Reevaluation of the Enigmatic Archosaur Dyoplax arenaceus from the Upper Triassic of Germany

Maisch, M. W., Matzke, A. T., and T. Rathgeber. 2013. Re-evaluation of the enigmatic archosaur Dyoplax arenaceus O. Fraas, 1867 from the Schilfsandstein (Stuttgart Formation, lower Carnian, Upper Triassic) of Stuttgart, Germany. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 267(3): 353-362
DOI: http://dx.doi.org/10.1127/0077-7749/2013/0317


Abstract - The holotype and only specimen of the small suchian archosaur Dyoplax arenaceus O. Fraas, 1867 from the Stuttgart-Formation (Schilfsandstein) of southwestern Germany is partially redescribed and re-evaluated. The type locality can be identified as Stuttgart, not Stuttgart-Feuerbach as erroneously suggested by previous authors. A re-description of the skull and the dorsal armour provides several new characters and a restoration of the skull is attempted for the first time. The phylogenetic placement of Dyoplax is discussed. Although it is agreed with previous authors that the taxon is not an aetosaur, its placement in Crocodylomorpha is questioned. Instead we demonstrate that Dyoplax has several important cranial and postcranial features in common with Erpetosuchus from the Late Triassic of Scotland and North America, and it is tentatively re-assigned to ?Erpetosuchidae.

Redescription of the Archosaur Parringtonia gracilis from the Middle Triassic Manda Beds of Tanzania

Here is another redescription of another of the enigmatic archosauriforms from the Middle Triassic Manda Formation. This study confirms the affinity of Parringtonia gracilis and Erpetosuchus granti, and reinforces the importance of redescribing these older taxa using an apomorphy-based approach and placing them in a modern phylogenetic analysis. Unfortunately the exact position of Erpetosuchidae with Archosauria was not able to be precisely determined.

Nesbitt, S. J., and R. J. Butler. 2012. Redescription of the archosaur Parringtonia gracilis from the Middle Triassic Manda beds of Tanzania, and the antiquity of Erpetosuchidae. Geological Magazine (FirstView article), 14p. doi:10.1017/S0016756812000362 

Abstract - Parringtonia gracilis Huene, 1939 is represented by both cranial and postcranial material collected from the lower Middle Triassic (Anisian) Lifua Member of the Manda beds in southwestern Tanzania. This aberrant taxon was previously proposed to have affinities with pseudosuchian archosaurs, and specifically with the enigmatic Erpetosuchus granti from the Upper Triassic of Scotland. Here, we confirm the close affinities of Parringtonia gracilis and Erpetosuchus granti based on the following unambiguous synapomorphies: mediolaterally expanded posterior portion of the maxilla, alveoli present only in the anterior half of the maxilla, and absence of tooth serrations. Furthermore, the two taxa share osteoderms with deep sculpturing, a deep fossa on the dorsal margin of the neural spines and a heavily waisted shaft of the scapula. We added both Parringtonia gracilis and Erpetosuchus granti into a comprehensive phylogenetic analysis of early archosaurs and found that these taxa are clearly referable to Archosauria but that relationships are poorly resolved at the base of this clade. However, our analysis demonstrates that Erpetosuchus granti is not closely related to Crocodylomorpha, as has been hypothesized previously. The Erpetosuchidae are a clade of small-bodied archosaurs that have a poor fossil record but have members from both northern and southern Pangaea, ranging temporally from the Middle to Late Triassic. Thus, Erpetosuchidae is part of the early archosaurian radiation.

Postcranial Skeletal Pneumaticity in Triassic Archosaurs

Butler, R. J., Barrett, P. M., and D. J. Gower. 2012. Reassessment of the evidence for postcranial skeletal pneumaticity in Triassic archosaurs, and the early evolution of the avian respiratory system. PLoS ONE 7(3): e34094. doi:10.1371/journal.pone.0034094


Abstract - Uniquely among extant vertebrates, birds possess complex respiratory systems characterised by the combination of small, rigid lungs, extensive pulmonary air sacs that possess diverticula that invade (pneumatise) the postcranial skeleton, unidirectional ventilation of the lungs, and efficient crosscurrent gas exchange. Crocodilians, the only other living archosaurs, also possess unidirectional lung ventilation, but lack true air sacs and postcranial skeletal pneumaticity (PSP). PSP can be used to infer the presence of avian-like pulmonary air sacs in several extinct archosaur clades (non-avian theropod dinosaurs, sauropod dinosaurs and pterosaurs). However, the evolution of respiratory systems in other archosaurs, especially in the lineage leading to crocodilians, is poorly documented. Here, we use mCT-scanning to investigate the vertebral anatomy of Triassic archosaur taxa, from both the avian and crocodilian lineages as well as non-archosaurian diapsid outgroups. Our results confirm previous suggestions that unambiguous evidence of PSP (presence of internal pneumatic cavities linked to the exterior by foramina) is found only in bird-line (ornithodiran) archosaurs. We propose that pulmonary air sacs were present in the common ancestor of Ornithodira and may have been subsequently lost or reduced in some members of the clade (notably in ornithischian dinosaurs). The development of these avian-like respiratory features might have been linked to inferred increases in activity levels among ornithodirans. By contrast, no crocodile-line archosaur (pseudosuchian) exhibits evidence for unambiguous PSP, but many of these taxa possess the complex array of vertebral laminae and fossae that always accompany the presence of air sacs in ornithodirans. These laminae and fossae are likely homologous with those in ornithodirans, which suggests the need for further investigation of the hypothesis that a reduced, or non-invasive, system of pulmonary air sacs may be have been present in these taxa (and secondarily lost in extant crocodilians) and was potentially primitive for Archosauria as a whole.

Smok wawelski, a New Late Triassic Archosaur from Poland

Interesting critter if all the material belongs to the same animal (and I can't see any reason right now why it wouldn't). The broadly expanded basisphenoid appears to be particularily autapomorphic. Taxonomically I'm going to put my bet on a fairly derived paracrocodylomorph, especially because the supratemporal fossa incises the posterior margin of the frontal; however, the presence of a postfrontal suggests it is not a crocodylomorph. I'll refrain from plugging this taxon into the Nesbitt (2011) matrix as the authors clearly state that the phylogenetic relationships are part of Grzegorz Niedźwiedzki's PhD project.

I do have a question regarding the specific name. If the specific name intends "from Wawel Hill" should the correct name be Smok wawelensis or maybe S. wawelhillensis? As I have stated before I don't think it is the best idea for new taxon names to be published online in accepted manuscript form because the authors may decide to change the name or modify it before final publication.

Niedźwiedzki, G., Sulej, T., and J. Dzik. in press A large predatory archosaur from the Late Triassic of Poland. Acta Palaeontologica Polonica. doi:10.4202/app.2010.0045

Abstract - We describe a new large predatory archosaur, Smok wawelski gen. et sp. nov., from the latest Triassic (latest Norian-early Rhaetian; approximately 205-200 Ma) of Lisowice (Lipie Śląskie clay-pit) in southern Poland. The length of the reconstructed skeleton is 5-6 m and that of the skull 50-60 cm, making S. wawelski larger than any other known predatory archosaur from the Late Triassic and Early Jurassic of central Europe (including theropod dinosaurs and rauisuchian crurotarsans). The holotype braincase is associated with skull, pelvic and isolated limb-bones found in close proximity (within 30 m), and we regard them as belonging to the same individual. Large, apparently tridactyl tracks that occur in the same rock unit may have been left by animals of the same species. The highly autapomorphic braincase shows large attachment areas for hypertrophied protractor pterygoideus muscles on the lateral surface and a wide, funnel-like region between the basal tubera and basipterygoid processes on the ventral surface. The skeleton (cranial and postcranial) possesses some features similar to those in theropod dinosaurs and others to those in large crocodile-line archosaurs (rauisuchians), rendering phylogenetic placement of S. wawelski difficult at this time.

A New Exhaustive Phylogenetic Analysis of Archosauria

I've been hinting at this for a bit but it is finally out. Congratulations to Sterling Nesbitt on an amazingly detailed and robust study of archosaurian relationships and their early evolution and distributions. This will be the new standard for archosaurian phylogeny and biogeography.

Nesbitt, S. J. 2011. The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades. Bulletin of the American Museum of Natural History 352:1-292. [Free Download]

Abstract - Archosaurs have a nearly 250 million year record that originated shortly after the Permian-Triassic extinction event and is continued today by two extant clades, the crocodylians and the avians. The two extant lineages exemplify two bauplan extremes among a diverse and complex evolutionary history, but little is known about the common ancestor of these lineages. Renewed interest in early archosaurs has led to nearly a doubling of the known taxa in the last 20 years.

This study presents a thorough phylogenetic analysis of 80 species-level taxa ranging from the latest Permian to the early part of the Jurassic using a dataset of 412 characters. Each terminal taxon is explicitly described and all specimens used in the analysis are clearly stated. Additionally, each character is discussed in detail and nearly all of the character states are illustrated in either a drawing or highlighted on a specimen photograph. A combination of novel characters and comprehensive character sampling has bridged previously published analyses that focus on particular archosauriform subclades.

A well-resolved, robustly supported consensus tree (MPTs  =  360) found a monophyletic Archosauria consisting of two major branches, the crocodylian-line and avian-line lineages. The monophyly of clades such as Ornithosuchidae, Phytosauria, Aetosauria, Crocodylomorpha, and Dinosauria is supported in this analysis. However, phytosaurs are recovered as the closest sister taxon to Archosauria, rather than basal crocodylian-line archosaurs, for the first time. Among taxa classically termed as “rauisuchians,” a monophyletic poposauroid clade was found as the sister taxon to a group of paraphyletic “rauisuchians” and monophyletic crocodylomorphs. Hence, crocodylomorphs are well nested within a clade of “rauisuchians,” and are not more closely related to aetosaurs than to taxa such as Postosuchus. Basal crocodylomorphs such as Hesperosuchus and similar forms (“Sphenosuchia”) were found as a paraphyletic grade leading to the clade Crocodyliformes. Among avian-line archosaurs, Dinosauria is well supported. A monophyletic clade containing Silesaurus and similar forms is well supported as the sister taxon to Dinosauria. Pterosaurs are robustly supported at the base of the avian line.

A time-calibrated phylogeny of Archosauriformes indicates that the origin and initial diversification of Archosauria occurred during the Early Triassic following the Permian-Triassic extinction. Furthermore, all major basal archosaur lineages except Crocodylomorpha were established by the end of the Anisian. Early archosaur evolution is characterized by high rates of homoplasy, long ghost lineages, and high rates of character evolution. These data imply that much of the early history of Archosauria has not been recovered from the fossil record. Not only were archosaurs diverse by the Middle Triassic, but they had nearly a cosmopolitan biogeographic distribution by the end of the Anisian.

Triassic Archosaur Taxonomic Diversity and Morphological Disparity: Redux.

Most analyses of taxonomic diversity and morphological disparity in the fossil record use a taxic approach, in which the researcher simply counts numbers of known taxa and uses these data for comparison. However, because the fossil record is spotty how can we be sure that these data are not affected by collecting biases?  Phylogentic analyses provide a useful tool, because they project 'ghost lineages' depicting where these missing data may occur.  Brusatte et al. (2008) looked at taxonomic diversity vs. morphological disparity in Triassic archosaurs using a taxic methodology.  In that study they found that diversity and rates of morphological change were decoupled in regards to comparisons between Triassic pseudosuchians and ornithodirans. 

In this paper Brusatte et al. provide a methodology for correcting the data phylogenetically, which for the most part consists of determining the character states for nodes within the MPTs and then adding these 'ancestors' to the diversity vs. disparity analysis.  They look at several case studies and find for the Triassic archosaur study generally the same overall results as Brusatte et al. (2008).

Brusatte, S. L., Montanari, S., Yi, H.-y., and M. A. Norell. 2011. Phylogenetic corrections for morphological disparity analysis: new methodology and case studies. Paleobiology 37:1-22. DOI: 10.1666/09057.1Abstract - Taxonomic diversity and morphological disparity are different measures of biodiversity that together can describe large-scale evolutionary patterns. Diversity measures are often corrected by extending lineages back in time or adding additional taxa necessitated by a phylogeny, but disparity analyses focus on observed taxa only. This is problematic because some morphologies required by phylogeny are not included, some of which may help fill poorly sampled time bins. Moreover the taxic nature of disparity analyses makes it difficult to compare disparity measures with phylogenetically corrected diversity or morphological evolutionary rate curves. We present a general method for using phylogeny to correct measures of disparity, by including reconstructed ancestors in the disparity analysis. We apply this method to discrete character data sets focusing on Triassic archosaurs, Cenozoic carnivoramorph mammals, and Cretaceous–Cenozoic euarchontogliran mammals. Phylogenetic corrections do not simply mirror the taxic disparity patterns, but affect the three analyses in heterogeneous ways. Adding reconstructed ancestors can inflate morphospace, and the amount and direction of expansion differs depending on the taxonomic group in question. In some cases phylogenetic corrections give a temporal disparity curve indistinguishable from the taxic trend, but in other cases disparity is elevated in earlier time intervals relative to later bins, due to the extension of unsampled morphologies further back in time. The phylogenetic disparity curve for archosaurs differs little from the taxic curve, supporting a previously documented pattern of decoupled disparity and rates of morphological change in dinosaurs and their early contemporaries. Although phylogenetic corrections should not be used blindly, they are helpful when studying clades with major unsampled gaps in their fossil records.
 
REFERENCE
 
Brusatte, S.L., Benton, M.J., Ruta, M., and G.T. Lloyd. 2008. Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs. Science 321:1485-1488.

Enigmatic Archosaur Remains from the Middle Triassic of the UK

Benton, M. J. 2010. Archosaur remains from the Otter Sandstone Formation(MiddleTriassic, late Anisian) of Devon, southern UK. Proceedings of the Geologists' Association (early online), doi:10.1016/j.pgeola.2010.08.004

Abstract - A new jaw from the Middle Triassic (Anisian) Otter Sandstone Formation of Devon confirms the existence of a derived archosaur(avesuchian). Numerous isolated teeth and vertebrae had already suggested the presence of archosaurs in the Otter Sandstone Formation, presumed predators on the fauna of temnospondyls, procolophonids, and rhynchosaurs, but the new fossil is the first to show some diagnostic characters. Other elements in the same block as the jaw, but not necessarily from the same animal, include a possible skull or pelvic bone, a slender long bone, a small tooth (perhaps prolacertiform), and two presumed archosaur dermal scutes. An additional scute is present, as well as the probable distal end of a pubis, perhaps from a large poposauroid archosaur like the Anisian-age Bromsgroveia or Arizonasaurus. The jaw and pubis represent animals of very different sizes, some 0.8m and 3m long in estimated body length respectively.

If Azendohsaurus is not a Sauropodomorph Then What is it?

Would you believe an basal archosauromorph?

Flynn, J. J., Nesbitt, S. J., Parrish, J. M., Ranivoharimanana, L., and A. R. Wyss. 2010. A new species of Azendohsaurus (Diapsida: Archosauromorpha) from the Triassic Isalo Group of southwestern Madagascar: cranium and mandible. Palaeontology 53:669-688.

As if the Triassic couldn't get any weirder.  If this discovery does not finally demonstrate the peril of assigning isolated jaw fragments and teeth to various dinosaurian subgroups, I do not know what will.  The placement of Azendohsaurus as a basal archosauromorph demonstrates that herbivory has evolved independently numerous times within Archosauromorpha and was actually much more common in this clade than previously believed.  Some of the primitive cranial features found in Azendohsaurus include a pineal opening, an incomplete lower temporal bar, and palatal teeth.  One unique feature of Azendohsaurus is that the palatal teeth are actually leaf-shaped with denticles, very similar to the marginal teeth.

Skull reconstruction of Azendohsaurus. From Flynn et al. 2010.

Abstract - Here, we describe a new species of Azendohsaurus from the Middle–Late Triassic of Madagascar, extending the geographical range of a taxon known otherwise only by a single species from Morocco. Although Azendohsaurus has consistently been regarded as an early dinosaur (based on various advanced dental and gnathic features resembling those characterizing certain dinosaur subgroups), the relatively complete skeletal material, now available from Madagascar, argues strongly against its dinosaurian affinities. Rather, the retention of numerous primitive cranial and postcranial features indicates a surprisingly early divergence of Azendohsaurus within Archosauromorpha and an unusual mosaic of characters in this taxon. Features considered diagnostic of Sauropodomorpha thus are inferred to occur homoplastically in at least one clade of nondinosaurian archosauromorphs, indicating a complex evolution and distribution of features traditionally thought to be derived within archosaurs. Azendohsaurus has teeth resembling those of both early sauropodomorph and ornithischian dinosaurs, yet also possesses numerous inarguable basal archosauromorph cranial and postcranial attributes. This highlights the risk of uncritically referring isolated, Middle–Late Triassic (or even later), ‘leafshaped’ teeth with denticles to the Dinosauria. Similarly, the occurrence of such teeth in an early diverging archosauromorph indicates that specializations for herbivory originated more frequently within this clade than conventionally assumed. For example, Azendohsaurus and numerous basal sauropodomorph dinosaur taxa share an array of convergently acquired features associated with herbivory, including tooth denticles, expanded tooth crowns, a downturned dentary and the articular located at the ventral margin of the mandible. Some of these features (denticles, expanded crowns and the ventrally deflected articular) are even more widespread among archosauromorphs, including aetosaurs, silesaurs and ornithischian dinosaurs. A downturned dentary also occurs in Trilophosaurus, a taxon further marked by unique specializations for herbivory, including transversely lophate, tricuspid teeth. An array of features associated with herbivory also occur in rhynchosaurs and certain crocodilians (e.g. Simosuchus). This distribution suggests that craniodental features associated with herbivory were much more pervasive across the archosauromorph clade than previously recognized, possibly evolving at least six to eight times independently.

New Paper on Archosaurian Locomotion Evolution

Livingston, V.J., Bonnan, M.F., Elsey, R.M., Sandrik, J.L., and D.R. Wilhite.2009. Differential limb scaling in the american alligator (Alligator mississippiensis) and its implications for archosaur locomotor evolution. The Anatomical Record 292:787-797. doi 10.1002/ar.20912

Abstract- Bipedalism evolved multiple times within archosaurs, and relatively shorter forelimbs characterize both crocodyliforms and nonavian dinosaurs. Analysis of a comprehensive ontogenetic sequence of specimens (embryo to adult) of the sauropodomorph Massospondylus has shown that bipedal limb proportions result from negative forelimb allometry. We ask, is negative forelimb allometry a pattern basal to archosaurs, amplified in certain taxa to produce bipedalism? Given the phylogenetic position of extant crocodylians and their relatively shorter forelimb, we tested the hypothesis that prevalent negative forelimb allometry is present in Alligator mississippiensis from a sample of wild specimens from embryonic to adult sizes. Long bone lengths (humerus, radius, ulna, femur, tibia, fibula, third metapodials) were measured with their epiphyseal cartilage intact at all sizes. Our results show an overall isometric pattern for most elements regressed on femur length, humerus length, or total limb length. However, negative allometry was prevalent for the ulna, and the third metapodials scale with positive allometry embryonically. These data suggest that the general forelimb proportions in relation to the hindlimb do not change significantly with increasing size in A. mississippiensis. The negative allometry of the ulna and embryonicaly positive allometry of the third metapodials appears to be related to maintaining the functional integrity of the limbs. We show that this pattern is different from that of the sauropodomorph Massospondylus, and we suggest that if bipedalism in archosaurs is tied, in part, to negative forearm allometry, it was either secondarily lost through isometric scaling, or never developed in the ancestor of A. mississippiensis.

Induan Dinosauromorphs Revised

Mickey commented on my earlier post regarding the recent proclamation of a dinosauromorph specimen from the Induan of Germany. In my original post I stated that crocodile-line archosaurs were present in the Induan in the form of Proterosuchus (see picture below - from Wikipedia). Of course Proterosuchus is a basal archosauriform and NOT a archosaur. I wish that

I could state that this was based on new research that shows that proterosuchids were more derived than previously thought, but of course this is not the case. In fact in an upcoming publication on the enigmatic form Vancleavea (Parker and Barton, in press) we include a phylogenetic analysis that also supports proterosuchids as basal archosauriforms. Thus I really have no excuse except that I simply rushed the original post. Upon reflection, based mainly on Mickey's comment, I realize that there is a little more to say about this find.

The discovery of a dinosauromorph from the Induan (earliest Triassic) would not "fill in" ghost lineages as I said in my earlier post, but rather would extend the ghost lineages for erythrosuchids and euparkeriids back from the Olenekian (late Early Triassic) into the Induan. It would also extend the ghost lineage for proterochampsids and pseudosuchians back from the Ladinian (late Middle Triassic) and Anisian (early Middle Triassic) respectively to the Induan. As a result the ornithodirans would hypothetically appear first in the fossil record in respect for the pseudosuchians. The earliest known described ornithodiran is from the Ladinian.

In my opinion one of the most interesting aspects of the Triassic fossil record is the diversity found in the crocodile-line archosaurs and the bauplan similarities between many pseudosuchians and ornithodirans. In fact I am fond of telling my students that, because the first pseudosuchian appears in the Anisian and the first ornithodiran in the Ladinian, the dinosaurs were simply "crocodile wannabees". I may have to rethink this if this Induan find is confirmed.

REFERENCE

Parker, W.G., and B.J. Barton. In Press. New information on the Upper Triassic archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica.