My research interest is in the field of paleoherpetology, the study of fossil amphibians and reptiles. For more than 25 years I have been primarliy interested in the biostratigraphy, phylogenetic systematics and taxonomy of fossil lizards, crocodylians, turtles and dinosaurs, as well as the controversial topic of dinosaur extinction. My stratigraphic focus has been on the fossil vertebrates on both sides of the Cretaceous-Tertiary boundary.
Here are some highlights of my current research:
KIRTLANDIAN LAND-VERTEBRATE "AGE"
Kirtland Formation (De-na-zin Member) near Alamo Wash, San Juan Basin, New Mexico
In 2003, I and my colleague, Dr. Spencer G. Lucas (New Mexico Museum of Natural; History and Science, Albuquerque), named the Kirtlandian land-vertebrate "age" for a 2.2 million gap in time between the Judithian and Edmontonian "ages." The Kirtlandian LVA is a vertebrate faunachron based on vertebrate faunas (Hunter Wash and Willow Wash local faunas) from the upper Fruitland and Kirtland formations, respectively, (Upper Cretaceous) San Juan Basin, New Mexico. The index taxon for the Kirtlandian LVA is ceratopsid dinosaur Pentaceratops sternbergii.
holotype skulls of the flat-headed pachycephalosaurid dinosaur Dracorex hogwartsia (left) and dome-headed Prenocephale prenes (right), from the Late Creatceous
Pachycephalosaurid dinosaurs are commonly referred to as the "dome-headed" dinosaurs, but a number of them are of the "flat-headed" variety. These include genera such as Goyocephale and Homalocephale (both from Asia) and well as the newly described Dracorex hogwastsia from the Late Cretaceous of South Dakota (Bakker et al., 2006).
My recent studies of the group (Sullivan, 2000; 2003; 2005 and 2006) plus that of Bakker et al. (2006), suggest that the "flat-headed" pachycephalosaurids are, in fact, derived, having reverted back to a more primitive skull morphology, expressed by having elongated skulls, open supratemporal fenestrae, among other features. More importantly, the characters that were used to unite pachycephalosaurids (i.e., pachycephalosaurs) with the ceratopsians into the monophyletic group Marginocephalia are no longer supported. Therefore, the phylogeny of pachycephalosaurids that has been embraced by paleontologists for the last 20+ years is invalid. Putative basal taxa, Stenopelix and Yaverlandia, are no longer considered pachycephalosaurs (the former is not a primitive "pachy" but rather an indeterminate ornithischian; the latter is probably a theropod). Moreover, the "Pachycephalosauria" and Pachycephalosauridae are diagnosed with the same charcters, making the former rank redundant. Therefore only the Pachycephalosauridae is accepted. It is clear, based on the fossil record, that fully domed pachycephalosaurids appeared first, and that flat-headed forms appeared later. Paleontologists are now forced to reconsider the phylogeny of this interesting group of Late Cretaceous dinosaurs.
Fossil anguid lizards from the middle Eocene of Grube Messel, Germany: Placosauriops (left) and x-ray of Ophisauriscus (left).
While I have always held a strong interest in dinosaurs, fossil lizards have long been particularly interesting to me from a phylogenetic and paleobiogeographic standpoint. For over thirty years I have studied fossil anguid lizards, especially those of the subfamily Glyptosaurinae. These lizards are heavily armored with osteoderms (osteoscutes) coverying their entire body. They evolved from Late Cretaceous stock (Odaxosaurus) and flourished in the Eocene and early Oligocene of North America and Europe. They became extinct in North America by mid-Oligocene time. Some of the well-known North American genera include: Glyptosaurus, Helodermoides, Melanosaurus and Peltosaurus; European genera include Placosaurus and Paraplacosauriops.
Fossil anguid lizards of the subfamily Anguinae, believed to be related to the modern legless lizard Ophisaurus, are also a group that I have worked on. These include Parophisaurus from North America and Ophisauriscus from Europe.