May 20, 2010

A new look for Typothorax

9:46 pm

Over the past year or so, I’ve been involved with reconstructing the aetosaur Typothorax coccinarum, based on two articulated skeletons found in eastern New Mexico. A paper describing these specimens and presenting the reconstruction has just been published in the Journal of Vertebrate Paleontology, and is freely available now through the Taylor and Francis JVP website. I’m happy to say that my color painting of this Triassic tank will be gracing the cover of the May issue of the journal:

Typhothorax coccinarum, painting by Matt Celeskey
Reconstruction of Typothorax coccinarum based on skeletons from eastern New Mexico
Image by Matt Celeskey

These skeletons allowed us to revise previously published reconstructions of this aetosaur. In particular, we now have good evidence of the total number of rows of armor, the arrangement of the scutes on the belly, new insight into the appendages (particularly the shoulder girdle and feet), a more domelike carapace based on extremely wide and gently curved paramedian scutes, and the very first aetosaur reconstruction to sport cloacal spikes.

This has been a fun project to be involved with, and I thank Dr. Andy Heckert (a former coworker at the NMMNH, now at Appalachian State University) for inviting me to assist in reconstructing this armor-plated Triassic reptile.

Update 5/21: At the request of commenter dmaas, I’m uploading a detail of the head of the reconstruction. Clicking on the thumbnail will bring it up at more than twice the size of the original painting.

—Matt Celeskey.

File under: Reptiles, The Day Job, Triassic.

Comments on record: (13)

March 3, 2010

Asilisaurus kongwe

2:52 pm


Asilisaurus kongwe (foreground). Image by M.H. Donnelly, Field Museum
From the Ruhuhu Basin Research Asilisaurus Page

No time to treat this with more than a passing mention, but a letter in today’s Nature presents a new, Middle Triassic silesaurid from Tanzania named Asilisaurus kongwe (“ancient ancestor lizard”). This adds another continent to the known range of the silesaurids, a group of plant-eating proto-dinosaurs previously found in Europe, South America and North America. It also extends the age of this group back 10 million years, which makes it not only the oldest-known silesaurid, but the oldest known reptile on the bird side of the bird-crocodile split.

Neat stuff, and particularly interesting after looking at Eucoelophysis…

For more information:

—Matt Celeskey.

File under: Dinosaurs, Recent Discoveries, Reptiles, Triassic.

Comments on record: (1)

February 13, 2010

The Paleobiology of Coelophysis Part III: Truly, Coelophysis?

11:22 am

or, An International Corpse of Mystery

Quick note: New readers might want to look at previous installments of the Paleobiology of Coelophysis (Parts 1 & 2) series before diving into this post.

In order to collect data from other specimens of Coelophysis bauri, some members of the research team (notably Larry Rinehart and Andy Heckert) visited several other museums to study the Whitaker quarry blocks in their care. After a trip to the Royal Tyrrell Museum of Paleontology in Alberta, Larry shared observations and photographs from one specimen that seemed a bit out of place compared to the other Coelophysis we had seen.


TMP 84-63-33, highlighted against the rest of the Tyrrell Museum block
Original photograph courtesy of Larry Rinehart

Specimen TMP 84-63-33 is the most easily seen skeleton on the Tyrrell Museum block. Although the front and back ends of the animal are missing, most of the middle is well-preserved, particularly the two hindlimbs. At first glance, it looks much like any other Coelophysis from the quarry.

Some details, however, led us to wonder about this. For instance, while Coelophysis has five sacral vertebrae connected to its hip, TMP 84-63-33 appears to have only four. Several features on the proximal end of the femur (that is, the part of the thigh bone that connects to the hip bones) also seemed notably different from what we saw on Coelophysis, and these features suggested an alternative identification.

Focus on the Femora
or, A Discomfiting Object Inserted in the Acetabulum


The proximal end of the left femur of TMP 84-63-33
articulated within the acetabulum (hip socket)
Photograph courtesy of Larry Rinehart

Some of the odd femoral features include:

  1. A well-defined groove on top of the head of the femur.
  2. The head of the femur is offset but appears to be completely rounded off. There is no sign of the hook-like prong seen in Coelophysis and other theropods.
  3. A distinct crest-like trochanter (or bump of bone) on the front (anterior) side of the femur toward the outside (lateral) edge, which has not been reported in Coelophysis.

Of these features, the first and (particularly) the third were a close match for features seen on NMMNH P-22298, the holotype specimen of Eucoelophysis baldwini. Eucoelophysis (“True Coelophysis” or “True Hollow Form”) also lacks a hook-like prong on the head of its femur, although it is quite different in overall shape from the rounded, offset femur head of TMP 84-63-33. This might be a real difference, or it might be due to the badly weathered condition of the Eucoelophysis holotype. In the end, based primarily on the similarities noted here, we assigned TMP 84-63-33 to Eucoelophysis sp.


Comparison of the femoral heads of Eucoelophysis baldwini, TMP 84-63-33, and Coelophysis bauri. Click for larger view. This figure is not from Rinehart et al. 2009, but drawn up as a visual aid to this post.

That’s where we left things for publication—a genus-level assignment based on some key characters that TMP 84-63-33 and NMMNH P-22298 have in common. Things get more interesting when you add a little background and some other fossils, so I’ll take the opportunity to explore some of those tangents here.

Eucoelophysis vs. Coelophysis: An Example of Interspecific Digression
or, Reflections on a Hollow Form of Truth

When Eucoelophysis was first described, it was considered to be a theropod dinosaur closely related to Coelophysis (Sullivan and Lucas 1999). However, a pair of later studies (Ezcurra 2006, Nesbitt et al. 2007) concluded that Eucoelophysis was a “non-dinosaurian dinosauriform”—not only was it not particularly close to Coelophysis, it lacked the requisite anatomical features needed to be included in the Dinosauria proper. Its closest companion in these dinosaur hinterlands appeared to be Silesaurus, a beaked, herbivorous reptile known from excellent skull and skeletal material from the Late Triassic of Poland (Dzik 2003).

The idea that Eucoelophysis might be a Silesaurus-style dinosauriform has received support from new and newly-recognized discoveries of other Silesaurus-like fossils in Late Triassic rocks from Arizona and New Mexico (Parker et al. 2006, Irmis et al. 2007). These include some blocky, angled femur heads whose overall shape is similar to that of both Silesaurus and the shape preserved in the Eucoelophysis holotype. And at least one of these femur heads (PEFO 34357) appears to have a Eucoelophysis-style anterolateral trochanter (=the dorsolateral trochanter noted by Nesbitt et al. 2007).

TMP 84-63-33, on the other hand, looks a lot more like Coelophysis than Silesaurus in many observable parts of its anatomy, especially the bones of its pelvis and feet. I wouldn’t say that our assignment of TMP 84-63-33 to Eucoelophysis reaffirms close relationship between Eucoelophysis and Coelophysis. But if this identification holds, then it doesn’t appear to do much for a Eucoelophysis-Silesaurus connection, either.

One last osteological nubbin of interest: the lesser trochanter (also referred to as the cranial or anterior trochanter) is a prong of bone that, in the animals we’re discussing, sits just below the head of the femur on the front-facing side. This trochanter is slender and crest-like on both Eucoelophysis and TMP 84-63-33. On observed and reported specimens of Coelophysis bauri, the lesser trochanter is thick, blocky, and connected to a well-developed shelf of bone that wraps around the outside of the femur. In other coelophysoids, both forms of lesser trochanter have been found within the same species—such as in the African species Coelophysis rhodesiensis, where the two different shapes may represent a difference between males and females (Raath 1990).


Comparison of the proximal left femurs of specimens mentioned in this post, in anterior (front) view, resized to similar widths. Inset shows silhouettes to scale. Redrawn from various sources. Click for larger view.

When a couple more femur heads are added to the previous figure, I begin to see a gradation of forms between the block-headed, slender-trochantered dinosauriform femora through to the hooked femur heads and robust trochanters of Coelophysis bauri. Note that I do not suggest that this shows any sort of evolutionary sequence. Instead, the continuum of shapes and features is probably due to a mix of phylogenetic differences, sexual dimorphism, age- and size-related changes, individual variation, and preservation quality.

The trick is to figure out what sort of meaningful divisions might be found within this femoral spectrum. In Rinehart et al. 2009, we made one division based on similarities between the femora TMP 84-63-33 and Eucoelophysis. I suspect that the wealth of fossils from the Whitaker quarry will have more to reveal on the topic, both from close evaluation of femur variation in the large Coelophysis population, and from comparing those variations with data gleaned from other parts of the skeleton.

Next time: Data Gleaned from other Parts of the Skeleton
With far less talk of femur variation, and perhaps even some actual paleobiology!

Previously:

Part I: Introduction
Part II: Other Critters in the Quarry

  • Main Reference: Rinehart, Larry F., Lucas, Spencer G., Heckert, Andrew B., Spielmann, Justin A. and Celeskey, Matthew D., 2009. The Paleobiology of Coelophysis bauri (Cope) from the Upper Triassic (Apachean) Whitaker quarry, New Mexico, with detailed analysis of a single quarry block. New Mexico Museum of Natural History and Science Bulletin 45, 260pp. Abstract [Rich text file]
  • Additional References: Dzik, Jerzy, 2003. A beaked herbivorous archosaur with dinosaur affinities from the Early Late Triassic of Poland. Journal of Vertebrate Paleontology, vol. 23 (3), pp. 556–574.
  • Ezcurra, Martín D., 2006. A review of the systematic position of the dinosauriform archosaur Eucoelophysis baldwini Sullivan & Lucas, 1999 from the Upper Triassic of New Mexico, USA. Geodiversitas, vol. 28 (4), pp. 649–684. [PDF]
  • Irmis, Randall B., Nesbitt, Sterling J., Padian, Kevin, Smith, Nathan D., Turner, Alan H., Woody, Daniel and Downs, Alex, 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science, vol. 317, no. , pp. 358–361. doi: 10.1126/science.1143325
  • Nesbitt, Sterling J., Irmis, Randall B. and Parker, William G., 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology, vol. 5 (2), pp. 209–243. doi: 10.1017/S1477201907002040
  • Parker, William G., Irmis, Randall B. and Nesbitt, Sterling J., 2006. Review of the Late Triassic dinosaur record from Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62, pp. 160–161.
  • Raath, Michael A. 1990. Morphological variation in small theropods and its meaning in systematics: evidence from Syntarsus rhodesiensis in Dinosaur Systematics: Perspectives and Approaches, Kenneth Carpenter and Philip J. Currie, eds. Cambridge University Press. pp. 91–105.
  • Sullivan, Robert M. and Lucas, Spencer G., 1999. Eucoelophysis baldwini, a new theropod dinosaur from the Upper Triassic of New Mexico, and the status of the original types of Coelophysis. Journal of Vertebrate Paleontology, vol. 19 (1), pp. 81–90.

—Matt Celeskey.

File under: Dinosaurs, Reptiles, Triassic.

Comments on record: (9)

January 27, 2010

The Paleobiology of Coelophysis Part II: Other Critters in the Quarry

6:43 am

or, Token Diversity in a Dinosaur Graveyard

Although fossils of the Triassic theropod Coelophysis bauri are by far the most numerous vertebrate remains preserved in blocks from the Whitaker quarry, several other animals are known from the site as well. Many have just been uncovered or described within the past ten years, and in the course of preparing the NMMNH block (and examining other blocks for comparison), several new fossils have come to light. A brief, annotated list of other fauna known from the quarry appears below:

Invertebrates: As mentioned last time, ostracods (Darwinula sp.) and conchostracans (Shipingia) were found in a sandy layer below the bones, remnants of temporary ponding at the site prior to the Coelophysis burial.


Synorichthys chased by Chinlea, sculpted by Gary Staab

Fish: Schaeffer (1967) reported paleonisciform and coelacanth fish in association with Coelophysis at Ghost Ranch. Both were found in the NMMNH block above the invertebrate layer and just below the lowest tetrapod bones: scaly little redfieldiid paleonisciforms, tentatively assigned to Synorichthys, and bits of fin, scale, and skull from the large coelacanth Chinlea sorenseni. As these fish would have been too large to thrive in the type of ephemeral pond indicated by the invertebrates, we posited that floodwaters washed them in to the site from a larger body of water.

Whitakersaurus bermani: This diminutive sphenodontian (my tiny-tuatara-based restoration at the left) is known from pieces of the upper and lower jaw found within 2 centimeters of the edge of the NMMNH block. The largest piece of the holotype, an incomplete right dentary preserving nineteen tooth-positions, is about 5 millimeters long (Heckert et al. 2008).

Drepanosaurs: Harris & Downs (2002) reported the first drepanosaur material from the quarry—a well-preserved (but isolated) shoulder girdle from the block at the Ruth Hall Museum of Paleontology. In a new review of the drepanosaurs, Renesto et al. (2010) assign that shoulder girdle to the genus Drepanosaurus, and describe a partially articulated but generically indeterminate foot on the edge of the NMMNH block (pictured below).


NMMNH P-57651, the foot of a small drepanosaur, preserved portion roughly 5cm long

Vancleavea campi: By far the most complete specimens of this armor-coated reptile are two beautifully articulated skeletons from the Ruth Hall Museum block, recently described by Nesbitt et al. (2009). Remains of a partial, disarticulated skeleton are associated with some characteristic Vancleavea armor in a mostly unprepared fossil removed from the NMMNH block.


Vancleavea sculpture by Phil Bircheff at the Ruth Hall Museum of Paleontology.

Phytosaurs: An ~80cm long phytosaur skull from the Whitaker/Coelophysis quarry (the holotype of Redondasaurus bermani Hunt and Lucas 1993) is housed at the Carnegie Museum. The skull and lower jaws of a juvenile phytosaur were found in the NMMNH block, currently exposed in left lateral/palatal view on a partially prepared jacket removed from the main block (photo below).


NMMNH P-44920, juvenile phytosaur (Redondasaurus?) skull and jaws, left lateral/palatal view. Photo courtesy of Larry Rinehart.

Postosuchus kirkpatricki: The Carnegie Museum of Natural History and the Ruth Hall Museum of Paleontology both have specimens of this rauisuchian on Whitaker/Coelophysis quarry blocks. These were partially figured in Long and Murry (1995, figs 145–146).


Effigia sculpture by Phil Bircheff
at the Ruth Hall Museum of Paleontology.

Effigia okeefeae: The type specimens of this bipedal suchian were recently discovered in jackets pulled from the quarry during the early excavations by the American Museum (Nesbitt & Norell 2006, Nesbitt 2007). A scapula and coracoid found in the NMMNH block might belong to this animal.

Hesperosuchus agilis: One of the best-preserved specimens of this early crocodylomorph is an articulated skull and partial skeleton now at the Carnegie Museum (Clark et al. 2000). Only a few armor scutes are known from the NMMNH block.

To the best of my knowledge, this little bestiary lists pretty much all the non-Coelophysis animals identified from Ghost Ranch Whitaker quarry fossils. All of them, that is, with one interesting exception, noticed while reviewing specimens on other Coelophysis blocks. Its story will make up the bulk of the next post.

Next time: Truly, Coelophysis?
or, The Mysterious Canadian

Previously:
Introduction

 

—Matt Celeskey.

File under: Bony Fish, Dinosaurs, Recent Discoveries, Reptiles, Triassic.

Comments on record: (2)

January 22, 2010

The Paleobiology of Coelophysis Part I: Introduction

10:26 pm

The Triassic dinosaur Coelophysis bauri is well-known from hundreds of fossils unearthed at Ghost Ranch in northern New Mexico, at a famous quarry first discovered by George Whitaker in 1947. Excavations at the quarry—initially by the American Museum of Natural History in the 1940s & 1950s, and subsequently by the Carnegie Museum in the 1980s—sent about thirty large blocks, each containing dozens of fossils, to museums across North America. Although it has been more than 25 years since the last major excavation, new discoveries continue to be made (e.g.: Nesbitt & Norell 2006, Heckert et al. 2008, Nesbitt et al. 2009) as fossils from the quarry are slowly prepared and interpreted.

One block, excavated by the Carnegie-led team in the 1980s, was given to the then-nascent New Mexico Museum of Natural History. In 2008, it was put on permanent exhibit. Larry Rinehart, who prepared the block for display, invited me to assist in the illustration and interpretation of some of the specimens it contained, as well as to reconstruct some of the different sizes and morphologies that were being uncovered through the analysis of dozens of specimens in this and other blocks from the quarry. The results of these investigations have been published in the past few weeks as a New Mexico Museum of Natural History and Science Bulletin (Rinehart et al. 2009).

In future posts, I hope to showcase some of the specimens, interpretations, and conclusions we came to as a result of these investigations. Note that everything I post here will be based on my own understanding and interpretation of the work, much of which was performed by the other authors. I don’t intend to misrepresent any of the procedures or findings, but if it happens, errors in reporting should be considered mine alone.

With that caveat in mind, I’d like to introduce the main object of the study: the NMMNH Coelophysis block.

NMMNH Coelophysis Block
The NMMNH Coelophysis block (Quarry # C-8-82) after preparation

The NMMNH block is a two ton wedge of reddish Chinle siltstone, trimmed down considerably from the original 12,000 pounds pulled out of the quarry in 1982. Across its exposed surface lie more than two dozen specimens referrable to Coelophysis bauri, plus a couple non-dinosaur bits & pieces. Several jackets removed from the block contain many more fossils of Coelophysis, along with an impressive sampling of Triassic fish and reptiles.


Schematic drawing of selected Coelophysis specimens on the exposed surface of the NMMNH block

Up from the Bottom
or, Lessons from a Triassic Turnover.

When looking at the prepared surface of the NMMNH block, everything you see is upside down—the block was flipped over and prepared from the bottom up. This was done to facilitate access to the primary bone layer by avoiding the reworked, poorer-quality bones found above this layer in other blocks. It also revealed new information about the environment where the Coelophysis were buried. Beneath the main Coelophysis beds were found layers containing rip-up clasts running in the same direction as most of the Coelophysis bones—evidence of rushing water that tore up chunks of the underlying mud as it swept the dinosaur bodies in.

A previous study of the taphonomy of the quarry (Schwartz & Gillette 1994) also concluded that water transported the Coelophysis to their burial site. Based on several lines of evidence, Schwartz and Gillette proposed that the animals died during a prolonged drought and were subsequently washed downstream by a flood, where their carcasses clogged a narrow channel prior to burial. But some tiny fossils near the bottom of the NMMNH block suggested a slightly different scenario to us.

Beneath the rip-ups were the fossils of tiny invertebrates—conchostracans and ostracods—often found in temporary bodies of water. They led us to consider that the quarry was once the site of an ephemeral pond—a topographic depression where water might collect during a wet season, followed by dry periods where the water would disappear. An unfortunate flock of Coelophysis, swept up in the flooding of a nearby river, might have been washed into this low spot in the landscape and buried.


The death/burial poses of the seven most complete Coelophysis specimens on the NMMNH block

As for the cause of death, there seemed little reason to suspect any agent beyond the flood that buried them. In fact, one observation previously cited as evidence for post mortem desiccation—the opisthotonic posture where neck and tail are flexed sharply upward and curled over the back—now seems more likely to indicate the death throes of animals suffering the final effects of disease, poison, or (in this case) lack of oxygen due to burial or drowning (Faux & Padian, 2007).

If the bonebed at the Whitaker/Coelophysis quarry is the end result of a single catastrophe, then it preserves an excellent sampling of a population of early dinosaurs. It may possibly be the largest (in terms of numbers of individuals) Mesozoic dinosaur population we know of. This provides an unparalleled opportunity to study growth and variation within a single dinosaur species. Before getting to that, however, I’d like to spend a post reviewing some of the other, non-Coelophysis fossils found within this and other quarry blocks.

Next time: It’s your Lagerstätte, I’m just buried in it
or, Token Diversity in a Dinosaur Graveyard.

—Matt Celeskey.

File under: Dinosaurs, The Day Job, Triassic.

Comments on record: (10)

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