Field of Science

Anatomy of the Late Triassic Ornithischian Eocursor parvus

Richard Butler's full description of the early ornithischian Eocursor parvus is now available online in the Zoological Journal of the Linnaean Society. As detailed by Richard, this taxon is of importance not only because of its phylogenetic position, but also because it represents only one of three purported ornithischian specimens from the Triassic Period and is the best preserved.

Unlike Pisanosaurus and the unnamed jaw fragment from Argentina assigned to the Heterodontosauridae, Eocursor can clearly be demonstrated to be a bona fide ornithischian. The potential problem with this specimen, and outlined in the paper, is debate on the stratigraphic position and thus the age of the specimen. There is indeed the possibility that this specimen may actually be Jurassic in age and hopefully some future studies on the provenance and age of the specimen can be done to verify the age. Splitting hairs? Maybe, but the exact age is very important when looking at tempo and mode of early dinosaur evolution. When you only have three physical specimens of a group from a specific time period, each one is of the utmost importance.

Nonetheless, this is still an extremely important specimen and congratulations to Richard on another solid contribution to vertebrate paleontology and early dinosaur studies.


Butler, R. J. 2009. The anatomy of the basal ornithischian dinosaur Eocursor parvus from the lower Elliot Formation(Late Triassic) of South Africa. Zoological Journal of the Linnaean Society. Published early online, doi: 10.1111/j.1096-3642.2009.00631.x

Abstract - Ornithischia is a morphologically and taxonomically diverse clade of dinosaurs that originated during the Late Triassic and were the dominant large-bodied herbivores in many Cretaceous ecosystems. The early evolution of ornithischian dinosaurs is poorly understood, as a result in part of a paucity of fossil specimens, particularly during the Triassic. The most complete Triassic ornithischian dinosaur yet discovered is Eocursor parvus from the lower Elliot Formation (Late Triassic: NorianRhaetian) of Free State, South Africa, represented by a partial skull and relatively complete postcranial skeleton. Here, the anatomy of Eocursor is described in detail for the first time, and detailed comparisons are provided to other basal ornithischian taxa. Eocursor is a small-bodied taxon (approximately 1 m in length) that possesses a plesiomorphic dentition consisting of unworn leaf-shaped crowns, a proportionally large manus with similarities to heterodontosaurids, a pelvis that contains an intriguing mix of plesiomorphic and derived character states, and elongate distal hindlimbs suggesting well-developed cursorial ability. The ontogenetic status of the holotype material is uncertain. Eocursor may represent the sister taxon to Genasauria, the clade that includes most of ornithischian diversity, although this phylogenetic position is partially dependent upon the uncertain phylogenetic position of the enigmatic and controversial clade Heterodontosauridae.

The Osteology and Relationships of Vancleavea campi (Reptilia: Archosauriformes)

Um...OK, this came out a bit earlier than I expected and is now available online from the Journal of the Linnaean Society. This is the long awaited full description of this taxon, which supersedes my 2008 publication which came out when this paper was in press and therefore not cited. Sterling felt that the new PEFO material should be dealt with in a separate publication, but unfortunately I did not get it out in time for him to use, and to ask him and his coauthors to go back through this manuscript in the proof stages and add info from my paper would have been too much. So we are left with a weird case in which two papers come out on the same taxon and neither cite each other, but I assure you that we were all aware of each other's work before anyone questions ethics. Sterling did compare the GR material to one of the PEFO specimens; whereas I did not describe or code any of the Ghost Ranch material, thus this new paper is up to date in that regard.

I guarantee you that this is one of the most bizarre archosauriforms you may ever encounter, see the above reconstruction courtesy of Sterling Nesbitt. Enigmatic since its initial discovery in 1962 (from Petrified Forest National Park) and initial description by Long and Murry in 1995, recent collection of better specimens has elucidated its osteology and broader relationships. Vancleavea is actually common at most levels of the Chinle Formation; however, due to the scrappy nature of most of the materials it is difficult to compare specimens across stratigraphic levels. Are we looking at one species or several? I congratulate Sterling, Michelle, Bryan, and Alex on a great discovery and paper.

Nesbitt, S. J., Stocker, M. R., Small, B. J., and A. Downs. 2009. The osteology and relationships of Vancleavea campi(Reptilia: Archosauriformes). Zoological Journal of the Linnaean Society 157:814–864 doi: 10.1111/j.1096-3642.2009.00530.x

Vancleavea campi Long & Murry, 1995, from the Late Triassic of western North America, represents the latest surviving non-archosaurian archosauriform known to date. We present here a detailed comparative description based on a nearly complete, articulated skeleton from the Coelophysis Quarry in north-central New Mexico and other fragmentary specimens. The unique combination of morphological features of Vancleavea is unparalleled within Reptilia; it has four unique morphologies of imbricated osteoderms covering the entire body, a short, highly ossified skull, relatively small limbs and morphological features consistent with a semi-aquatic lifestyle. Vancleavea is placed in a rigorous phylogenetic analysis examining the relationships of non-archosaurian archosauriforms, and is found to be more closely related to Archosauria than both Erythrosuchus and Proterosuchus, but outside of the crown group. The analysis confirms previously hypothesized relationships, which found Euparkeria to be the closest sister taxon of Archosauria. It is not clear whether specimens referred to Vancleavea campi represent a single species-level taxon or a clade of closely related taxa that lived through much of the Late Triassic of North America, given the poor fossil record of the taxon.

Cruxicheiros newmanorum, a new Middle Jurassic Theropod from England

Benson, R. B. J., and J. D. Radley. in press. A new large-bodied theropod dinosaur from the Middle Jurassic of Warwickshire, United Kingdom. Acta Palaeontologica Polonica, available online 20 Nov 2009.

Abstract - Previously undocumented postcranial material from the Chipping Norton Limestone Formation (Middle Jurassic: Lower Bathonian) of Cross Hands Quarry, near Little Compton, Warwickshire represents a new large-bodied theropod dinosaur, distinct from the contemporaneous Megalosaurus bucklandii. Cruxicheiros newmanorum gen. et sp. nov. is diagnosed by a single autapomorphy, the presence of a proximomedially inclined ridge within the groove that marks the lateral extent of the posterior flange of the femoral caput (trochanteric fossa). C. newmanorum shows three tetanuran features: widely separated cervical zygapophyses, a swollen ridge on the lateral surface of the iliac blade and an anterior spur of the caudal neural spines. However, due to fragmentary preservation its affinities within Tetanurae remain uncertain: phylogenetic analysis places it as the most basal tetanuran, the most basal megalosauroid (= spinosauroid) or the most basal neotetanuran.

Galileo's Fingers Reportedly Resurface After a Century

Interesting, yet slightly bizarre, story out today. Especially the part that discusses how this type of dismemberment is commonly done with Catholic saints, whose body parts purportedly possess sacred powers.

http://www.cnn.com/2009/WORLD/europe/11/23/galileo.fingers/index.html

BTW...doesn't "Galileo's Fingers" sound like a good name for a rock band?

Petrified Forest Landmarks - Eagle Nest Rock

For the first 35 years of its existence as a National Monument, one of the major attractions of the Petrified Forest was Eagle Nest Rock in Jasper Forest. As seen below in an undated National Park Service photo. Eagle Nest Rock was a erosional pillar of sandstone in the Jasper Forest bed of the Sonsela Member (Chinle Formation).

The photo [NPS] below is a closer view, showing how remarkable (and delicate) this feature was. It stood about 15 feet (5 meters) tall.

A gravel road detoured off of the main park road and wound its way through the Jasper Forest (numerous petrified logs) turned to the southwest and at its terminus curved around Eagle Nest Rock.



This was a very popular attraction and visitors (and park rangers) would climb up and have their photo taken next to the pillar. The photo [courtesy of the UCMP]. below was taken of Clyde Polacca (of the Hopi Reservation) in 1923 by Charles L. Camp during his paleontological research of the Petrified Forest area


Sadly Eagle Nest Rock "landmark in the First Forest, fell January 25 and 26 [1941] following a month of considerable rain and some high winds.” - Howard R. Stagner, monthly naturalist report.
The old Jasper Forest road was closed afterwards to protect fossil resources (petrified wood) and now only an overlook exists from which you cannot see the Eagle Nest Rock site. The old road was never removed and you can still follow it today. Below is a reshoot of Camp's photo from 2006.


New Blog

Back at the beginning of the year I had this post on my American Civil War interests. Much of this stems from the letters and diary I have from an ancestor who fought in the war. I've been researching these letters and his regiment for almost 20 years now in hopes of someday putting it in book form. I've realized that this is not going to happen anytime soon, so what I have decided to do in the meantime is put portions of this research in blog form to share with other enthusiasts.

So I am happy to announce the launch of a new blog titled "three month men" which essentially is a study of the First Connecticut Volunteer Regiment in the American Civil War, chiefly through the words of one of the participants. What will be presented is my ancestor's role in the American Civil War, portraying events from the end of 1860 through the days after the First Battle of Bull Run in April of 1861. I will present excerpts from his diary and letters and the posts will be ordered chronologically and cover a day or two of time. Thus I hope that people will be interested in following his story as the blog progresses. Furthermore, I hope to provide information on a poorly known military unit. As this will truly be a side project I hope to post new material once a week or biweekly. If any of my Triassic readers have a soft spot for American history, in particular the Civil War, I welcome you to follow along. I hope that you will find it of interest.

www.threemonthmen.blogspot.com

Ectothermic Bovids? Is Nothing Sacred?

OK...this is pretty far out from the Triassic, but really interesting so I thought I'd share.

Kohler, M., and S. Moya-Sola. 2009. Physiological and life history strategies of a fossil large mammal in a resource-limited environment. Early online, PNAS. doi: 10.1073/pnas.0813385106

Abstract - Because of their physiological and life history characteristics, mammals exploit adaptive zones unavailable to ectothermic reptiles. Yet, they perform best in energy-rich environments because their high and constant growth rates and their sustained levels of resting metabolism require continuous resource supply. In resource limited ecosystems such as islands, therefore, reptiles frequently displace mammals because their slow and flexible growth rates and low metabolic rates permit them to operate effectively with low energy flow. An apparent contradiction of this general principle is the long-term persistence of certain fossil large mammals on energy-poor Mediterranean islands. The purpose of the present study is to uncover the developmental and physiological strategies that allowed fossil large mammals to cope with the low levels of resource supply that characterize insular ecosystems. Long-bone histology of Myotragus, a Plio-Pleistocene bovid from the Balearic Islands, reveals lamellar-zonal tissue throughout the cortex, a trait exclusive to ectothermic reptiles. The bone microstructure indicates that Myotragus grew unlike any other mammal but similar to crocodiles at slow and flexible rates, ceased growth periodically, and attained somatic maturity extremely late by ~12 years. This developmental pattern denotes that Myotragus, much like extant reptiles, synchronized its metabolic requirements with fluctuating resource levels. Our results suggest that developmental and physiological plasticity was crucial to the survival of this and, perhaps, other large mammals on resource-limited Mediterranean Islands, yet it eventually led to their extinction through a major predator, Homo sapiens.

This is open access from:

www.pnas.orgcgidoi10.1073pnas.0813385106

Aardonyx celestae, a New Transistional Sauropodomorph from the Early Jurassic of South Africa

Yates, A. M., Bonnan, M. F., Neveling, J., Chinsamy, A., and M. G. Blackbeard. 2009. A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism. Proceedings of the Royal Society B. doi:10.1098/rspb.2009.1440. Published online

Abstract - Aardonyx celestae gen. et sp. nov. is described from the upper Elliot Formation (Early Jurassic)of South Africa. It can be diagnosed by autapomorphies of the skull, particularly the jaws, cervical column, forearm and pes. It is found to be the sister group of a clade of obligatory quadrupedal sauropodomorphs (Melanorosaurus + Sauropoda) and thus lies at the heart of the basal sauropodomorph–sauropod transition. The narrow jaws of A. celestae retain a pointed symphysis but appear to have lacked fleshy cheeks. Broad, U-shaped jaws were previously thought to have evolved prior to the loss of gape-restricting cheeks. However, the narrow jaws of A. celestae retain a pointed symphysis but appear to have lacked fleshy cheeks, demonstrating unappreciated homoplasy in the evolution of the sauropod bulk-browsing apparatus. The limbs of A. celestae indicate that it retained a habitual bipedal gait although incipient characters associated with the pronation of the manus and the adoption of a quadrupedal gait are evident through geometric morphometric analysis (using thin-plate splines) of the ulna and femur. Cursorial ability appears to have been reduced and the weight bearing axis of the pes shifted to a medial, entaxonic position, falsifying the hypothesis that entaxony evolved in sauropods only after an obligate quadrupedal gait had been adopted.

Defining Silesauridae

The new paper by Max Langer and collaborators (Langer et al., 2009) provides a phylogenetic definition for the clade Silesauridae. Silesaurids are gracile, quadrupedal, hebivorous/omnivorous dinosauriforms whose first announcement in 2003 (Dzik, 2003) generated a large amount of interest in early dinosaur phylogeny. In fact there were at least five presentations at this years SVP meeting alone that dealt with silesaurs.

Langer et al. (2009) define Silesauridae as "all archosaurs closer to Silesaurus opolensis, than to Heterodontosaurus tucki and Marasuchus lilloensis". Currently it is pretty much agreed that Silesauridae contains Silesaurus opolensis, Sacisaurus agudoensis, Eucoelophysis baldwini, and Pseudolagosuchus major (Nesbitt et al., 2005; Ezcurra, 2006; Irmis et al., 2007b; Langer et al., 2009). Silesaurus also possibly contains Lewisuchus admixtus, Technosaurus smalli, and an isolated specimen from Petrified Forest National Park (Irmis et al., 2005; Parker et al., 2006; Nesbitt et al., 2007; Langer et al., 2009).

Since their first publication it has been debated whether this clade represent dinosauriforms (e.g., Parker et al., 2006; Irmis et al., 2007a; Nesbitt et al., 2007) or possibly basal ornithischians (e.g., Ferigolo and Langer, 2007; Dzik and Sulej, 2007). Currently there is much more support for placement as the sister taxon of Dinosauria (Ezcurra, 2006; Irmis et al. 2007b, Langer et al., 2009). Nonetheless, silesaurids most likely filled an ecological niche later filled by ornithischian and thus possibly supressed early ornithischian diversity through the Late Triassic.

REFERENCES

Dzik, J. 2003. A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23:556-574.

Dzik, J., and T. Sulej. 2007. A review of the Early Triassic Krasiejow biota from Silesia, Poland. Paleontologia Polonica 64:3-27.

Ezcurra, M. D. 2006. A review of the systematic position of the dinosauriform archosaur Eucoelophysis baldwini from the Upper Triassic of New Mexico, U.S.A. Geodiversitas 28:649-684.

Ferigolo, J. and M. Langer. 2007. A late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology 19:23-33.

Irmis, R. B., Parker, W. G., and S. N. Nesbitt, 2005. Critical review of the Late Triassic dinosaur record, part 2: Ornithischia. Society of Vertebrate Paleontology 25(3):73A.

Irmis, R. B., Parker, W. G., Nesbitt, S. J., and J. Liu, 2007. Early ornithischian dinosaurs: the Triassic Record. Historical Biology 19:3-22.

Irmis, R. B., Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H., Woody, D., and A. Downs. 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317:358-361.

Nesbitt, S. N., Irmis, R. B., and W. G. Parker, 2005. Critical review of the Late Triassic dinosaur record, part 3: Saurischians of North America. Society of Vertebrate Paleontology 25(3):96A.

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

Parker, W. G., Irmis, R. B., and S. J. Nesbitt. 2006. Review of the Late Triassic dinosaur record from Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62:160-161.

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.