Fins to Feet

Reigning Reptiles – The Archosaurs

Many great dynasties of reptiles rose and fell during the early phases of the Mesozoic Era (a hefty block of geological time, sometimes called the “Age of Reptiles”, that lasted from 250 to 65 million years ago). The beaked rynchosaurs, the agile, erect-walking, predatory rauisuchians, the crocodile-like phytosaurs and armored aetosaurs all had their heydays within the first 50 million years of the Mesozoic.

The names of these lineages – successful though they were – are probably only familiar to professionals and die-hard enthusiasts. The same cannot be said of the ever-popular Dinosauria, whose first (relatively unassuming) representatives also appeared on the scene during this early-Mesozoic time interval, around 230 million years ago. The Dinosaur clade has attained tremendous public notoriety for its evolutionary experiments in large body-size, defensive weaponry and bipedal carnivory, as well as for its sudden fall from planetary dominance about 65 million years ago. Dinosaurs have penetrated the public perception of prehistoric life so completely that the casual museum-goer is prone to indiscriminately identify any large, extinct reptile (or, worse, any large, extinct animal) on exhibit as a dinosaur. The term “Dinosaur” refers to the members of a specific branch on the greater tree of reptile evolution – it is not a blanket descriptive term for anything extinct, gigantic and ostensibly reptilian. None of the early Mesozoic reptiles I mentioned in the opening paragraph are dinosaurs.

But what distinguished the earliest Dinosaurs from their non-Dinosaurian reptile contemporaries? Are modern-day crocodiles close relatives of dinosaurs? Answering these questions is not trivial and will require a fair amount of phylogenetic tree-climbing.

Step 1: Dinosaurs as Diapsids

Dinosaurs have two openings on each side of the skull behind the eye-orbits. These holes, called temporal fenestrae, have margins that provide secure anchorage points for muscles involved in jaw function. The presence of these paired holes in the skull roof places them among the Diapsida. All living reptiles –with the possible exception of turtles and tortoises – are Diapsids by ancestry.

The Diapsida may be sectioned up into two distinct groups of reptiles:

- A group of reptiles that includes lizards and snakes in addition to a number of now-vanished forms, such as the spectacular marine Mosasaurs. This group is formally called the “Lepidosauromorpha” and is of no further concern to us in this account of Dinosaur origins.

- A group that includes modern crocodillians, avian and non-avian dinosaurs, flying reptiles called pterosaurs and a bewildering assortment of lesser known extinct reptiles with varied body-plans, diets, native habitats and temporal ranges. These are the “Archosauromorpha“.

Step 2: Dinosaurs as Archosaurs

Archosaurs display a number of skeletal features that distinguish them from other reptiles:

- An opening in front of each eye-orbit called the “anorbital Fenestra”.  These holes may have served to accommodate additional muscle-mass or, alternatively, housed air sinuses or a salt gland.
- An opening on each side of the jaw called the “Mandibular fenestra”
- Teeth that are “laterally compressed” – that is, flattened from side to side – and serrated.
- Teeth set in sockets
- A large protrusion on the femur called the fourth trochanter that serves as an attachment point for a set of thigh retracting muscles.
- A high narrow skull
- Various anatomical characteristics that indicate an upright or semi-upright limb posture, as opposed to the sprawling limb posture observed in modern lizards.

One of the main thrusts of my post is this: the earth was already inhabited by a wide variety of impressive, large terrestrial reptiles – most of them archosaurs or close relatives of archosaurs – when the first dinosaurs made their appearance on the evolutionary stage.

Phytosaurs were superficially crocodile-like archosaurs of semi-aquatic habit from the early Mesozoic. They had elongate snouts and jaws rimmed with conical serrated teeth. Unlike modern-day crocodilians, they had nasal openings positioned high up on the skull – close to the eye-orbits in many cases – rather than at the tip of the snout. Only its highly-placed nostrils and eyes would be visible from above, were it wading just below the water-surface. It could have snagged unsuspecting prey on the edges of rivers and lakes with the same adeptness that modern crocodiles display.

Aetosaurs were herbivorous archosaurs armored in heavy, interlocking dermal plates – it is tempting to think of them as prehistoric armadillos. Many were armed with fearsome spikes and stout claws.  Rynchosaurs were strange plant-eating relatives of the archosaurs that had barrel-shaped bodies, stocky limbs and strong beaks that could cut through plant matter.

Before the ecological ascent of the dinosaur lineage that produced Tyrannosaurus rex and Velociraptor (the theropoda), the biggest land predators on the planet were the Rauisuchians. They are generally understood to have been active, toothy, largely quadrupedal* archosaurs related to modern crocodiles. They were erect-walking animals with limbs oriented vertically below the body, rather than jutting out sideways (as in lizards) – one of several traits they share in common with dinosaurs. The head of the femur fits into a downward-facing socket in the hip girdle called the acetabulum. This is known as a “pillar-erect” configuration as the femur bears the weight of the skeleton above in it in much the same way that a vertical column bears the weight of a roof-top. The largest raiusuchians hit a length of over 7 meters, far bigger than the carnivorous dinosaurs of their time, and hunted large game.

The first “true crocodiles” also appeared during the early Mesozoic. Modern-day crocodiles and alligators are built for a low-energy, semi-aquatic lifestyle. Crocodiles walk, for the most part, with a sprawling, lizard-like gait and spend hours basking on river-banks in a state of absolute intertia. Many early Crocodillians were, by contrast, slender-legged and fleet-footed terrestrial creatures – some of them were even herbivores! Crocodile evolution is a fascinating story in its own right and will be dealt with more fully in a future post.

Step 3: Dinosaurs as erect-walking animals

Crocodillians, Raiusuchians, Aetosaurs and Phytosaurs can be readily distinguished from dinosaurs on the basis of their ankle morphology. In the former groups, the functional ankle joint is situated between two foot bones of similar size: the astragalus (an ankle-bone; in these archosaurs it is sutured to the two bones of the lower leg, the fibula and tibia) and the calcaneum (heel bone). This configuration allows for considerable rotation of the foot during locomotion.

In Dinosaurs, however, the articulation between the ankle bones is modified to produce a simple hinge joint. The functional ankle-joint is between the upper bones of the ankle (the calcaneum and astragalus) and the bones immediately distal to it (refer to diagram). The calcaneum was very small compared to the astragalus. The important implication of this anatomical design is that the foot was restricted to motion in a vertical plane. In other words, it could move back-and-forth but could not be twisted to any considerable extent. This is a clear adaptation for upright, speedy running. A similarly structured ankle is also found in pterosaurs. Pterosaurs are aerially adapted reptiles and close relatives of dinosaurs.

Both the Raiusuchians and the Dinosaurs independently evolved an erect gait (where the limbs were positioned directly below the body rather than splayed outwards), but their anatomical approaches to this condition differed.

Three bones make up the pelvis: the pubis, the ischium and the illium. At the junction of these three bones, there is a socket into which the head of the femur fits called the acetabulum. In Dinosaurs, the head of the femur is bent – that is, offset at an angle to the main body of the bone. The head fits into the acetabulum. The dinosaur acetabulum, rather than being a solid bone-pocket, actually has an opening in it (an open acetabulum is one of the defining features of the dinosaur clade).  The dinosaur femoral head is rather more barrel-shaped that the bulbous head of the human femur, limiting thigh motion to a plane parallel to the body.

The oldest known dinosaurs from the fossil record are lithe, bipedal carnivores like Staurikosaurus and Herrerasaurus. One of the interesting implications of this ancient bipedal body plan is that all quadrupedal dinosaurs from succeeding ages – horned Triceratops and mighty Brachiosaurus among them – belong to lineages that, at some point, had to revert to walking on four legs from a state of two leged-ness.

On the road to upright-walking bipedalism, the dinosaurs evolved a host of anatomical features that, taken in summation, set them apart from the rest of reptiledom – including, among other traits, a distinct neck, a backward pointing shoulder joint, a hole in the center of the acetabulum, longer hind-limbs than forelimbs and long lower foot bones and digits.

Dinosaurs did not comprise a significant portion of terrestrial animal diversity for much of the early Mesozoic, but they eventually supplanted the raisuchians, aetosaurs, rynchosaurs and other reptile groups as the dominant vertebrates on land. Their spectacular success would last them through the middle and later Mesozoic – and beyond, given that birds are modern day descendants of dinosaurs.

But what made the ascent of the dinosaurs to ecological preeminence possible?

Some researchers have suggested that the upright-walking dinosaurs may have been more efficient terrestrial locomotors than their sprawling gaited (or semi-sprawling gaited) contemporaries – and that this was crucial to their long-term evolutionary success. Others have opined that the evolution of warm-blooded physiology in early dinosaurs may have given them a “competitive edge” over other reptile groups.

It seems likely, however, that the meteoric rise of this scrappy upstart clade had more to do with luck than any inherent superiority. After all, a number of other archosaurs evolved an erect gait but did not diversify to anywhere near the extent that the dinosaurs did, nor did they survive past the early Mesozoic. The fossil record suggests that two large-scale mass extinction events took place during the early Mesozoic, successively culling the landscape of various groups of large reptiles and transforming the vegetation. Early dinosaurs just happened to scrape through these tough times (mass extinctions are often more random in eliminating groups of species than ecologically selective) and inherited a world deserted of major competitors.

*There is evidence for at least facultative bipedalism in a number of raiusuchians.

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