Thursday, September 29, 2011

Rauisuchian Osteoderms

Scheyer, T. M. and J. B. Desojo. 2011. "Palaeohistology and external microanatomy of rauisuchian osteoderms (Archosauria: Pseudosuchia)." Palaeontology. DOI: 10.1111/j.1475-4983.2011.01098.x

The presence of postcranial dermal armour is plesiomorphic for Archosauria. Here, we survey the external microanatomy and histology of postcranial osteoderms (i.e. dorsal paramedian and caudal osteoderms) of rauisuchians, a widely distributed assemblage of extinct predatory pseudosuchians from the Triassic. The osteoderms of eight rauisuchian taxa were found to be rather compact bones, which usually lack significant bone remodelling or large areas of cancellous bone. The presence of highly vascularized woven or fibrolamellar bone tissue deposited in the core areas indicates higher growth rates during earlier life stages, whereas a more compact parallel-fibred bone matrix indicates reduced growth rates in later development. This pattern of change corroborates earlier studies on long bone histology. With the exception of a bone tissue found in the sample of Batrachotomus kupferzellensis, which might be the result of metaplastic ossification, the general mode of skeletogenesis is comparable with intramembraneous ossification. The lack of cancellous bone tissue and remodelling processes associated with bone ornamentation, as well as the predominantly intramembraneous mode of ossification, indicates that rauisuchian osteoderm formation differs profoundly from that of the osteoderms of the only extant pseudosuchian lineage, the crocodylians.

Thursday, September 15, 2011

Taxonomy of Diplocynodon and Tooth Wear in Caiman latirostris

Martin, Jeremy E. and Martin Gross. 2011. "Taxonomic clarification of Diplocynodon Pomel, 1847 (Crocodilia) from the Miocene of Styria, Austria" Neues Jahrbuch für Geologie und Paläontologie - Abhandlunge, 261:2 DOI: 10.1127/0077-7749/2011/0159

A re-examination of the original type series of Prangner (1845) and Hofmann (1887a) of the primitive alligatoroids from the middle Miocene of Styria, led to a reappraisal of the taxonomy of the following species of Diplocynodon: D. steineri and D. styriacus. Of unsettled affinities, Enneodon ungeri was also re-examined. It is here demonstrated that it belongs to the same taxon of the specimens described by Hofmann (1887a). These taxa are in fact junior synonyms of the previously erected Enneodon ungeri. Moreover, comparison with other European alligatoroids reveals that the Austrian specimens described by Prangner (1845) and Hofmann (1887a) belong to the same genus: Diplocynodon Pomel, 1847. According to the rules of the International Commission on Zoological Nomenclature (ICZN), Diplocynodon has priority over Enneodon. Under the principle of priority, it is therefore proposed to rename all the Miocene remains of alligatoroids from Styria as Diplocynodon ungeri Prangner, 1845. Comparison of almost complete skulls from various Miocene contemporaneous localities reveals that there is no reason to erect another taxon for the French specimens of D. "styriacus" described in Ginsburg & Bulot (1997). However, these specimens need to be redefined as D. ungeri as well. D. ungeri was coded and included in a character matrix to cladistically test its affinities with other alligatoroids. A total of six species of Diplocynodon were analysed including: D. ratelii, D. hantoniensis, D. tormis, D. muelleri, D. darwini and D. ungeri. The results are consistent with previous studies and favour a monophyletic diplocynodontid clade. D. ungeri is the first species of the genus to be recognized from distant coeval European deposits, namely the Paris and the Pannonian Basins.

Ősi, Attila and Paul M. Barrett. 2011. "Dental wear and oral food processing in Caiman latirostris: analogue for fossil crocodylians with crushing teeth" Neues Jahrbuch für Geologie und Paläontologie - Abhandlunge, 261:2 DOI: 10.1127/0077-7749/2011/0161

Almost all of the 23 extant species of crocodylians are opportunistic predators that consume their food without extensive intraoral food processing. Posterior bulbous crushing teeth with heavy dental wear in two specimens of Caiman latirostris, however, indicate that oral food processing can be an important factor during feeding. Wear pattern analysis in two specimens of C. latirostris clearly indicates crushing of hard food items that produced large wear surfaces on tooth crowns in the posterior part of the tooth row. This type of wear suggests that the diet was predominantly composed of durable, hard-shelled prey (e.g.molluscs, crustaceans, turtles), a supposition confirmed by recent studies on the stomach contents of several C. latirostris specimens. The absence of similar wear patterns in other ontogenetically mature specimens of C. latirostris, however, indicates that specific, possibly regional differences in food resources might affect the degree and type of dental wear. The dental features we report in C. latirostris can provide an important extant analogue for fossil forms with similar dentitions (e.g. Bernissartia, Unasuchus and globidontan eusuchians).

Friday, September 2, 2011

Summer Field Work 2011 - Part 4 - Ghost Ranch

My 10th and final week at PEFO was actually spent mostly outside of PEFO, at Ghost Ranch in New Mexico. At the last minute (with a little pushing from Bill), we interns decided to go join the fun (Bill had wilderness training, so he couldn't join us). We left late on Sunday, arriving Monday morning. We arrived just in time to meet up with Alan Turner, Michelle Stocker, and crew as they surveyed the lab and quarry. We would be working in the Hayden Quarry (know for its wealth of data associated with dinosaur origins as well as from the paintings of Georgia O'Keefe) and would soon be joined by a rather large field crew comprised of paleontologists from University of Utah (Randy Irmis et al), UC Berkely (Kevin Padian et al), and several other institutions. Due to the large number of people, Rachel, Chuck, and I would have to camp at the quarry instead of the Ghost Ranch campsite.

Hayden Quarry 2 and 4 as viewed from 3.

Last year, they had started working on a large block of Typothorax material, so our fist goal was to get that ready to be pulled out. Step 1 was cleaning off all the material that had fallen from the quarry wall over the year (lots and lots of shoveling involved). Then we had to add several more layers of plaster and continue excavating around the base of the jacket. This took quite a while because of the amount of bone that was in the rock around and underneath the jacket. We just collected the bone on top of the jacket whilst removing the "underburden" (read Bill's post on when a large quarry block becomes furniture).

Unfortunately, we couldn't stay for longer than a few days. And although we were still working in the Chinle Formation, it was a vastly different experience compared to working the Chinle of PEFO. We returned to the park in time for me to get to spend one last day in the field. Reflecting on those 10 weeks, I can certainly say that the Chinle can be a frustrating formation to work in but its wealth of data us well worth it. I just hope that I get to spend many more field seasons working in the Chinle, especially the PEFO Chinle.

Summer Field Work 2011 - Part 3

Week 7
Surface bone. What could it be? (Photo by Andrea Thomer)

Phytosaur skull.
This was a somewhat slow week between office days and unproductive prospecting. Friday was spent back at Billings Gap where we had noticed several surface exposures of bone previously. Most of the bone we were finding was fragmentary and did not go far under the surface, a common frustration in the Chinle, but rule #7 is that if something catches your eye, you must examine it. So that afternoon, after prospecting for a while I started walking back to where Rachel was working on an aetosaur osteoderm, and something caught my eye. In the Chinle of PEFO, highly weathered bone is typically a dark purple or maroon, but what caught my eye was much lighter in color - a good sign. I started digging, and digging, and digging, and before long I had uncovered a complete phytosaur cranium. What had caught my eye turned out to be the parietals. Since it was getting late and the skull was so big, we would have to come back another day to finish excavating it and collect it. We dug a drainage ditch and covered it with a plastic bag. We'd be back.