A Beginner’s Guide to Theropods, Part 3: Giant Killers and Lesser Tyrants

By the beginning of the Late Jurassic, the global supercontinent had begun to break up. Seaways had split off the Northern and Southern landmasses, Asia was an island, and Europe was a Tethys-spanning archipelago. And all of it, from the Chinese Shaximiao to the Tanzanian Tendaguru, was ruled by allosauroids.

Allosaurus itself was a very common theropod. It spanned from Portugal to Western North America, where it made up as much as 75% of all theropod specimens (Foster). One site, the Cleveland-Lloyd Quarry, preserves the remains of at least 45 individuals of all ages. What were they all doing there? It was probably a “predator trap”, formed by herbivorous dinosaurs getting stuck in the mud and attracting scavenging Allosaurus, which themselves would get stuck and fed on by other Allosaurus (Madsen). (Similar examples of opportunistic cannibalism are known from Majungasaurus and Tyrannosaurus).

Allosaurus had a hard life. Bones marked from healing injuries are common (Molnar), including one with a Stegosaurus-shaped puncture-hole in the crotch (Carpenter et al)! But how did it hunt? That’s still up for debate. We know the teeth were thin and bladelike, and the skull was narrow and full of holes. This means they were poorly adapted to crushing bone or gripping onto struggling prey. But the neck was strong and vertically mobile, the jaws had a gape of over 90⁰, and the skull was flexible enough to withstand a surprising amount of force (Snively et al; Lautenschlager). This has led to the suggestion that it was a kind of living hatchet, slashing off chunks of meat and leaving its prey to bleed out (Rayfield et al). Other researchers think this would cause too much stress on the jaws, and that it may have hunted more like a big cat, grabbing prey with its piercing claws and making short, powerful bites (Foster). It scavenged, too, especially on the abundant sauropods of the Morrison Formation (Lei et al). But whatever it was doing, it must have been very successful.

Two carcharodontosaurs, 6-meter Concavenator and 12-meter Acrocanthosaurus (Paul).

This worldwide success continued into the Early Cretaceous, with the increasingly massive, increasingly specialized, sauropod-killing carcharodontosaurs. The name means “shark lizards,” and it's easy to see why: these are allosauroid flesh-tearing adaptations taken to the extreme, with rows of spines supporting muscular necks and backs, and long, deeply fenestrated skulls for taking enormous bites. Unlike those of abelisaurids, carcharodontosaurs’ tiny arms seem to have retained some of their original functionality, being able to hold prey tightly against their bodies while the jaws did the work (Senter and Robins).

The biggest carcharodontosaurs, like Giganotosaurus, regularly approached (Hartman) or exceeded the size of the biggest tyrannosaurs (Spinosaurus was longer but more lightly built). It's likely this represents a kind of “size ceiling”, beyond which it's impossible to grow a mechanically efficient bipedal predator (Henderson; D'Emic et al). They had to be big, because they're almost invariably found with the biggest sauropods: Acrocanthosaurus with Sauroposeidon, Carcharodontosaurus with Paralititan, Mapusaurus with Argentinosaurus (Holtz et al). A dinosaur that large can't be brought down like any other animal, so they had to adopt some special strategies. A set of trackways from Texas’ Paluxy River seems to show an attack by several Acrocanthosaurus on a herd of Sauroposeidon. At one point, one theropod seems to skip a step, maybe to latch onto its still-moving quarry (Thomas and Farlowe). Another clue to their behavior comes from Mapusaurus, of which several individuals were found buried at the same time and place, suggesting they might have been hunting together (Corria and Currie).

Despite their success, allosauroids did not see the end of the Cretaceous, disappearing with the cooling climate around 90 mya. Exactly why is uncertain; it may have been the extinction of giant titanosaurs (Holtz et al). Or it may have been a bunch of local extinctions, combined with the fragmentation of the continents preventing any further dispersal (Sereno). Whatever the cause, their extinction left an ecological gap to be filled. And what stepped in to fill that gap?

The Hollow Lizards

Allosauroids share with the remaining theropods, the coelurosaurs, a number of important features. Other theropods had air sacs, but in these advanced forms, they permeate the skull and vertebrae, making a series of complex chambers identical to those of modern birds, which help them breathe efficiently, regulate their body temperature and keep their lungs moist (Sereno et al). Another, more controversial, feature is the presence of feathers, which have been inferred in the allosauroid Concavenator based on what looked to its describers like quill knobs on the upper arm (Ortega et al). Other researchers think they're in the wrong position and were more likely attachment points for ligaments. But definite feathers, both filamentous and branched, are present on ornithischians (Godefroid et al) and probably pterosaurs (Cincotta et al), so it's safe to assume they could've been on other theropods as well.

Stages of feather evolution and their distribution (Ksepka). Feathers first appeared as simple filaments, likely for insulation in small animals. Later on, in ornithomimosaurs and therizinosaurs, they became long, branching structures, used for brooding or display. Oviraptorosaurs, dromaeosaurs, and troodontids had feathers of basically modern construction, with fractally branching filaments coming off a central shaft, hooked together to form a protective sheath.

Compsognathus, the “ur-type” coelurosaur, was a lightly built, rather generic-looking hunter of small animals, and, until the 2000s, was the smallest known nonavian theropod. But its relative Sinosauropteryx, from the Early Cretaceous of China, made history twice over: in 1996, it was the first feathered dinosaur ever described, and the first of many from the fine-grained mudstone of the Yixian Formation (Ji and Ji). The preservation in these sediments is so fine that, in 2010, researchers (Zhang et al) were able to identify the shape of its melanosomes, making it the first dinosaur with a reconstructed color scheme! From its counter-shaded pattern and ringed tail, we can infer that it was camouflaged for more open environments than its forest-dwelling contemporaries (Smithwick et al). From this sort of slender, agile, (relatively) bug-brained hunter of small, fast prey came the whole diversity of coelurosaurs, from herbivorous ornithomimids to birdlike dromaeosaurs to giant, bone-crushing tyrannosaurs.

For most of their history, tyrannosauroids were fairly unremarkable mid-sized predators. Look at the 3-meter, late Jurassic Guanlong (Hartman) and you'd never put together that its relatives would become some of the biggest predators to ever live. But look closer and you may start to see some resemblances: under that crest, the nasal bones are fused together, strengthening the skull. The snout is blunt, and the premaxillary teeth aren't bladelike as in allosauroids, but rounded, built for puncturing and scraping. 5-meter Eotyrannus, from the Early Cretaceous of England, is a little more familiar-looking: the head is more robust; the legs are longer; and the arms, though still very much in use, aren't quite as long or strong (Holtz et al).

Often obscured by their massive proportions is just how leggy even the biggest tyrannosauroids are. In derived forms, like the 7-meter Appalachiosaurus (Carr et al), this is accompanied by a middle metatarsal “pinched” between the other two, effectively turning the whole foot into one big shock absorber (also found in ornithomimids, alvarezsaurs, troodontids and some oviraptorosaurs). They needed to be fast, because they spent their whole history surrounded by other long-legged coelurosaurs. Even the big hadrosaurs and ceratopsians they hunted came from ancestors that, like them, were more agile than the allosauroids, sauropods, and armored dinosaurs they lived alongside (Holtz). Tyrannosaurus itself was almost certainly too heavy to run like its smaller relatives, but with these energy-saving adaptations, it could easily outpace (Dececchi et al) and outmaneuver (Snively et al) its faster prey.

Until the tail end of the Cretaceous, tyrannosauroids played minor roles in their ecosystem, living alongside a whole range of similar-sized or larger predators (Schroeder). There were rare exceptions, like Yutyrannus (Xu et al), the local usurper of the Yixian, but for the most part, they were relegated to hunting smaller, faster prey than their allosauroid competitors. Once those had gone extinct, the only other large competitors were other tyrannosaurs.

Incidentally, Yutyrannus is so far the largest dinosaur preserved with feather impressions, overlapping in size with smaller tyrannosaurids. Traces of scales are known from large portions of the body, mainly around the legs and tail, in several tyrannosaurids, and it's been suggested that the loss of feathers may be due to gigantism (Bell et al). But these remains are few and far between, and no tyrannosaurid lived anywhere with comparable preservation. It's possible that, like owls today, follicles grew between the scales, and, like large mammals, juveniles were more insulated than adults, mixing sparse patches of fuzz with large areas of exposed skin (Holtz et al).

Mysterious Megaraptorans

While the famous tyrannosaurids were busy taking up the role of dominant predators in the Northern Hemisphere, an altogether more poorly-known group was doing the same in the South. When their remains - bits of enormous hand claws - were first discovered, no one quite knew what to make of them. Suggestions ranged from giant dromaeosaurs to late-surviving megalosauroids. Later, their robust, exceptionally pneumatized skeletons were discovered, and a theory formed that they were a second radiation of long-legged, big-armed allosauroids (Benson et al). Finally, in 2013 came the surprising discovery that the skull was not allosaur-like at all, but long, slender, and strangely… tyrannosauroid. The arms, though uncharacteristically long and robust, show tyrannosauroid characters, too: the first two fingers are much larger than the third (Novas). Perhaps there were two separate radiations of tyrannosauroid apex predators, living at the same time on opposite ends of the globe, completely opposite in adaptations and independently achieving enormous sizes. With the continents fragmented by the end-Cretaceous, tyrannosauroids and their coelurosaur compatriots were unable to achieve the same sort of uniform, cosmopolitan success the allosauroids enjoyed. But, as we'll see soon, with geographic isolation comes plenty of room for experimentation.

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