A Portrait of Thomas Holtz

by Steve Brusatte

By Steve Brusatte dinosaur research, and chairs the Earth, Life, and Time Program, also for honors students. After finding a steady job at the University of Maryland, Holtz plowed full-speed into writing. Up until now, he has published a plethora of papers, pamphlets, and books. In 1994 Holtz published two of his most important and influential papers.

The first regarded the phylogenetic position of the Tyrannosauridae. Published in the Journal of Paleontology, this 18-page paper helped transform the modern views on Tyrannosaurus and its kin. Members of the theropod clade are generally classified into one of two groups: carnosauria and coelurosauria. These names were used for the duration of dinosaur paleontology to divide large and small theropods.However, this division did not take into affect phylogeny, only size.

Up until Holtz\'s paper, it was generally thought that Tyrannosaurus, Albertosaurus, Gorgosaurus, and other tyrannosaurids were carnosaurs. This was assumed because of their large size, according to Holtz, but never tested. In order to compensate for years of misunderstandings, Holtz did the research and testing himself.

This research culminated in his paper, which bluntly stated that tyrannosaurids were not Allosaurus-like carnosaurs, but rather bird-like coelurosaurs. As would be expected, this paper caused quite a stir. However, it withheld the scrutiny of peer-review and the paleontological community as a whole. Today, a mere seven years after publication, the hypothesis is nearly universally accepted. In recent years, Paul Sereno has backed up the theory using his own research into dinosaur evolution. Much of this can be read in a 1999 paper published in Science.

The second of the 1994 duo regarded the arctometatarsalian pes (foot) in Cretaceous theropods. One may recognize this paper as having nearly the same title as Holtz\'s Yale dissertation. It is a widely accepted notion that theropods became faster runners over time, and the legs of Cretaceous theropods are generally longer than earlier species. They are also proportioned differently, with the bones of the lower leg and metatarsus being elongated.

In addition, this increased speed may have also been caused by the development of a more elastic and flexible metatarsus for reducing and distributing the stresses caused by impact of the foot with the ground during running. This modified foot was the topic of Holtz\'s second 1994 paper. Basically, Holtz measured theropod limb bones, including those of Tyrannosaurus, and compared them to the feet and limbs of birds and mammals.

What he discovered was that Tyrannosaurus and its kin were very efficient in absorbing shock and transmitting force while running. Because of this, Holtz postulated that Tyrannosaurus was one of the fastest large theropods. A nice summary of Holtz\'s work can be read in Toby White\'s 2000 Paleozoica article. Holtz published several additional papers during the years 1994-1998, including those on dinosaurian biogeography, feeding habits in Troodon, and the osteology of dinosaurs.

However, none of these papers caused as much press coverage as his 1998 Suchomimus commentary in Science. In the same issue, Paul Sereno and his colleagues officially named Suchomimus, a large spinosaurid dinosaur from Africa with a strange snout that was likely an adaptation used for catching and eating fish. In his short commentary, Holtz argued that Suchomimus\' odd snout played a major role in the ecology and environment of Cretaceous Africa.

According to Holtz, the ecomorphology, or the interpreting of anatomy and how this anatomy is used to interact with other animals, of Suchomimus and its contemporaries is fascinating. It is basically assumed by many that all theropods ate meat. By studying and analyzing mammals, Holtz inferred that Suchomimus\' fish-eating adaptation could be used to explain theropod diversity in Cretaceous Africa. Currently, several large theropods are known from the same general geographic and temporal range as Suchomimus. However, in modern environments it is rare to see even two large meat-eaters inhabiting the same space. But, the fact that Suchomimus ate predominately fish means that it could have filled a different niche than its other large carnivorous contemporaries.

It is certainly plausible that two large carnivores can live together if one eats meat and another eats fish. The two are not competing for the same food, and, therefore, are not threatening each other. This is what Holtz hypothesized about Suchomimus in his 1998 paper. A more recent research interest for Holtz is the evolution of birds. As a morphologist who specializes in the anatomy and evolution of theropods, Holtz can be considered an expert on the rise of birds.

It is generally accepted that birds evolved from some sort of small dinosaur, likely during the Early Jurassic. However, this hypothesis was \"threatened\" by a 2000 paper, written by Terry Jones and his colleagues, on the enigmatic Asian reptile Longisquama. Longisquama, which means \"long scales,\" is believed by some to be an early archosaur, and by others to be nothing close to the archosaur lineage.

The interesting aspect of Longisquama, though, is a series of feather-like appendages preserved along its back. Jones and his colleagues interpreted these appendages as early feathers and, therefore, proclaimed that dinosaurs did not evolve into birds. Although he has never been a frequent researcher of the dinosaur-bird link, Holtz does find the Longisquama debate interesting. He even went as far as to compare Longisquama to other archosaurs, and plans to write a paper on his research. Much of this early research was highlighted in a post to the Dinosaur Mailing List.

Basically, Holtz wrote, Longisquama is fascinating, but the small Triassic reptile is not an archosaur. \"The neck vertebrae of all archosaurs are specialized, and this guy doesn\'t have specialized vertebrae,\" he said in a University of Maryland alumni magazine interview.

Longisquama is infamous for its incompleteness, as there are no hands or hindquarters, which would also be helpful to Holtz and other scientists trying to place it within the family tree of reptiles. However, despite the absence of specialized vertebrae and the lack of much of its anatomy, the authors of the Longisquama paper not only assigned the animal to the archosaur stock, but also wrote that the appendage impressions are early feathers.

This is also quite shocking to some, given the fact that the \"feathers\" are quite dim and hard to see, and show no real feather-like structures. One paleontology student even went as far as to write that they may be \"palm fronds or other vegetation.\" Holtz wrote that they \"look more like continuous sheets, thin enough to wrinkle.(and) they might be derived from scales, as feathers are, but they don\'t necessarily tell us anything about how feathers developed.\" The Longisquama paper caused such a stir in the national media-it garnered articles in Newsweek and other magazines and newspapers-is that it directly attacked the dinosaur-bird link, one of the most widely supported hypotheses in dinosaur paleontology.

Because these structures are \"feathers,\" Jones and his colleagues wrote, then birds could not have evolved from dinosaurs, but likely some archosaur during the Triassic. Holtz does not support this view, and plans to attack it in his paper. \"They (the people who support the Longisquama feather view) make assumptions about the rate of evolution in birds; they make assumptions about the impossibility of dinosaurs being tree-dwellers.

But the assumptions are untestable and they put forward no alternative hypothesis to the evolutionary development of birds. These people have an antithesis, not a hypothesis.\" Well done, Dr. Holtz, but you could have said it in simpler terms: Longisquama had nothing to do with the rise of feathers and does not hamper the dinosaur-bird link! Because of the melee caused by Longisquama, Holtz and colleague John Merck are in the process of doing a cladistic analysis of Longisquama that will compare it to similar specimens of the same era as well as to dinosaur and primitive bird species. This project may take a year, but should culminate in the aforementioned paper.

This research brings us to the present. In addition to the Longisquama analysis, Holtz also plans to continue his work on the phylogeny and functional morphology of theropods, ecomorphology of terrestrial predators, locomotion in theropods, Mesozoic biogeography, and phylogenetic taxonomy of the theropods. However, according to the University of Maryland magazine article, Holtz\'s real goal, other than being a dinosaur, is to discover the direct ancestor of Tyrannosaurs. However, he said, \"but if I found it, I couldn\'t name it after myself.\" Brazen, conceited, egotistic? Hardly! If anyone deserves the ancestor of Tyrannosaurus to be named after themselves, Holtz is the leading candidate. After all, few know more about large theropods than this boyish-looking but genius Maryland professor.

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