Good tidings and well-wishes faithful readers!
Sometimes, to paraphrase in a Cosby-esque style, evolution does the darndest things! Occasionally, a species which seems perfectly well-adapted for a given lifestyle will suddenly undergo massive physical alterations and inhabit a new and radically different niche. Such is the case for the sphenosuchians…the unlikely anscestors of modern crocodylians.
As loyal readers (again, assuming they exist) can probably tell, I’ve had crocodylians on my mind for a little while now, having reviewed David Schwimmer’s book on Deinosuchusa few posts back. The group has undergone a fascinating history which has seen the rise and fall of a dazzling array of characters. Thus, it is a mistake to claim that crocodylians have remained “unchanged for millions of years”, as the anscestors of the modern species with which we are all familiar have spawned from some of the most unlikely creatures imaginable.
Take a look at the image above. This depicts a Hesperosuchus, the quintessential North American sphenosuchian (since my knowledge in this area extends only to North American creatures, I’ll abstain from discussing their European, African, and Asian counterparts). At first glance, it seems almost impossible to believe that such a beast gave rise to the same crocodiles seen grappling with zebras on the Discovery Channel. It’s a well-known fact that modern crocodiles are not exactly the most energetic members of the animal kingdom…one can easily mistake captive specimens for stuffed dummies when walking beside their enclosures at the local zoo. In stark contrast, everything about Hesperosuchusseems to shout “speed deamon”! The animal’s limbs are long and lanky: perfect for short (or even long) distance sprinting, its skull is lightly-built and its tail (which unfortunately can’t be viewed in this photograph) is long and wispy, rather than stout and heavy as in modern crocodylians.
Upon closer examination, however, the similarities between this animal and a modern counterpart are made apparent.
Crocodylians are distinguished from other reptiles by their fairly non-kinetic (unmovable) skulls: while snakes, lizards and even dinosaurs posses some cranial mobility, crocodylians maintain a comparatively rigid skull, which greatly increases the power of their infamous biting capabilities. Other group features include:
-The back of the skull (the posterior end) has a unique ‘squared off’ shape.
-The quadrate bone (which forms part of the jaw joint) extends far downward ventrally (towards the creature’s belly) and firmly contacts most of the posterior jaw bones. This allows crocodylians to widely open their jaws and provides an extended hinge for the musculature responsible for the group’s notoriously-strong jaw closure. It should also be noted that modern species, in addition to the traditional jaw adductor muscles (jaw-closing muscles, see the picture below) sport another pair of muscles which may be found inside of these called the pterygoideus muscles. These provide a quick pull on the mandible (lower jaw) which, combined with the slower perpendicular contraction of the jaw adductor muscles, produces their multi-ton jaw-closing power.
-Most vertebrates which posses a reasonably-advanced sense of hearing (such as birds, mammals, and most reptiles) sport what’s known as a Eustachian tube in their ears (see the picture below of a human ear).
In most archosaurs, such as dinosaurs, this tube is accommodated by a mere skull cavity. However, in crocodylians, birds and mammals, a specific canal in the skull, appropriately called the eustacian canal houses its namesake tube. This feature is what forced the scientific community to recognize sphenosuchians as anscestral crocodylians, as a Chinese specimen of a species called Dibothrosuchus sported a recognisably-crocodylian eustacian canal.
Sphenosuchians, like many Triassic archosaurs, possessed scutes (armor-like external plates of bone). However, these were arranged in two rows along their spines. This brings me to an interesting point. As I brought up in my last post, David Schwimmer in his book “King of the Crocodylians” made the observation that living crocodylians don’t use their scutes for defense so much as for physical support. Modern species (like the one below) occasionally engage in what’s known as a ‘high walk’ wherein their bellies are held off the ground as their limbs are placed directly underneath their bodies rather than sprawling out to their sides.
Their scutes attach to the top of the animal’s ribcage via a series of muscles which, when placed in this position, actually pull on the animal’s body and help lift it off the ground, similar to how suspension cables hold up the Brooklyn bridge. This feature is a throwback to their sphenosuchian ancestors, who made extensive use of it as they ran about during their energetic lives. Thus, it is believed that scutes evolved initially for support and were only later utilized for defensive purposes.
There has been considerable debate as to whether sphenosuchians were bipedal (walking and running on two legs) or quadrupedal (walking and running on all fours). In most species, the hind limbs are more than twice the length of the forelimbs, however, some researchers maintain that they predominantly fell into the latter category for most of their activities. Perhaps they would walk and jog on all fours but revert to their back legs when the need was felt to run at an especially-fast pace.
Besides Hesperosuchus, some paleontologists contend that a second late Triassic North American species, Dromicosuchus, exists as well. The type specimen of the latter was unearthed in North Carolina in 2003. The animal’s discovery was interesting because it was found directly underneath a large rauisuchian (see the picture below).
This fact in and of itself isn’t especially likely to raise any eyebrows; after all, skeletons of various animals are frequently found atop each other at dig sites, particularly those which hail from a river deposit. However, the Dromicosuchus specimen bore damage from rauisuchian teeth on its neck! Since neither of these animals are particularly common, this suggests that the Dromicosuchuswas killed by the larger rauisuchian before it too perished somehow. Fascinating, no?
Personally, I don’t think that the differences given between Dromicosuchus and Hesperosuchus in the scientific paper which followed the discovery explaining why the former should be considered its own genus are justified. One of the explanations given was the fact that the “Dromicosuchus” specimen lacks a lone triangular scute at the base of its skull found in Hesperosuchusspecimens from the American southwest, however this is probably due to the injuries sustained by the rauisuchian bites. The other differences given are, for lack of a better term, “nit-picky” and aren’t substantial enough to justify a new genus (though it may possibly represent a new species or subspecies of Hesperosuchus).
These fleet-footed creatures likely dined on insects and small vertebrates, yet surprisingly they co-existed with early theropods for millions of years in the Late Triassic (such as Coelophysis, the New Mexican state fossil, pictured below), even though by every indication they too were dining on similar prey items.
Sphenosuchians are also noteworthy because they, along with dinosaurs and a handful of other archosaurs, survived the mass-extinction which occurred at the end of the Triassic period. This extinction wiped out the phytosaurs (pictured below), which were superficially crocodile-like and occupied the large semi-aquatic predator niche through much of the period.
Their extinction, coupled with the increased diversity of theropods which occurred during the Jurassic (the succeeding period), likely drove the sphenosuchians to abandon their traditional role as speedy terrestrial predators and evolve larger, more streamlined bodies to inhabit the waterways as had the phytosaurs before them. As is often the case in evolution, no niche goes unfilled for long.
May the fossil record continue to enchant us all.