Field of Science

New Volume on Triassic Archosaurs/ New Cranial Material of Poposaurus gracilis

There is a new volume coming out that is sure to be of interest to all Triassic aficionados. It's titled "Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin" edited by Sterling Nesbitt, Julia Desojo, and Randall Irmis and published in the Special Publication series of the Geological Society of London. The volume stems from a symposium held in 2011 in San Juan, Argentina at the IV Congreso Latinoamericano de Paleontología de Vertebrados.  It was a great meeting and you can read some more about it here [in Spanish].

The new volume includes overviews of many archosaurian clades including Euparkeriidae, Phytosauria, "Rauisuchia", Ornithosuchidae, and of course Aetosauria. There are also a plethora of other more specific papers.  Currently the papers are being released "online first" and not all are up yet. I'm not even sure how many there are to be in the final volume, but you can consider this volume to be the " The Dinosauria" volume for non-ornithodiran archosauriforms and pseudosuchians. Keep checking back as more papers are released and at some point the printed volume should be available.

My own contribution is up. In 2003 a group from Yale University, assisted with staff from the Petrified Forest, excavated what turned out to be a nearly complete, articulated skeleton of the carnivorous pseudosuchian Poposaurus gracilis (more here and here). This specimen has been covered in several papers now (Gauthier et al., 2011; Schachner et al., 2011) and provides us with more information about poposauroids and their amazing convergence with theropod dinosaurs. Unfortunately, the skull of this specimen has eroded prior to dicovery. This was unfortunate because poposauroids show an amazing diversity of forms from presumably quadrupedal, sail-backed toothed forms (e.g., Arizonasaurus babbitti) , to bipedal edentulous forms (Effigia okeeffeae), and of course a quadrupedal sail-backed, edentulous form (Lotosaurus adentus) just to make things interesting. What is poorly understood is the congruence of the aquisition of these characters in poposauroid phylogeny. In this question, Poposaurus gracilis plays a key role as according to recent phylogenetic analyses of Archosauria (Nesbitt, 2011; Butler et al., 2011) it is a mid-grade poposauroid. It is clear from the Yale specimen that P. gracilis was bipedal and lacked a sail. A fragment of premaxilla found with the specimen suggested the presence of teeth but conformation was needed.

In 2008 Petrified Forest paleontology staff (Kate Hazlehurst and Jeff Martz) discovered a beautifully preserved ilium and pubis of Poposaurus gracilis from the base of the Sonsela Member in the park. Associated with this were a partial maxilla, dentary, and strangely a prearticular.  These elements were not complete, but they were enough to show that the skull was very similar to other poposauroids like Arizonasaurus babbitti, but more importantly it confirmed that P. gracilis was toothed.  The new paper by myself and colleague Sterling Nesbitt describes this new material and discusses its implications for specific character acquisition in the poposauroids.  Essentially we find that character acquisition is very complex and evolving quickly within the group with a strong suggestion of convegent evolution not only with theropod dinosaurs but also within the clade Poposauroidea as well.

Parker, W. G., and S. J. Nesbitt. 2013. Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution. From: Nesbitt, S. J., Desojo, J. B. & Irmis, R. B. (eds) Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin. Geological Society, London, Special Publications, 379, http://dx.doi.org/10.1144/SP379.3

Abstract - The partial postcrania of Poposaurus gracilis, a bipedal poposauroid convergent with theropod dinosaurs, has been known for nearly a century, but the skull of P. gracilis has proven elusive. P. gracilis is part of a clade of morphologically divergent pseudosuchians (poposauroids) whose members are sometimes bipedal, lack dentition (i.e. beaks) and some have elongated neural spines (i.e. sails). However, the timing and acquisition of these character states is unknown given the uncertainty of the skull morphology of the ‘mid-grade’ poposauroid P. gracilis. Here, we present the first confirmed skull remains of P. gracilis directly associated with diagnostic pelvic elements that overlap with the holotype. The incomplete skeleton (PEFO 34865) from the Chinle Formation of Petrified Forest National Park (Arizona, USA) includes a left maxilla with a large, mediolaterally compressed tooth, left dentary, right prearticular and a partial postcranium. The character states of P. gracilis (bipedal, ‘sail-less’ and toothed) demonstrate that the evolution of bipedalism, the origin/loss of a dorsal ‘sail’ and the shift to an edentulous beak are complex in poposauroids. P. gracilis is widespread in the Upper Triassic formations in the western USA and is restricted temporally prior to the Adamanian–Revueltian faunal turnover during the Norian.


Passing of William J. Breed

Museum of Northern Arizona geologist and Distinguished Fellow William J. Breed passed away in Flagstaff Arizona on January 22, 2013 after a long and accomplished career predominantly studying the geology and paleontology of the southwestern U.S.  He is also well known for his work with Edwin (Ned) Colbert and Jim Jensen in Antarctica in 1969 when they discovered fossil evidencesupporting the idea of Continental Drift. Among many honors Breed was also a Fellow of the Geological Society of America.

http://azdailysun.com/news/local/obituaries/william-breed-renowned-mna-geologist-dies-in-flagstaff/article_41857e06-1387-53d0-bf6f-f3c469222f65.html

New Spider from the Upper Triassic of Italy

Regardless of the higher taxonomic uncertainty, the Triassic record for spiders is poor and just got a little bit better.

Dalla Vecchia, F. M., and P. A. Selden. 2013. A Triassic spider from Italy. Acta Palaeontologica Polonica forthcoming paper. http://dx.doi.org/10.4202/app.2011.0132

Abstract - A new fossil spider from the Triassic (Norian) Dolomia di Forni Formation of Friuli, Italy, is described as Friularachne rigoi gen. et sp. nov. This find brings the number of known Triassic spider species to four. The specimen is an adult male, and consideration of various features, including enlarged, porrect chelicerae, subequal leg length, and presence of a dorsal scutum, point to its identity as a possible member of the mygalomorph superfamily Atypoidea. If correct, this would extend the geological record of the superfamily some 98–115 Ma from the late Early Cretaceous (?Albian, c. 100–112 Ma) to the late middle–early late Norian (c. 210–215Ma).


 
 
 
 
 Left: Drawing of the fossil of Friularachne rigoi (from Dalla Vecchia & Selden, 2013). Right: Reconstruction of Friularachne rigoi; Credits Lukas Panzarin.
 

The First Published Geological Map of Petrified Forest National Park

Today marks a momentious day in the geological research history of Petrified Forest National Park in Arizona. 35 years ago work began on a draft geological map of the park. Unfortunately the draft, completed in the early 1980s, was never finalized although it served as the basic guide for work through the early 2000s. Around this time the need to update the map was considered and some minor revisions were accomplished.  In 2005 the National Park Service contracted with Northern Arizona University to finally complete the map; however, the work was stalled when it was realized that the current stratigraphic schemes used by the park (proposed in 2002 and 2003) could not recreated by the mapping and therefore something was wrong.

In 2008, Jeff Martz and I decided to tackle both the stratigraphic problems as well as revising the park map in the progress.  As we discussed in our 2010 stratigraphic revision paper (available for free here), mapping is a crucial part of solving stratigraphic problems, because it involves detailed walking out of beds and really tests your proposed correlations.  We walked kilometer after kilometer that first summer, arguing the whole time, but really working out the correlations.  Subsequently Jeff took the lead on completing the mapping and after literally wearing his boots down to 'sandals' (they proudly hang in my office today) completed this work in 2010. His map was then painstakingly digitized and finalized by Lisa Skinner at NAU. The report includes some of Jeff's amazing artwork that he is developing a reputation for in the scientific realm.  I especially find the cross-section to be particularly telling and useful for individuals to place themselves in the stratigraphy as they progress through the park.

The finished product is now available on-line courtesy of our partners at the Arizona Geological Survey as part of their Contributed Map series, and the map and associated report is available as a free download from here.  I'd like to thank Arizona State Geologist Lee Allison for allowing the map to be distributed as part of their publication series and to Mike Conway for putting up with all of our edits through the whole process.  I think that Jeff and Lisa in particular did an amazing job finalizing the product and finally after 35 years there is an official map of the park.  Now we just need to add the park expansion areas added in 2007 to the present.....

Two New Temnospondyl Papers - Phylogeny of Major Clades and Suction Feeding in Gerrothorax

Schoch, R. R. 2013. The evolution of major temnospondyl clades: an inclusive phylogenetic analysis. Journal of Systematic Palaeontology DOI:10.1080/14772019.2012.699006http://www.tandfonline.com/doi/full/10.1080/14772019.2012.699006

Abstract - Phylogenetic analysis of a large dataset (72 taxa, 212 characters) focuses on the in-group relationships of temnospondyls, the largest lower tetrapod clade. Representatives of all clades and grades were considered, spanning the entire stratigraphical range of temnospondyls from the Early Carboniferous through to the Early Cretaceous. Several major groups are defined phylogenetically (node or branch-based) rather than by apomorphies. The following groups were unequivocally found to be monophyletic: Edopoidea (node), Dvinosauria (stem, excl. Brachyopidae), Dissorophoidea (node), Eryopidae (stem), and Stereospondyli (node). The latter encompass three well-defined, branch-based taxa: Rhinesuchidae, Trematosauria and Capitosauria. Trematosauria (stem) contain Trematosauroidea (node), which includes the classic trematosaurids, metoposaurids, and possibly part of the rhytidosteids (Peltostega) but their in-group relationships remain unsettled; most other short-snouted stereospondyls (chigutisaurids, brachyopids, Laidleria and the plagiosaurids) are probably monophyletic and likely nest in some form with trematosauroids. Capitosauria (stem) include the Capitosauroidea (node) spanned by Parotosuchus and Mastodonsaurus, with the successive stem taxa Edingerella, Benthosuchus, Wetlugasaurus and Watsonisuchus. In all variant analyses, edopoids form the basalmost temnospondyl clade, followed by a potential clade (or grade) of small terrestrial taxa containing Balanerpeton and Dendrerpeton (‘Dendrerpetontidae’). All taxa higher than Edopoidea are suggested to form the monophyletic stem taxon Eutemnospondyli, tax. nov. The remainder of Temnospondyli fall into four robust and undisputed clades: (1) Dvinosauria; (2) Zatracheidae plus Dissorophoidea; (3) Eryopidae; and (4) Stereospondyli. These taxa are together referred to as Rhachitomi (node). Eryopidae and Stereospondylomorpha are probably monophyletic, here referred to as Eryopiformes (tax. nov.). The position of Dissorophoidea + Zatracheidae is still ambiguous; it may either form the sister taxon of Dvinosauria, or nest between Dvinosauria and Eryopiformes, whereas there is no support for Euskelia (Dissorophoidea + Eryopidae) after basal taxa of each clade are better understood.



Witzmann, F. and R. R. Schoch. 2012. Reconstruction of cranial and hyobranchial muscles in the Triassic temnospondyl Gerrothorax provides evidence for akinetic suction feeding. Journal of Morphology DOI: 10.1002/jmor.20113
http://onlinelibrary.wiley.com/doi/10.1002/jmor.20113/abstract

Abstract - The cranial and hyobranchial muscles of the Triassic temnospondyl Gerrothorax have been reconstructed based on direct evidence (spatial limitations, ossified muscle insertion sites on skull, mandible, and hyobranchium) and on phylogenetic reasoning (with extant basal actinopterygians and caudates as bracketing taxa). The skeletal and soft-anatomical data allow the reconstruction of the feeding strike of this bottom-dwelling, aquatic temnospondyl. The orientation of the muscle scars on the postglenoid area of the mandible indicates that the depressor mandibulae was indeed used for lowering the mandible and not to raise the skull as supposed previously and implies that the skull including the mandible must have been lifted off the ground during prey capture. It can thus be assumed that Gerrothorax raised the head toward the prey with the jaws still closed. Analogous to the bracketing taxa, subsequent mouth opening was caused by action of the strong epaxial muscles (further elevation of the head) and the depressor mandibulae and rectus cervicis (lowering of the mandible). During mouth opening, the action of the rectus cervicis muscle also rotated the hyobranchial apparatus ventrally and caudally, thus expanding the buccal cavity and causing the inflow of water with the prey through the mouth opening. The strongly developed depressor mandibulae and rectus cervicis, and the well ossified, large quadrate-articular joint suggest that this action occurred rapidly and that powerful suction was generated. Also, the jaw adductors were well developed and enabled a rapid mouth closure. In contrast to extant caudate larvae and most extant actinopterygians (teleosts), no cranial kinesis was possible in the Gerrothorax skull, and therefore suction feeding was not as elaborate as in these extant forms. This reconstruction may guide future studies of feeding in extinct aquatic tetrapods with ossified hyobranchial apparatus.


Comparing the Tooth Enamel Microstructure of the Pseudosuchian Revueltosaurus and the Proposed Triassic Ornithischian Krzyzanowskisaurus

This is a new paper testing the relationships between the pseudosuchian Revueltosaurus callenderi and the hypothesized ornithischian Krzyzanowskisaurus hunti (originally Revueltosaurus hunti) utilizing tooth enamel microstructure. The study finds that the tooth enamel structure of these two taxa share many characters not found in other taxa and thus they are probably closely related.  The authors advocate that generic distinction should be maintained until the skeletal remains of K. hunti are discovered. However, the teeth of K. hunti were recovered from a microvertebrate deposit in the Blue Mesa Member of the Chinle Formation near St. Johns Arizona.  As mentioned by Parker et al (2005) and Irmis et al. (2007) the same deposit included an autapomorphic squamosal of Revueltosaurus as well as numerous osteoderms also referable to the taxon.  Thus the assemblage possibly contains Revueltosaurus hunti or Revueltosaurus as well as a second taxon with Revueltosaurus-like teeth called Krzyzanowskisaurus. Heckert and Miller-Camp argue that this could be circumstantial given the purported lack of element association; however, no evidence exists either that they weren't originally found in association. There is simply just drawers of microvertebrate material from the same quarry. I and my colleagues have just maintained that the former is more parsimonious (Revueltosaurus hunti) and the shared characteristics of the teeth revealed by this study seem to support that hypothesis. In any case there still is no strong evidence for an ornithischian dinosaur affinity for K. hunti.

Heckert, A. B., and J. A. Miller-Camp. 2013. Tooth enamel microstructure of Revueltosaurus and Krzyzanowskisaurus (Reptilia:Archosauria) from the Upper Triassic Chinle Group, USA: Implications for function, growth, and phylogeny. Palaeontologia Electronica Vol. 16, Issue 1; 1A,23p;  palaeo-electronica.org/content/2013/344-revueltosaurus-tooth-enamel
 
Abstract - Tooth enamel microstructure can carry significant phylogenetic, ontogenetic, and functional information within amniotes. Here we provide the first descriptions of the tooth enamel microstructure of two Late Triassic taxa, the crurotarsan Revueltosaurus callenderi Hunt and the putative ornithischian Krzyzanowskisaurus hunti (Heckert), which some consider closely related. To test the hypotheses that enamel thickness corresponds to function and/or phylogeny we analyzed the enamel of each at various scales, measuring enamel thickness and examining microstructural features throughout both longitudinal and cross-sectional thickness using previously established techniques to facilitate comparisons. Both taxa possess thick (up to ~150 µm) enamel for their size (< 20 mm crown height). Enamel in R. callenderi ranged from ~5-152 µm across a premaxillary tooth in longitudinal section, and ~42-92 µm in a maxillary/dentary tooth transverse section. K. hunti enamel thickness was ~18-155 µm longitudinally and ~29-75 µm transversely. Both also had well-developed basal unit layers (BUL) and weakly developed columnar microstructure. Well-developed lines of incremental growth (LIG) are present in both taxa, through which the columnar enamel grades into parallel crystallite enamel. Their enamel microstructure is therefore grossly similar to that of several ornithischian taxa, especially ankylosaurs, with which they are strongly convergent, and also compares well to rauisuchids and tyrannosaurids. The relatively unique combination of microstructural characteristics in the schmelzmuster of R. callenderi and K. hunti supports the hypothesis that they are closely related, but does not conclusively preclude a different taxonomic placement for K. hunti so we retain its separate generic designation.

Macropredatory ichthyosaur, Thalattoarchon saurophagis, from the Middle Triassic of Nevada

New in PNAS. I assume by terrestrial apex predators in the Carnian they are thinking about phytosaurs and/or large rauisuchids as they (p. 3) state that "Although large predators such as the rauisuchians Erythrosuchus and Ticinosuchus appear in the terrestrial rock record in the Anisian (Nesbitt, 2011), it has been suggested that full recovery on land was not reached until the Late Triassic, 30 My after the P/T extinction (Sahney & Benton, 2008)" [full references and edit added]. Unfortunately this ignores a good bit of new evidence published since 2008 that has changed our understanding of the timing of recovery on land (Xilousuchus, Nyassasaurus). However, I still think the existance of giant (almost 9 meter long) ichthyosaurs is incredible. More than a decade ago I got to see the 21 meter long holotype specimen of Shastasaurus sikanniensis (Late Triassic, British Columbia) when it was being prepared at the Royal Tyrell Museum. To get a photograph of just the rear portion of the skull I had to stand at the top of a fully extended ladder. I used a meter stick for the scale bar! This new specimen is not quite 9 meters in length, but is still huge as the average is 2-4 meters, and it has 12 cm long teeth.

Fröbisch, N. B., Fröbisch, J., Sander, P. M., Schmitz, L., and O. Rieppel. 2013. Macropredatory ichthyosaur from the Middle Triassic and the origin of modern trophic networks. PNAS Early Edition, January 7, 2013. doi:10.1073/pnas.1216750110

 Abstract -
The biotic recovery from Earth’s most severe extinction event at the Permian-Triassic boundary largely reestablished the preextinction structure of marine trophic networks, with marine reptiles assuming the predator roles. However, the highest trophic level of today's marine ecosystems, i.e., macropredatory tetrapods that forage on prey of similar size to their own, was thus far lacking in the Paleozoic and early Mesozoic. Here we report a top-tier tetrapod predator, a very large (>8.6 m) ichthyosaur from the early Middle Triassic (244 Ma), of Nevada. This ichthyosaur had a massive skull and large labiolingually flattened teeth with two cutting edges indicative of a macropredatory feeding style. Its presence documents the rapid evolution of modern marine ecosystems in the Triassic where the same level of complexity as observed in today’s marine ecosystems is reached within 8 My after the Permian-Triassic mass extinction and within 4 My of the time reptiles first invaded the sea. This find also indicates that the biotic recovery in the marine realm may have occurred faster compared with terrestrial ecosystems, where the first apex predators may not have evolved before the Carnian.