Overview of the Aetosaurs

Desojo, J. B., Heckert, A. B., Martz, J. W., Parker, W. G., Schoch, R. R., Small, B. J., and T. Sulej. 2013. Aetosauria: a clade of armoured pseudosuchians from the Upper Triassic continental beds; 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 Publication 379: doi:10.1144/SP379.17

Abstract - Aetosauria is a clade of obligately quadrupedal, heavily armoured pseudosuchians known from Upper Triassic (late Carnian–Rhaetian) strata on every modern continent except Australia and Antarctica. As many as 22 genera and 26 species ranging from 1 to 6 m in length, and with a body mass ranging from less than 10 to more than 500 kg, are known. Aetosauroides scagliai was recently recovered as the most basal aetosaur, placed outside of Stagonolepididae (the last common ancestor of Desmatosuchus and Aetosaurus). Interrelationships among the basal aetosaurs are not well understood but two clades with relatively apomorphic armour – the spinose Desmatosuchinae and the generally wide-bodied Typothoracisinae – are consistently recognized. Paramedian and lateral osteoderms are often distinctive at the generic level but variation within the carapace is not well understood in many taxa, warranting caution in assigning isolated osteoderms to specific taxa. The aetosaur skull and dentition varies across taxa, and there is increasing evidence that at least some aetosaurs relied on invertebrates and/or small vertebrates as a food source. Histological evidence indicates that, after an initial period of rapid growth, lines of arrested growth (LAGs) are common and later growth was relatively slow. The common and widespread Late Triassic ichnogenus Brachychirotherium probably represents the track of an aetosaur.

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 + Desmatosuchinae. This discovery indicates that other reports of Aetosaurus across Laurasia based on osteoderms should be reassessed. Similar confusion with the osteoderms of other non-typothoracisine/desmatosuchine aetosaurs such as Aetosauroides, Stagonolepis and Calyptosuchus suggests that osteoderms are not always reliable taxonomic indicators.

Dental Microwear of the Late Triassic Dinosauriform Silesaurus opolensis

Kubo, T., and M. O. Kubo. 2013. Dental microwear of a Late Triassic dinosauriform, Silesaurus opolensis. Acta Palaeontologica Polonica (in press).
doi: http://dx.doi.org/10.4202/app.2013.0027

Abstract - Silesaurus opolensis belongs to Silesauridae, the closest sister group
to dinosaurs. The present study analyzed the dental  microwear patterns of Silesaurus opolensis. Low pit-to-scratch ratios imply they did not feed on hard objects. Unimodal distributions of both wear-facet and non-facet scratch orientations indicate simple orthal jaw movement. Scratch orientation and density differ between microscopic regions in Silesaurus, and unlike hadrosaurid dinosaurs, the microwear patterns of small areas are not identical to those of whole teeth.

Analysis of Triassic Archosauriform Trackways

Kubo, T. and M. O. Kubo. 2013. Analysis of Triassic archosauriform trackways: difference in stride/foot ratio between dinosauromorphs and other archosauriforms. Palaios 28: 259-265 doi:10.2110/palo.2012.p12-099r
Abstract - Fossilized trackways have rarely been analyzed quantitatively to examine major trends and patterns in evolution despite their potential utility, especially in understanding locomotory evolution. In the present study, trackways of Triassic archosauriforms were analyzed. The analyses showed foot and stride lengths of archosauriforms increased from the Early to Middle Triassic, especially those of dinosauromorphs, which tripled. Dinosauromorphs were much smaller in foot length and stride length compared to other archosauriforms during
the Early Triassic. They reached similar stride length compared with other archosauriforms during the Middle Triassic and similar foot length in the Late Triassic. Stride/foot ratio is significantly higher in dinosauromorphs compared to other archosauriforms throughout the Triassic. This relatively long stride length of dinosauromorphs is attributed to either faster speed or higher relative hip height that was probably caused by their digitigrade foot posture. Analyses of trackway data sets, especially in combination with precise trackmaker assignment and age determination, would bring us more thorough knowledge about locomotory evolution of tetrapods that complements body fossil evidence.

The Early Evolution of Synapsids

Brocklehurst, N., Kammerer, C. F., and J. Frobisch. 2013. The early evolution of synapsids, and the influence of sampling on their fossil record. Paleobiology 39:470-490. DOI: 10.1666/12049

Abstract - Synapsids dominated the terrestrial realm between the late Pennsylvanian and the Triassic.
Their early evolution includes some of the first amniotes to evolve large size, herbivory, and macropredators. However, little research has focused on the changes in diversity occurring during this early phase in their evolutionary history, with more effort concentrating on later events such the Permo-Triassic extinction. Here we assess synapsid diversity, at both the species and genus levels, between the Carboniferous (Moscovian) and the Middle Permian (Capitanian). A raw, taxic diversity (richness) estimate is generated, and we use two separate methods to correct for sampling biases in this curve. To remove the effect of anthropogenic sampling bias, we apply a recently published modification of the residual diversity method, and then generate a supertree, using matrix representation with parsimony to infer ghost lineages and obtain a phylogenetic diversity estimate. The general diversity pattern reflects the initial diversification of synapsids in the late Pennsylvanian and early Cisuralian, which was followed by an extinction event during the Sakmarian. Diversity recovered during the Artinskian and Kungurian, coinciding with the radiation of Caseidae, although other families begin to decline. A second extinction event occurred across the Kungurian/Roadian boundary, in which Edaphosauridae and Ophiacodontidae died out although Caseidae and Therapsida diversified. The sampling-corrected curves reveal further extinction during the Roadian, although therapsids were again unaffected. Pelycosaurian-grade synapsids survived during the Wordian and Capitanian, but were a minor part of an otherwise therapsid-dominated fauna. Evidence of significant anthropogenic sampling bias calls
into question previous diversity studies that have not employed sampling correction.

New Hypothesis on the Evolutionary Origin of the Turtle Shell (with video)

This recent early online paper has been getting a lot of attention lately regarding the origin of turtles and their phylogenetic relationships. There is also this associated video on YouTube depicting this new hypothesis of the evolution of the turtle shell.

Lyson, T. R., Bever, G. S., Scheyer, T. M., Hsiang, A. Y.,  and J. A. Gauthier. 2013. Evolutionary Origin of the Turtle Shell. Current Biology. doi: http://dx.doi.org/10.1016/j.cub.2013.05.003Abstract - The origin of the turtle shell has perplexed biologists for more than two centuries. It was not until Odontochelys semitestacea was discovered, however, that the fossil and developmental data could be synthesized into a model of shell assembly that makes predictions for the as-yet unestablished history of the turtle stem group. We build on this model by integrating novel data for Eunotosaurus africanus—a Late Guadalupian (~260 mya) Permian reptile inferred to be an early stem turtle. Eunotosaurus expresses a number of relevant characters, including a reduced number of elongate trunk vertebrae (nine), nine pairs of T-shaped ribs, inferred loss of intercostal muscles, reorganization of respiratory muscles to the ventral side of the ribs, (sub)dermal outgrowth of bone from the developing perichondral collar of the ribs, and paired gastralia that lack both lateral and median elements. These features conform to the predicted sequence of character acquisition and provide further support that E. africanus, O. semitestacea, and Proganochelys quenstedti represent successive divergences from the turtle stem lineage. The initial transformations of the model thus occurred by the Middle Permian, which is congruent with molecular-based divergence estimates for the lineage, and remain viable whether turtles originated inside or outside crown Diapsida.

Revisiting the Dicynodonts of Western North America

Kammerer, C. F., Fröbisch, J., and Angielczyk, K. D. 2013. On the validity and phylogenetic position of Eubrachiosaurus browni, a kannemeyeriiform dicynodont (Anomodontia) from Triassic North America. PLoS ONE 8(5): e64203. doi:10.1371/journal.pone.0064203

Abstract - The large dicynodont Eubrachiosaurus browni from the Upper Triassic Popo Agie Formation of Wyoming is redescribed. Eubrachiosaurus is a valid taxon that differs from Placerias hesternus, with which it was previously synonymized, by greater anteroposterior expansion of the scapula dorsally and a very large, nearly rectangular humeral ectepicondyle with a broad supinator process. Inclusion of Eubrachiosaurus in a revised phylogenetic analysis of anomodont therapsids indicates that it is a stahleckeriid closely related to the South American genera Ischigualastia and Jachaleria. The recognition of Eubrachiosaurus as a distinct lineage of North American dicynodonts, combined with other recent discoveries in the eastern USA and Europe, alters our perception of Late Triassic dicynodont diversity in the northern hemisphere. Rather than being isolated relicts in previously therapsid-dominated regions, Late Triassic stahleckeriid dicynodonts were continuing to disperse and diversify, even in areas like western North America that were otherwise uninhabited by coeval therapsids (i.e., cynodonts).

Chirotherium Trackways from the Middle Triassic of China

Xing, L., Klein, H., Lockley, M. G., Li, J., Zhang, J., Matsukawa, M., and J. Xiao. 2013. Chirotherium trackways from the Middle Triassic of Guizhou, China. Ichnos 20: 99-107. DOI:10.1080/10420940.2013.788505

Abstract - Triassic tetrapod footprints from China are less well known than those from the Jurassic and Cretaceous. Archosaurian trackways of the ichnogenus Chirotherium were found in the Middle Triassic Guanling Formation in Zhenfeng County (Guizhou Province) at the southwestern edge of the Yangtze plate in the early 1960s but were not correctly identified and adequately described until 40 years later. Here we give a detailed re-description and review of the trackways, which are known from two localities near the villages of Niuchang and Longchang. They occur on the bedding surface of a mud-cracked argillaceous dolostone deposited in a near-shore, shallow-water environment. Their morphology and general trackway pattern indicate that they pertain to the ichnospecies Chirotherium barthii, well known from Middle Triassic
track surfaces of Europe, North and South America, and northern Africa. A peculiarity of the trackways from China are the low pace angulation and stride length, reflecting slow-moving trackmakers, which were basal crown-group archosaurs, possibly early
representatives of the dinosaur-bird line or, alternatively, stem-group crocodylians. These tracks constitute the only chirotheriid record known from Asia thus far and indicate a Pangea-wide distribution for this ichnotaxon. Biostratigraphically, assemblages with C. barthii are characteristic of the early Anisian, an age assignment already supported for the Guanling Formation based on conodont and bivalve biostratigraphy. In contrast, however, radiometric data from an interlayered ash bed indicate a Ladinian age.

High Environmental and Metabolic Plasticity as a Successful Evolutionary Strategy in a Long-lived Homeostatic Triassic Temnospondyl.

Sanchez S., and R. R. Schoch. 2013. Bone histology reveals a high environmental and metabolic plasticity as a successful evolutionary strategy in a long-lived homeostatic Triassic temnospondyl. Evolutionary Biology (early online) DOI: 10.1007/s11692-013-9238-3

Abstract - Evolutionary stasis (long-term stability of morphology in an evolving lineage) is a pattern for which explanations are usually elusive. The Triassic tetrapod Gerrothorax pulcherrimus, a gill-bearing temnospondyl, survived for 35 million years in the Germanic Basin of Central Europe persisting throughout the dinosaur-dominated Late Triassic Period. This evolutionary stasis coincides with the occurrence of this species in a wide range of habitats and environmental conditions. By the combination of palaeoecological and palaeohistological analyses, we found great ecological flexibility in G. pulcherrimus and present substantial evidence of developmental and
metabolic plasticity despite the morphological stasis. We conclude that G. pulcherrimus could show the capacity to settle in water bodies too harsh or unpredictable for most other tetrapods. This would have been made possible by a unique life history strategy that involved a wide reaction norm, permitting adjustment to fluctuating conditions such as salinity and level of nutrients. Growth rate, duration of juvenile period, age at maturity, and life span were all subject to broad variation within specimens of G. pulcherrimus in one single lake and in between different lakes. In addition to providing a better understanding of fossil ecosystems, this study shows the potential of such a methodology to encourage palaeobiologists and evolutionary biologists to consider the mechanisms of variation in extant and fossil organisms by using a similar time-scope reference.

A New Late Triassic Phytogeographical Scenario in Westernmost Gondwana.

Césari, S. N.,  and Colombi, C. E. 2013. A new Late Triassic phytogeographical scenario in westernmost Gondwana. Nature Communications 4, Article number: 1889 doi:10.1038/ncomms2917

 Abstract - Floral provincialism within the Southern Hemisphere during the Late
Triassic (230 Ma) is characterized by the Ipswich and Onslow provinces, recognized originally in eastern Gondwana. However, new palynological assemblages from the Ischigualasto Formation, northwestern Argentina (231–225 Ma), change the phytogeographic interpretation for the Carnian–Norian in the westernmost Gondwana,
which was previously considered part of the southern floral Ipswich province. Here we show the presence of diagnostic Euramerican species within assemblages dominated by Gondwanan taxa that allows us to refer the palynofloras to the Onslow province. Our new data extend the Onslow floral belt, previously recognized from the western edge of Tethys to Timor, to the western margin of South America. This has implications for palaeophytogeography, palaeoclimate reconstructions and the palaeoecology of a Triassic ecosystem, which has yielded significant vertebrate remains and is regarded important in the early evolution of groups such as the Dinosauria.

New Stratigraphy, Geochronology, and Paleontology from the Late Triassic of Laos

Note that 225-221 Ma is now considered to be Norian...

Blanchard, S., Rossignol, C., Bourquin, S., Dabard, M.-P., Hallot, E., Nalpas, T., Poujol, M., Battail, B., Jalil, N.-E., Steyer, J.-S., Vacant, R., Véran, M., Bercovici, A., Diez, J. B., Paquette, J.-L., Khenthavong, B. and Vongphamany, S. 2013. Late Triassic volcanic activity in South-East Asia: new stratigraphical, geochronological and paleontological evidence from the Luang Prabang Basin (Laos). Journal of Asian Earth Sciences 70-71: 8–26.

Abstract - In South-East Asia, sedimentary basins displaying continental Permian and Triassic deposits have been poorly studied. Among these, the Luang Prabang Basin (North Laos) represents a potential key target to constrain the stratigraphic and structural evolutions of South-East Asia. A combined approach involving sedimentology, palaeontology, geochronology and structural analysis, was thus implemented to study the basin. It resulted in a new geological map, in defining new formations, and in proposing a complete revision of the Late Permian to Triassic stratigraphic succession as well as of the structural organization of the basin. Radiometric ages are used to discuss the synchronism of volcanic activity and sedimentation.

The Luang Prabang Basin consists of an asymmetric NE-SW syncline with NE-SW thrusts, located at the contact between Late Permian and Late Triassic deposits. The potential stratigraphic gap at the Permian–Triassic boundary is therefore masked by deformation in the basin. The Late Triassic volcaniclastic continental deposits are representative of alluvial plain and fluvial environments. The basin was fed by several sources, varying from volcanic, carbonated to silicic (non-volcanic). U–Pb dating of euhedral zircon grains provided maximum sedimentation ages. The stratigraphic vertical succession of these ages, from ca. 225, ca. 220 to ca. 216 Ma, indicates that a long lasting volcanism was active during sedimentation and illustrates significant variations in sediment preservation rates in continental environments (from ∼100 m/Ma to ∼3 m/Ma). Anhedral inherited zircon grains gave older ages. A large number of them, at ca. 1870 Ma, imply the reworking of a Proterozoic basement and/or of sediments containing fragments of such a basement. In addition, the Late Triassic (Carnian to Norian) sediments yielded to a new dicynodont skull, attributed to the Kannemeyeriiform group family, from layers dated in between ∼225 and ∼221 Ma (Carnian).

A New Non-mammaliaform Eucynodont from the Ischigualasto Formation of Argentina

Martínez, R. N., Fernandez, E., and O. A. Alcober. 2013. A new non-mammaliaform eucynodont from the Carnian-Norian Ischigualasto Formation, Northwestern Argentina. Revista Brasileira de Paleontologia 16: 61-76. doi:10.4072/rbp.2013.1.05
 
Abstract - The record of non-mammaliaformes eucynodonts from the Carnian-Norian Ischigualasto Formation is diverse and abundant, including a medium to large size herbivore and small carnivores. Here is described a new small eucynodont from the Ischigualasto Formation, on the basis of a partial skull. The new taxon is characterized by palatal process of the premaxilla extending posterior to the level of the first postcanine; deep and large maxillary laterodorsal fossa that opens at the level of the root of the upper canine; and postorbital bar diverging posterolaterally at very low angle (35.6°) from the anteroposterior axis of the skull. Results from a phylogenetic analysis supports the new genus placement as a probainognathian eucynodont, more derived than Probainognathus Romer, and more closely related to Ecteninion Martinez, May & Forster and Trucidocynodon Oliveira, Soares & Schultz than to any other eucynodont. Ecteniniidae is proposed as a new clade including the new genus, Ecteninion and Trucidocynodon, and in the phylogenetic hypothesis represents the sister-group of Prozostrodontia (Prozostrodon Bonaparte & Barberena, Tritylodontidae and Mammaliaformes). Additionally, the new taxon from the Ischigualasto Formation shows that the Scaphonyx-Exaeretodon-Herrerasaurus biozone has similar cynodont diversity than the supposedly contemporaneous Hyperodapedon Assemblage Zone of Santa Maria 2 Sequence, in Southern Brazil.

 

 

 

 

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.geobios.2013.01.004

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.

Provincialization of Terrestrial Faunas Following the End-Permian Mass Extinction.

Sidor, C. A., D. A. Vilhena, K. D. Angielczyk, A. K. Huttenlocker, S. J. Nesbitt, B. R. Peecook, J. S. Steyer, R. M. H. Smith, and L. A. Tsuji. 2013. Provincialization of terrestrial faunas following the end-Permian mass extinction. Proceedings of the National Academy of Sciences (advance online publication)
doi: 10.1073/pnas.1302323110

Abstract -  In addition to their devastating effects on global biodiversity, mass extinctions have had a long-term influence on the history of life by eliminating dominant lineages that suppressed ecological change. Here, we test whether the end-Permian mass extinction (252.3 Ma) affected the distribution of tetrapod faunas within the southern hemisphere and apply quantitative methods to analyze four components of biogeographic structure: connectedness, clustering, range size, and endemism. For all four components, we detected increased provincialism between our Permian and Triassic datasets. In southern Pangea, a more homogeneous and broadly distributed fauna in the Late Permian (Wuchiapingian, ~257 Ma) was replaced by a provincial and biogeographically fragmented fauna by Middle Triassic times (Anisian, ~242 Ma). Importantly in the Triassic, lower latitude basins in Tanzania and Zambia included dinosaur predecessors and other archosaurs unknown elsewhere. The recognition of heterogeneous tetrapod communities in the Triassic implies that the end-Permian mass extinction afforded ecologically marginalized lineages the ecospace to diversify, and that biotic controls (i.e., evolutionary incumbency) were fundamentally reset. Archosaurs, which began diversifying in the Early Triassic, were likely beneficiaries of this ecological release and remained dominant for much of the later Mesozoic.