A Beginner’s Guide to Theropods, Part 1: Setting the Scene
Theropods (meat-eating dinosaurs) permeate popular culture. Anyone who’s ever had a passing interest in dinosaurs knows T. rex and Velociraptor. They fill books and movies; they’re perpetual objects of childhood fascination for their size, power, and ferocity. They’re windows into a world lost forever, that we can only ever look dimly into. And, in the form of birds, they’re still around today. In this series, we’ll be looking at the Mesozoic through the eyes of theropods, taking a walk up the tree of life and through time to track the ever-changing Mesozoic world and our changing knowledge of it. We’ll see the roles they played in their ecosystems and look at their evolution and diversity, along with a number of historically important discoveries that helped enrich our view of the Age of Reptiles and the predators that stalked through it.
Tree of Theropods through Time. Modified from Rauhut et al. Template courtesy of Thomas Holtz. Key: 1) Herrerasaurs, 2) Coelophysids, 3) “Dilophosaurs”, 4) Ceratosaurs, 5) Abelisauroids, 6) Megalosauroids, 7) Spinosaurids, 8) Allosauroids, 9) Carcharodontosaurs, 10) Tyrannosauroids, 11) Megaraptorans, 12) Tyrannosaurids, 13) Compsognathids, 14) Ornithomimosaurs, 15) Alvarezsaurs, 16) Therizinosaurs, 17) Oviraptorosaurs, 18) Scansoriopterygids, 19) Dromaeosaurs, 20) Microraptorines, 21) Unenlaginians, 22) Troodontids, 23) Birds (including Anchiornithes)
What is a Theropod? What is a Dinosaur?
Everyone has heard the word “dinosaur,” but it’s rare for anyone to actually know what sets dinosaurs apart from other animals. Go to a museum, and you might see some guests use the word for any big charismatic extinct animal, like pterosaurs, marine reptiles, and even Ice-Age mammals. Others might be able to sort out Brontosaurus and Triceratops from Pteranodon and Plesiosaurus, but not know why, or what they have in common. And it'd be especially rare to find anyone who'd include the taxidermied songbirds or the chicken from the museum cafeteria as well. Before we start our look at theropod dinosaurs, it’s important to know what we’re looking at in the first place.
In 1842, when Richard Owen coined Dinosauria, dinosaurs were represented by a small handful of poorly known species, among which was the theropod Megalosaurus (his restoration, next to a modern one, below.) Even with such little to work with, Owen realized that these new discoveries were distinct from any other reptile then known.
Owen characterized dinosaurs as reptiles convergent on mammals, with columnar legs held straight under the body. He used these as evidence for a long-gone “Age of Reptiles,” in which the fantastic rulers of the land, air, and sea had since degenerated into the lowly lizards and snakes of today. To him, this lack of progression was direct evidence against evolution. Around 30 years later, Owen’s rival and Darwin's early supporter Thomas Huxley, working from slightly better material (including the complete skeleton of the much smaller Compsognathus), recognized that Megalosaurus was not an elephantine quadruped but a biped - and one with a surprisingly birdlike locomotor system. Under ‘Darwin’s bulldog’, dinosaurs were reinterpreted from proof of degeneration to evidence for evolution.
To understand what a dinosaur is, we should first look at their place in the broader tree of life, and at what the world around them looked like when they first appeared. Dinosaurs are reptiles - but that doesn’t mean much more than that they are egg-laying (amniotic) vertebrates more closely related to other living reptiles than to mammals. The most we can say about all reptiles is that they’re “lower” amniotes, separated from birds and mammals by negative characters (eg. “cold-bloodedness”) rather than shared novel features (Holtz) - a concept that gets very blurry when dealing with the whole diversity of life, especially when it comes to dinosaurs.
Among living animals, birds and crocodiles are most closely related to each other in a group called archosaurs (“ruling reptiles”), characterized by features like an antorbital fenestra (a hole in front of the eyes, which makes the skull lighter and airs out the brain). Studies of living crocodiles show they also seem to be united by a number of behavioral features, including nest-building, complex vocal communication, and even extensive parental care (Holtz)- not bad for a “lower” animal! Critically for animals that have survived two of the biggest mass extinctions in Earth history (about which more here), they also share features of the respiratory and circulatory systems, including a four-chambered heart, a system of air sacs that keep deoxygenated air out of the lungs, and a set of “belly ribs,” which allow the ribcage to expand and contract while the animal breathes in and out. These would have been vital for getting as much oxygen as possible in the deoxygenated Early Triassic environment (Benton), but they also allow for sustained levels of activity in stabler times.
Finally, all archosaurs walk more upright than most other reptiles, allowing them to breathe and move at the same time (in sprawlers, the same muscles do both.) The dinosaur locomotor system is the logical conclusion of this: all dinosaurs share a series of adaptations, including a hole in the acetabulum and a matching femur head, that allow the legs to be brought straight under the body, cutting the sideways leg motion of all other reptiles and making their strides as efficient as possible (Benton; Holtz). This upright stance is supported by a digitigrade foot with a mesotarsal (“straight-hinge”) ankle, which allows the foot to swing back and forth rather than side-to-side, as in flat-footed crocodiles’ crurotarsal (“cross-ankle“) joints. In theropods, this digitigrade foot is specialized even further, with the fifth digit being lost completely and the fourth shrinking to a little stump (although not lost; in many birds, you can see this toe re-enlarged and turned backward). While even the biggest dinosaurs were bipedal ancestrally, theropods committed fully to their bipedal adaptations, freeing up their hands for all kinds of prey-catching specializations - including, eventually, flight.
The first meat-eating dinosaurs and the world they inhabited
If the world of dinosaurs seems unfamiliar to us today, imagine how much stranger the already crowded Late Triassic world they entered must have been! There were no small mammals, but there were large carnivorous and herbivorous cynodonts, and even bigger dicynodonts. There were no frogs or lizards, but there were giant swimming temnospondyls and strange plant-eating rhynchosaurs. And there were no crocodiles, but there were croc-line archosaurs of all shapes and sizes, from scurrying sphenosuchids to herbivorous aetosaurs, croc-like phytosaurs to bipedal poposaurs (Brusatte et al). The Triassic was the Age of Crocs, and the massive predatory rauisuchians were rulers of the Age. 230 million years ago, in the Ischigualasto Formation of Argentina, the most common dinosaur, the mid-sized predator Herrerasaurus, played second fiddle to the bigger Saurosuchus, hunting the rare smaller dinosaurs (Martinez et al).Ten million years later, in the Chinle Formation of Arizona, the lean desert-dwelling Coelophysis appeared to be in much the same situation with the big phytosaurs and rauisuchids it shared its environment with. But under the surface, something in their ecology was already beginning to change.
Fauna of the Ischigualasto and Chinle Formations (from Tamura; Parker and Martz)
Around the time of Herrerasaurus, volcanic eruptions near British Columbia led to a period of increased rainfall and a stabilization of the hot Triassic climate. Over much of this new, temperate world, forests replaced the earlier low-growing desert vegetation. With them came a new abundance of big, high-browsing herbivorous dinosaurs, feeding off the plants that other herbivores couldn't reach (Dal Corso et al). Arizona was not so lucky: it was inland and close to the equator, at a time when the continents were joined together and CO2 concentrations ranged 3-5 times today's levels. Forest fires raged, and the climate and vegetation fluctuated wildly, from wet-weather fern forests to conifers and extreme drought (Whiteside et al). In this environment, heat-resistant croc-relatives still thrived, while in the cooler, wetter, more stable high latitudes, herbivorous dinosaurs were becoming more common.
Coelophysis was, however, successful in its own way: since the 1940s, a single quarry in Ghost Ranch, Arizona has produced over a thousand specimens of all ages, seemingly buried all at once in a flash flood (Colbert). This has allowed us to study everything from its growth and variation to its feeding ecology. Most famously, some specimens have been claimed to show evidence of cannibalism. These were probably either misidentified remains of other animals (Nesbitt et al) or individuals that got buried on top of one another (Gay).
Variation in Coelophysis, showing age and possible sexual dimorphism (Paul.)
Coelophysids were mainly lightly built predators of small animals, keeping out of direct competition with the bigger croc-relatives. Some, like the 5-meter Liliensternus, may have been big enough to prey on at least some of the large herbivorous dinosaurs they lived with. Critically, they were probably more adaptable, needing less food than the macropredatory rauisuchians, allowing them to survive into the newly-emptied world of the Early Jurassic. Dinosaurs were beneficiaries of two major climatic changes that killed off other organisms, giving them a virtual monopoly on faunas the world over and allowing them to diversify explosively. As we'll see in future installments, the world changes constantly, sometimes devastatingly, but nature always bounces back.
Image credits
Coelophysis from Paul, G. S. (2016). The Princeton Field Guide to Dinosaurs. Princeton University Press.
Megalosaurus by Scott Hartman from https://www.skeletaldrawing.com/theropods
Ischigualasto fauna by Nobu Tamura from https://spinops.blogspot.com/2018/03/ischigualasto-formation.html?m=1
Works cited:
Benton, Michael (2020). The origin of endothermy in synapsids and archosaurs and arms races in the Triassic. Gondwana Research. DOI: 10.1016/j.gr.2020.08.003.
Brusatte, Stephen et al (2008). Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs. Science 321. doi:10.1126/science.1161833.
Colbert, Edwin (1989). The Triassic Dinosaur Coelophysis. Museum of Northern Arizona Bulletin. 57: 160.
Dal Corso et al (2020). Extinction and dawn of the modern world in the Carnian (late Triassic). Science Advances, 6(38). science.org/doi/10.1126/sciadv.aba0099.
Gay, R.J. (2010). Notes on Early Mesozoic Theropods. ISBN 978-0-557-46616-0.
Holtz, Thomas S et al (2007, last ed. 2015) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages.
Holtz, Thomas S (2021). Lectures for GEOL-104. Dinosaurs: A Natural History. University of Maryland. youtube.com/playlist?list=PLzO5aKfW25o1YGvDbUJC8yT9mD0TQn3MX
Huxley, Thomas H (1868). On the Animals Which are Most Nearly Intermediate Between Birds and Reptiles. Annals and Magazine of Natural History, series 4, vol 2.
Martínez, Ricardo et al (2013). "Vertebrate succession in the Ischigualasto Formation". Journal of Vertebrate Paleontology. 32. doi:10.1080/02724634.2013.818546.
Nesbitt, S. J. et al (2006). "Prey choice and cannibalistic behaviour in the theropod Coelophysis". Biology Letters. 22. 2 (4): 611–614. doi:10.1098/rsbl.2006.0524.
Owen, Richard (1842). Report on British Fossil Reptiles. Eleventh Meeting of the British Association for the Advancement of Science.
