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Naashoibito : New Mexico

Bob and I are pretty much standing on the lower conglomerate. This marks the contact between the Denazin member of the Kirtland Fm (dark grey-purple mudstones) and the overlying pale sandstones of the Naashoibito Mbr of the Ojo Alamo Sandstone (foreground). (photo by WP Fowler)

The Naashoibito Member of the Ojo Alamo Sandstone is a thin (~25m) yet important unit in the southwestern US.

The age of the Naashoibito has been controversial. Originally it was thought to have been deposited in the uppermost Maastrichtian, about ~66-65 Ma. However, there is no justification for this view. Units in Utah and Texas that share a comparable fauna were similarly thought to be uppermost Maastrichtian and contain the K-T boundary. Recent radiometric and magnetostratigraphic research has shown that these units are not uppermost Maastrichtian (see Fowler & Sullivan, 2011 and references therein). rather they are probably closer to ~69Ma, about 2-3 million years older than the classic Hell Creek and Lance Fm faunas in the northern US.

These photos show just a few highlights of the collecting from this important unit. With a larger crew, we could do so much more, but these tantalizing pieces give us a good idea of what we can expect.

See below for more info on the fauna.

There are large exposures of the Naashoibito member at Alamo Wash. (R Sullivan, left; D Fowler, right; photo by WP Fowler).

The most commonly encountered dinosaur is probably the sauropod Alamosaurus; a large, long necked herbivore. Here I am excavating an Alamosaurus fibula that I found in 2003. This site was especially memorable for me as while digging this bone up, I twice sat directly on a cactus (the same cactus!).

It's not always fossils that we find. This vintage 1960's 7-up bottle turned up in Alamo Wash, nearby a ceratopsid site (see later).

In 2002 Bob found this jugal (cheekbone) from a lambeosaurine hadrosaurid. This specimen is important as it is the first lambeosaurine known from the Naashoibito, and a late-suriviing member of the clade.

REF: Sullivan et al. (2011) The first lambeosaurin (Dinosauria, Hadrosauridae, Lambeosaurinae) from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico. New Mexico Museum of Natural History and Science Bulletin 53:405-417. [PDF]

Here the jugal is highlighted on a skull of Hypacrosaurus exhbited at the Tyrrell Museum (Drumheller, Alberta). Hypacrosaurus remains are known from the near-contemporaneous Horseshoe Canyon Fm (Canada). The Naashoibito taxon is probably something very similar. (photo adapted from Wikipedia).

I think Bob found this specimen in 2006; a Tyrannosaurus scapulocoracoid (shoulder blade). It was pretty shattered, but the breaks were clean, so we carefully bagged up all the pieces, paying special attention to which were next to each other so that it could reconstructed back in the lab (below).

(Left): my work in progress on the bone jigsaw in the PA State Museum Lab. (Right): Bob finished the preparation to make a large and almost complete scapulocoracoid. The Naashoibito Tyrannosaurus species probably isn't T. rex, because T. rex is from the Hell Creek Fm (and equivalents) which are about 3 million years younger. Without exception, dinosaur species are observed to change over this amount of time, so it is likely that this specimen is from an older, so far unnamed species of Tyrannosaurus. However, we currently lack enough complete material to diagnose a new taxon.

My dad found this Tyrannosaurus tooth in 2003. It is quite stout, with slightly twisted carinae, so it probably comes from the front of the lower jaws (anterior dentary).

The dinosaurs from this time period (~69Ma, early part of the mid-Maastrichtian) are not well known, especially in the southern US.

The dinosaur fauna includes early versions of more famous upper Maastrichtian species, like the ceratopsid Ojoceratops (related to Ticeratops), an unnamed dromaeosaur and other indeterminate little carnivorous coelurosaurs (including Richardoestesia), and undefined species of the duckbill Edmontosaurus and Tyrannosaurus. Alongside these expected taxa are the unusual ankylosaur Glyptodontopelta, the newly described oviraptorid Ojoraptorsaurus, a late surviving lambeosaurine (a different group of duckbills), and the sauropod Alamosaurus, which is thought to have immigrated from South America.

A review of the Naashoibito fauna was recently completed by Jasinski et al (2011), including descriptions of many of the specimens shown here.

Jasinski, Sullivan & Lucas (2011) Taxonomic composition of the Alamo Wash local fauna from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member) San Juan Basin, New Mexico. New Mexico Museum of Natural History and Science Bulletin 53:216-271. [PDF]

Ceratopsids! So far, only chasmosaurine ceratopsid remains are known from the Naashoibito. (Left) I came across this left squamosal (part of the frill) in 2004. (Middle) My dad found this parietal midline bar (very important specimen, also part of the frill). (Right) Bob digging out a predentary bone (support for the beak on the lower jaw) from the Pinocchio site where he found a partial skull (including the nasal horn).

On the hike back from the squamosal site, I came across this ceratopsid right dentary (lower jaw), eroding out of the riverbank.

We came back and dug it up, along with other bones from the same bonebed (including juvenile material). The site was called "the 7-up sandbar", after a vintage 1960's 7-up bottle that was found nearby.

in 2010, Bob and Spencer (Lucas) described our new ceratopsid material as a new species: Ojoceratops fowleri (they were kind enough to name it after me!).

The holotype specimen is the squamosal that I found. It is uniquely "squared off" at the distal end; something that you don't see in Triceratops at any growth stage.

Another important specimen is the midline parietal that my dad found; this has a unique bar, remniscent of ealier chasmosaurines and unlike any Triceratops.

Finally, aspects of the nasal horn morphology of "Pinocchio" (the specimen Bob found) are consistent with what we would expect a more basal Triceratops "ancestor" to look like, fitting with the earlier stratigraphic positioning of Ojoceratops. I presented on this at SVP 2010, and it forms part of my PhD research.

Sullivan & Lucas (2010) A new chasmosaurine (Ceratopsidae, Dinosauria) from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico; pp 169-180 in Ryan et al (eds): The Horned Dinosaurs. Indiana University Press.

Ojoceratops fowleri skull recontruction (composite, including the nasal horn from Pinocchio)

In the Naashoibito, we quite frequently find isolated osteoderms / scutes (armour plates) from ankylosaurs (armored dinosaurs). In 2003 however, my dad found a site ("Warwick's ankylosaur hill") that yielded over a hundred osteoderms and fragments. We quickly set about collecting them all.

Here Bob (left) and my dad (right) sift through the loose sand, collecting bags of ankylosaur scutes from the site.

Eventually we recovered 4 trays of material, many of which still need to be glued together. The specimens were described and assigned to Glyptodontopelta by Mike Burns in 2008 (J. Vert. Paleo. 28:1102-1109).

Behind the Glyptodontopelta site my dad also found a couple of tail vertebrae from the giant sauropod Alamosaurus.

This is one of three Alamosaurus fibulae (lower hindlimb) I have collected. This is the most complete specimen. It's from a fairly modest-sized individual.

In 2004, Bob found this enormous Alamosaurus cervical (neck) centrum at Alamo Wash. It is only fragmentary, but it is much larger than other Alamosaurus material, and comparable to some of the giant sauropods known from South America.

The cervical vertebra (SMP VP-1850) suggests that we had previously underestimated the maximum body size of Alamosaurus. (Image adapted from Lehman & Coulson, 2002)

Our new paper (Fowler & Sullivan, 2011; click below for link to paper and press info) describes some of the giant Alamosaurus material that we have collected, including the huge neck vertebra. By cross-comparing with other fragmentary giant specimens, we determined that Alamosaurus grew to sizes comparable to the biggest sauropods on other continents, including Argentinosaurus (widely considered to be the largest dinosaur so far discovered).

This follows other recent research by the Horner Paleo Lab (Museum of the Rockies) where we are finding that many dinosaur species are defined on immature individuals. How this affects our interpretation is not yet clear, but often dinosaurs do not take on their adult characteristics until quite late in ontogeny. hence defining species from immature specimens may not be helpful.

FOWLER, D.W., & SULLIVAN. R.M. (2011) The first giant titanosaurian sauropod from the Upper Cretaceous of North America, Acta Palaeontologica Polonica 56(4): 685-690

Based on the enormous cervical vertebra, Alamosaurus was comparable in size to the largest sauropods in the world, including Argentinosaurus. (Figure only intended as a rough indication of size)

(Left) A very large shaft of a shattered Alamosaurus femur that we left in the field. (Right) Another partial Alamosaurus femur that we collected (Mike Burns, photographed). this specimen was next to a pubis and ischium (pelvis bones), suggesting that more of the skeleton may be present..

The photo on the left is a Tyrannosaurus tooth that we collected from next to some Alamosaurus vertebral material back in 2006.

Like all dinosaurs, Tyrannosaurus would have shed teeth from time to time; replacing old teeth with new ones. This may have occurred during feeding, as the strains on the tooth during biting cause it to loosen and fall out.

When isolated teeth like this are found in association with skeletal remains of other specimens, it is sometimes presented as evidence that a carnivore lost teeth while feeding on a carcass (typically following detailed analysis of the sediment that they were preserved in, and other factors). This specimen is consistent with the hypothesis that it represents a tooth lost during feeding, although without more detailed investigation this remains speculative.

Even so, such associations present us with the vivid mental picture of an enormous Tyrannosaurus preying upon Alamosaurus. That must have been quite a sight!