New Paper on the Origin and Early Evolution of Dinosaurs

Langer, M.C., Ezcurra, M.D., Bittencourt, J.S., and F.E. Novas. 2009. The origin and early evolution of dinosaurs. Biological Reviews 84:1-56. doi:10.1111/j.1469-185X.2009.00094.x

ABSTRACT-The oldest unequivocal records of Dinosauria were unearthed from Late Triassic rocks(approximately 230 Ma)accumulated over extensional rift basins in southwestern Pangea. The better known of these are Herrerasaurus ischigualastensis, Pisanosaurus mertii, Eoraptor lunensis, and Panphagia protos from the Ischigualasto Formation, Argentina, and Staurikosaurus pricei and Saturnalia tupiniquim from the Santa Maria Formation, Brazil. No uncontroversial dinosaur body fossils are known from older strata, but the Middle Triassic origin of the lineage may be inferred from both the footprint record and its sister-group relation to Ladinian basal dinosauromorphs. These include the typical Marasuchus lilloensis, more basal forms such as Lagerpeton and Dromomeron, as well as silesaurids: a possibly monophyletic group composed of Mid-Late Triassic forms that may represent immediate sister taxa to dinosaurs. The first phylogenetic definition to fit the current understanding of Dinosauria as a node-based taxon solely composed of mutually exclusive Saurischia and Ornithischia was given as ‘‘all descendants of the most recent common ancestor of birds and Triceratops’’. Recent cladistic analyses of early dinosaurs agree that Pisanosaurus mertii is a basal ornithischian; that Herrerasaurus ischigualastensis and Staurikosaurus pricei belong in a monophyletic Herrerasauridae; that herrerasaurids, Eoraptor lunensis, and Guaibasaurus candelariensis are saurischians; that Saurischia includes two main groups, Sauropodomorpha and Theropoda; and that Saturnalia tupiniquim is a basal member of the sauropodomorph lineage. On the contrary, several aspects of basal dinosaur phylogeny remain controversial, including the position of herrerasaurids, E. lunensis, and G. candelariensis as basal theropods or basal saurischians, and the affinity and/or validity of more fragmentary taxa such as Agnosphitys cromhallensis, Alwalkeria maleriensis, Chindesaurus bryansmalli, Saltopus elginensis, and Spondylosoma absconditum. The identification of dinosaur apomorphies is jeopardized by the incompleteness of skeletal remains attributed to most basal dinosauromorphs, the skulls and forelimbs of which are particularly poorly known. Nonetheless, Dinosauria can be diagnosed by a suite of derived traits, most of which are related to the anatomy of the pelvic girdle and limb. Some of these are connected to the acquisition of a fully erect bipedal gait, which has been traditionally suggested to represent a key adaptation that allowed, or even promoted, dinosaur radiation during Late Triassic times. Yet, contrary to the classical ‘‘competitive’’ models, dinosaurs did not gradually replace other terrestrial tetrapods over the Late Triassic. In fact, the radiation of the group comprises at least three landmark moments, separated by controversial (Carnian Norian, Triassic-Jurassic) extinction events. These are mainly characterized by early diversification in Carnian times, a Norian increase in diversity and (especially) abundance, and the occupation of new niches from the Early Jurassic onwards. Dinosaurs arose from fully bipedal ancestors, the diet of which may have been carnivorous or omnivorous. Whereas the oldest dinosaurs were geographically restricted to south Pangea, including rare ornithischians and more abundant basal members of the saurischian lineage, the group achieved a nearly global distribution by the latest Triassic, especially with the radiation of saurischian groups such as ‘‘prosauropods’’ and coelophysoids.

More Free PDFs

Paläontologische Zeitschrift, now published by Springer, is offering open access for the month of November only.

http://www.springerlink.com/content/0031-0220?sa_campaign=email/PROM/PSE11800_V1

The New Mexico Museum of New Mexico and Science is also offering free PDFs of many of its published bulletins. Obviously there are lots of free Triassic PDFs here but the one that I highly recommend is Bulletin #4 (Long and Murry, 1995 - Late Triassic Carnian and Norian Tetrapods from the Southwestern United States). Although becoming dated it is still one of the quintessential references for anyone interested in Late Triassic vertebrates of the American southwest (and one of the most cited Triassic references ever). This has been out of print for awhile and those who have copies know that they were poorly bound and thus most existing copies are in tatters. Here is your chance to get the volume in its entirety.

http://econtent.unm.edu/cdm4/document.php?CISOROOT=/bulletins&CISOPTR=659&REC=6

BTW...you'll have to turn off your pop-up blocker to get the bulletin.

Tyrannosauroid from the Bathonian (Middle Jurassic) of England

Important new Jurassic paper for those of you not subscribed to the VRTPALEO listserve...

RAUHUT, O.W.M., A.C. MILNER, and S. MOORE-FAY. 2009. Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi(Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society Early View doi: 10.1111/j.1096-3642.2009.00591.x

ABSTRACT
The cranial osteology of the small theropod dinosaur Proceratosaurus from the Bathonian of Minchinhampton, England, is described in detail, based on new preparation and computed tomography (CT) scan images of the type, and only known, specimen. Proceratosaurus is an unusual theropod with markedly enlarged external nares and a cranial crest starting at the premaxillary-nasal junction. The skull is highly pneumatic, with pneumatized nasals, jugals, and maxillae, as well as a highly pneumatic braincase, featuring basisphenoid, anterior tympanic, basipterygoid, and carotid recesses. The dentition is unusual, with small premaxillary teeth and much larger lateral teeth, with a pronounced size difference of the serrations between the mesial and distal carina. The first dentary tooth is somewhat procumbent and flexed anteriorly. Phylogenetic analysis places Proceratosaurus in the Tyrannosauroidea, in a monophyletic clade Proceratosauridae, together with the Oxfordian Chinese taxon Guanlong. The Bathonian age of Proceratosaurus extends the origin of all clades of basal coelurosaurs back into the Middle Jurassic, and provides evidence for an early, Laurasia-wide, dispersal of the Tyrannosauroidea during the late Middle to Late Jurassic.

Enigmatic Triassic Fossils - Agnosphitys cromhallensis

Alton Dooley over at Updates from the VPL has some excellent photos of the holotype material of Agnosphitys cromhallensis from the Upper Triassic fissure fills of England. The name, 'unknown begetter', reflects upon the unclear phylogentic relationships of this taxon. The original describers, Fraser et al. (2002), considered it to be a dinosauromorph more derived than Eoraptor and Herrerasaurus, but just outside of Dinosauria. It is known from an isolated left ilium, a left maxilla, a right humerus and a left astragalus. There is also a referred tooth. The association of this material was questioned with Langer (2004) considering it a nomen dubium.

The ilium possesses a distinct brevis shelf and 'open' ventral portion of the acetabulum (so it was slightly perforated), yet only has scars for two sacrals. The subrectangular deltopectoral crest on the humerus is typical for dinosauromorphs and the astragalus is very similar to that of basal saurischians (Nesbitt et al., 2007). Thus these specimens contain a melange of basal and derived characters making its phylogenetic placement uncertain (Fraser et al., 2002). This is also a very small animal as the humerus length is under 35mm. Unfortunately, more material needs to be found to elucidate the relationships of this animal, but what is present is not only very well preserved but extremely interesting.

REFERENCES

Fraser, N. C., Padian, K., Walkden, G. M., and Davis, A. L. M. 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology 45:79–95.

Langer, M.C. 2004. Basal Saurischia. In: Weishampel, D.B., Dodson, P., & Osmolska, H. (Eds.). The Dinosauria (2nd Edition). Berkeley: University of California Press. Pp. 25–46.

Nesbitt, S. J., Irmis, R. B. & Parker, W. G. 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5, 209-243.

Why I Use Pseudosuchia Instead of Crurotarsi for the Crocodile-line Archosaurs

This issue was briefly mentioned on the Vertebrate Paleontology List-server about a week ago and I feel like discussing this a bit further. A quick perusal of archosaurian literature will show that there is a dichotomy in the nomenclature when referring to the lineage of Archosauria that leads to the crocodylians. Some workers will refer to this clade as Pseudosuchia and others as Crurotarsi, with the latter seeming to be the most popular. However, I along with some of my closest Triassic collaborators prefer Pseudosuchia over Crurotarsi, and often when I review papers I will suggest that workers using Crurotarsi switch. Why? Well for me it comes down to two issues, priority and stability.

This problem has been reviewed by several workers, such as Brochu (1997) and most recently Senter (2005). Gauthier and Padian (1985) defined Pseudosuchia as "crocodiles and all archosaurs closer to crocodiles than to birds"; however, Pseudosuchia has a much longer history dating back to the late 1800s, and although it has always had the intent of containing non-dinosaurian archosaurs, the membership of this clade has changed through the years and at some points even included some of what are now considered to be non-dinosaurian ornithodirans (the bird-line clade).

Several authors disliked Gauthier and Padian's (1985) redefining of what they considered to be an "ill-defined and misused" name. Furthermore, the name is in a sense contradictory as Pseudosuchia means "false-crocodiles" yet includes crocodiles as members. Accordingly Benton and Clark (1988) suggested a new name, "Crocodylotarsi", and although they did not explicitly define this clade they inferred that it was the same as Pseudosuchia. Sereno and Arcucci (1990) proposed a third name, Crurotarsi, which Sereno (1991) defined as "Parasuchia, Ornithosuchidae, Prestosuchus, Suchia, and all decendents of their common ancestor".

However, just because a group once contained members that have since been recognized as belonging to other groups does not warrant abandonment. Indeed if this were the case very few names would be valid, including Dinosauria. Likewise, contradictory names also are not grounds for dismissal, for example the name phytosaur means 'plant-reptile' although they surely ate everything but. Despite this we are still stuck with the contradictory name. Furthermore, there is no confusion among modern workers as to the meaning of Pseudosuchia, so statements to the contrary are moot. When I say something is a pseudosuchian, those familiar with archosaurs clearly understand what I mean.

Crocodylotarsi has been used by some workers but has since fallen out of usage with most modern workers using either Pseudosuchia or Crurotarsi. As all three groups currently have the same membership it has been argued by some that Pseudosuchia should be used as it has precedence. I agree in principle that the first defined name should have priority and this is the reason that I use Pseudosuchia instead of Crocodylotarsi. However, as noted by Brochu (1997) and Senter (2005) the definitions of Pseudosuchia and Crurotarsi are not the same.

Pseudosuchia is stem-based and thus is flexible to future changes, as any archosaur that is not an ornithodiran is included in this group. Crurotarsi, however, is defined as a node-based taxon and thus has an explicit membership, most notably phytosaurs (parasuchians), ornithosuchids, Prestosuchus, aetosaurs, "rauisuchians", and crocodylomorphs. This definition is much less flexible. In fact let's just suppose that the basal most group of crurotarsans, the phytosaurs, fell outside of the crown-clade Archosauria, and were instead considered to be derived archosauriforms. What would happen to Pseudosuchia and Crurotarsi? The content of Pseudosuchia would be pretty much the same except that phytosaurs would no longer be constituents. In contrast, because the base of Crurotarsi is specified by phytosaurs, Crurotarsi would now include phytosaurs plus all of Archosauria. Thus dinosaurs (including birds) would be Crurotarsans by definition. As you can see this definition is much less stable, another reason why I prefer and highly recommend that all workers use Pseudosuchia over Crurotarsi. Admit it, having to say "non-phytosaurian and non-ornithiodiran crurotarsan" is pretty clunky!

Actually, if this ever did happen ;), in my eyes Crurotarsi might actually now be a useful name when discussing phytosaurs, as you could now simply say that phytosaurs are the basalmost crurotarsans and still be correct. This is probably just slightly more explicit than simply saying they are derived archosauriforms.

For much more detail on this issue read Brochu (1997) and Senter (2005). You can also check out this page for a different opinion.

REFERENCES

Benton, M.J., and J.M. Clark. 1988. Archosaur phylogeny and the relationships of the Crocodylia. Pp. 295–338 in M.J. Benton (ed.). The Phylogeny and Classifi cation of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Clarendon Press, Oxford.

Brochu, C.J. 1997. Synonymy, Redundancy, and the name of the crocodile stem group. Journal of Vertebrate Paleontology 17:448-449.

Gauthier, J., and K. Padian. 1985. Phylogenetic, functional, and aerodynamic analyses of the origin of birds and their flight. Pp. 185–197 in M.K. Hecht, J.H. Ostrom, G. Viohl, and P.
Wellnhofer (eds.). The Beginnings of Birds. Freunde des Jura- Museums, Eichstätt.

Senter, P. 2005. Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades. PaleoBios 25:1–7.
Sereno, P.C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Society of Vertebrate Paleontology Memoir 2:1–53.

Sereno, P.C., and A.B. Arcucci. 1990. The monophyly of crurotarsal archosaurs and the origin of bird and crocodile ankle joints. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen
180:21–52.

Phytosaur Taxonomy Flowchart

I remember sitting down one evening back in 2001 and putting this together. It proceeds through time from left to right. This is why nobody wants to research phytosaur taxonomy!

Rhaetian Magneto-biostratigraphy from the Southern Alps (Italy): Constraints on Triassic Chronology

This is fairly significant as it argues for quite a long Rhaetian Stage of the latest Triassic. If this is correct it means that a sizable portion of the upper part of the Chinle Formation could actually be Rhaetian in age. Not bad considering not too long ago workers were arguing if the Rhaetian was really even applicable outside of the marine realm.

Muttoni, G., Kent, D.V., Jadoul, F., Olsen, P.E., Rigo, M., Galli, M.T., and A. Nicora. 2009. Rhaetian magneto-biostratigraphy from the Southern Alps (Italy): Constraints on Triassic chronology. Palaeogeography, Palaeoclimatology, Palaeoecology. doi: 10.1016/j.palaeo.2009.10.014.

ABSTRACT: New Late Triassic-earliest Jurassic magneto-biostratigraphic data have been obtained from three overlapping sections in the Southern Alps, Italy (Costa Imagna, Brumano, Italcementi Quarry), comprised of ~ 520 m of shallow marine carbonates outcropping in stratigraphic continuity. Characteristic magnetic components of presumed depositional age record a sequence of 9 normal and reverse polarity magnetozones (as defined by at least three stratigraphically superposed samples) linked by conodont and palynofloral evidence from this study and the literature to Rhaetian to Triassic–Jurassic boundary age. This represents a significantly larger number of polarity zones than previously recognized in more condensed
Rhaetian sections from the literature, and by inference represents more time. These data are placed in a broader Late Triassic temporal framework by means of correlations to published magneto-biostratigraphic data from the Tethyan marine Pizzo Mondello section and the Newark astronomical polarity time scale (APTS). This framework is consistent with a position of the
Norian–Rhaetian boundary (as defined at Brumano and Pizzo Mondello by the first appearance of Misikella posthernsteini) within Newark magnetozones E17r–E19r in the ~ 207–210 Ma time interval, in basic agreement with the position originally estimated in the Newark using pollen biostratigraphy (E18 at 208–209 Ma). This framework is also consistent with the position of
the Triassic–Jurassic boundary interval (placed at Italcementi Quarry at the acme of Kraeuselisporites reissingeri coincident with a negative carbon isotope excursion) correlative to just above Newark magnetozone E23r and just below the oldest CAMP lavas dated at ~ 202 Ma. Hence, we estimate the duration of the Rhaetian to be ~ 5.5–8.5 Myr (or even longer if the
Triassic–Jurassic boundary is instead placed above the negative carbon isotope excursion as at Kuhjoch, which is the designated GSSP for the base of the Hettangian), and encompassing 9 magnetozones. This duration contrasts with a duration of ~ 2 Myr and only ~ 4 magnetozones in several alternative published magneto-biostratigraphic schemes.