosaur armor morphology. Then two things happened; 1) we discovered juvenile Typothorax (another aetosaur) material at Petrified Forest National Park that clearly showed that little if any change occurred through ontogeny; 2) Bill Mueller at Texas Tech University turned us on to a lateral plate from a larger individual that was clearly referable to the same taxon, and possessed the same characters as the UMMP specimen. Still, we needed to be sure so we recruited Randy Irmis to conduct histological work to determine an ontogentic stage for the material. Randy was able to conclude that although the specimen was not fully grown, it was not exactly a very young juvenile either. Thus we felt confident enough to complete the study and erect a new taxon.
This is the first published study that attempts ontogentic stage determination in an aetosaur using the histology of osteoderms, and we hope that this will become a very important tool for future studies. For those who are wondering, the name Sierritasuchus is from Sierrita de la Cruz Creek near where the specimen was found. The species name S. macalpini honors the late Archie MacAlpin who collected the specimen in 1939. MacAlpin was a student of Ermine Cowles Case who published much on the Late Triassic of Texas. MacAlpin was later a geology professor at the University of Notre Dame. Sierritasuchus is currently only known from two specimens from the Tecovas Formation (Dockum Group) of Texas. It differs from both Desmatosuchus and Longosuchus (the two best known desmatosuchines) by various characters of the osteoderms and vertebrae. The picture above shows the majority of the holotype material (UMMP V60817). The reconstruction for this post was generously completed by Jeff Martz.
REFERENCES
Parker, W.G. 2001. An enigmatic aetosaur specimen from the Upper Triassic Dockum Formation of Texas. Western Association of Vertebrate Paleontologists with Mesa Southwest Museum and Southwest Paleontological Society Abstracts 2001:23.
Parker, W.G. 2005. A new species of the Late Triassic aetosaur Desmatosuchus (Archosauria: Pseudosuchia). Compte Rendus Palevol 4:327-340.
Parker, W.G. 2007. Reassessment of the aetosaur “Desmatosuchus” chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). Journal of Systematic Palaeontology 5:41-68.
Parker, W.G. 2008. Description of new material of the aetosaur Desmatosuchus spurensis (Archosauria: Suchia) from the Chinle Formation of Arizona and a revision of the genus Desmatosuchus. PaleoBios 28:1-40.
Parker, W.G., Stocker, M.R., and R.B. Irmis. 2008. A new desmatosuchine aetosaur (Archosauria: Suchia) from the Upper Triassic Tecovas Formation (Dockum Group) of Texas. Journal of Vertebrate Paleontology 28:692-701.
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