Theropoda (Late Triassic - Recent) includes the classic bipedal carnivorous dinosaurs plus some interesting surprises. Theropods represent an interesting combination of diversity and uniformity:
- Uniformity: Committed to obligate bipedality for their entire history.
- Although they started small, they repeatedly occupied the large-bodied dominant predator niche. Some were much larger than any previous terrestrial amniote.
- Diversity: The theropod cladogram is complex and contains creatures that, within the strictures of bipedality, achieve great diversity of locomotor and feeding mechanisms
Literary evolution of theropod phylogeny.
Older literature broke theropods down into
Both "groups" ended up being polyphyletic. Oops.
And yet cladists have struggled with theropod phylogeny, also. A cladist of the mid 1990s would have confidently divided them into "Ceratosauria" and Tetanurae. Alas, recent analyses all have broken Ceratosauria, as originally conceived, into a paraphyletic grade. (Note: We still use the name, but apply it to a much smaller group.)
- "carnosaurs" - the big ones
- "coelurosaurs" - the little ones
Survey of Theropod Diversity
Eoraptor. (Late Triassic) Near the base of the theropod tree:
- Considered either to be a basal theropod or a basal saurischian
- A small predatory obligate biped with serrated blade-like teeth.
- A tall but narrow "hatchet-head" snout. A snout shape common to many ancient predators that lacked secondary palates. (See Dimetrodon, rauisuchians.)
Coelophysoidea. (Late Triassic - Early Jurassic):
- E.G.: Coelophysis - small (but larger than Eoraptor). Common fossil dinosaurs of the Petrified Forest
- Ecologically not so different from Eoraptor but possess many derived anatomical features typical of proper theropods:
Ceratosauria (in the strict sense) (Early Jurassic - Latest Cretaceous):
- Medium - large predators that start out rare but blossom in diversity in the southern continents during the late Cretaceous.
- Includes the well-known Late Jurassic North American Ceratosaurus
- But the real glory of ceratosaurs are creatures like Majungasaurus (right) from the Cretaceous of Madagascar.
- These later ceratosaurs represent a very distinctive ecological morph with:
- Short, rounded, and very deep snouts.
- Improbably slender jaws
- Rugose (horny) texture of facial bones.
- Thick squat teeth
- Highly reduced forelimbs (radius and ulna hardly bigger than the wrist bones) attached to strong, robust pectoral girdles. WTF?
Spinosauroidea (Middle Jurassic - Late Cretaceous): Includes one of the two first dinosaurs to be studied - Megalosaurus - (not highly specialized), as well as specialized long-snouted forms. Generally speaking, spinosauroids reflect derived features that they share with other groups of more derived theropods:
Apart from these, derived spinosauroids had their own unique features, including:
- Enlarged hands.
- Tooth row restricted to the snout in front of the eye socket.
- Interlocking tail vertebrae.
Carnosauria (Late Jurassic - Late Cretaceous): Includes such classics as the Late Jurassic North American Allosaurus. During the last twenty years, a great many other carnosaurs have become well known. Carnosaurs share with coelurosaurs (the other more derived theropods) a key feature:
Apart from these, carnosaurs are characterized by complex extra openings in the bones of the snout.
They include the largest known carnivorous dinosaur and largest biped in history - Giganotosaurus (right)
Coelurosauria (Middle Jurassic - Recent): Includes a great diversity of small - medium-sized animals, including birds. Indeed, coelurosaurs seem, ancestrally, to have been small creatures like Sinosauropteryx. Synapomorphies include:
Examples of relatively primitive coelurosaurs include:
- Enlarged brains: at least twice the size of those of other theropods of the same body size
- Long slender three-fingered hand
- Long narrow metatarsals
- Protofeathers: simple apparently hollow
down-like tufts on the body as well as scales
- Tyrannosaurs: (Late Jurassic to Latest Cretaceous)Interesting small dinosaurs that "decided to get big." Limited to east Asia and North America. The earliest known tyrannosauroids were person-sized, and in one case, definitely known to have protofeathers. Derived members of the group were large - huge (6 tons) with reduced two-fingered forelimbs. Tyrannosaur snouts are relatively low and broad. Probably related to the presence of a secondary palate (unique among dinosaurs).
- Ornithomimisaurs: (Early - Late Cretaceous) Slender, lightly built omnivores with small, often toothless heads. Reminiscent of the weird Triassic rauisuchian Effigia on one hand, and of modern ratite birds on the other.
Size - how big can a biped be?
Although all theropod lineages except Coelophysoidea and Eoraptor (if it counts) produced very large animals, members of Spinosauroidea, Carnosauria, and Coelurosauria have members that compete for the title of largest biped ever:
What's interesting is that they all weigh in at roughly six tons. Could this represent the mechanical limit for a biped? What are the limiting factors? Some possibilities:
- For a long time, Tyrannosaurus rex was the largest known biped, until...
- Giganotosaurus, a carnosaur, was discovered. It seems to hold the title of most massive biped for the moment.
- but the spinosauroid Spinosaurus is not far behind and, being slenderer, may be the longest biped ever.
- The strength of limb bones. Obviously, all of these animals could stand and walk, but normal locomotion is not where the skeleton experiences peak loads. That comes in running, cornering, tripping and falling. Recent biomechanical analyses suggest that for a creature the size of Tyrannosaurus a fall would be potentially fatal, and not to be risked. In all probability the largest theropods, even the ones with cursorial adaptations, probably only walked fast when in hot pursuit.
- The ventilation of lungs. In size, the largest theropods outclass even the largest quadrupedal mammals. What did they have that we don't? One possibility: The air-sack and flow-through lung system found in birds was arguably present in the last common ancestor of saurischians and leaves many osteological traces in theropods and sauropods. This system is known to be more efficient than the tidal flow system of mammals and powers the most metabolically active vertebrates on earth (birds). Maybe it was also useful in allowing their extinct relatives to attain great size and still oxygenate tissues.
Diversity of table-manners
Like other archosaurian predators, theropods ancestrally took bites out of their prey with serrated blade-shaped teeth born in narrow but deep snouts. This hatchet-head shape was well-suited to slicing meat, but very poorly adapted to withstanding side-to-side bending forces. It seems probable that even the very big carnosaurs with this skull shape hunted by wounding their prey with their forelimbs then eating it after it could no longer struggle. (Indeed, the enlarged hand of derived theropods may have been an adaptation to this.) But there were variations:
- Derived ceratosaurs took the hatchet-head morph and exaggerated it, with their deep narrow snouts, small teeth, and dinky jaws. Their heads sat on very robust necks, and their neck muscles originated from robust shoulder girdles. Perhaps they really were chopping into their prey. They certainly weren't wounding it with their tiny arms.
- Derived spinosauroids seem to have done the opposite, abandoning chunk-biting in favor of taking smaller prey. The spinosuar Baryonyx has been found with fish as gut contents. Were these theropod "herons" competing with metasuchians for fish?
- Derived tyrannosaurs also gave up the use of their hands in prey capture and relied on their snouts. Although their teeth are serrated, they are stout "serrated bananas" not blade like as in hatchet-heads. Furthermore, their snouts are broad, low, and structurally more solid, thanks to their secondary palates. Perhaps they could grab and shake their prey, like a wolf of hyaena.