Flat Frogs 101

3 12 2008

Good tidings and well-wishes!

While much ado is traditionally made regarding the paleontology and evolution of prehistoric reptiles (including everyone’s favorite group of ancestral poultry), relatively little attention is paid to their amphibian counterparts. Despite this, reptiles do not in any sense possess a monopoly on interesting (or even downright bizzare) life-forms, and the amphibians themselves sport a rich and diverse evolutionary history featuring a colorful cast of characters. Since MCC is located in a region of New Mexico primarily recognized for its late Triassic deposits, I shall dedicate this post to arguably the most interesting group of amphibians found in such localities, the Metoposaurs, or, as my instructor likes to call them, “flat frogs”.

Apachesaurus gregorii, image from Wikipedia.

Apachesaurus gregorii, image from Wikipedia.

As the above illustration demonstrates, these creatures have earned their nickname due to their distinctive dorso-ventrally-compressed (“flattened from the top and bottom” in layman’s terms) skulls. To get a better idea of just how flat the skulls of these beasts were, check out the image below:

Mastodonsaurus skull (side view).

Mastodonsaurus skull (side view).

Before one can fully appreciate metoposaurs, one must possess some basic understanding of amphibian taxonomy. These creatures belong to an extinct group of amphibians known as Temnospondyls, which translates to “split-spines”.

In modern amphibians, vertebral bones are developed in two sets. During development, either one of these sets is reduced while the other becomes the centrum (the main body of the vertebrae) or both become the centrum. Temnospondyls sport the latter orientation. In reptiles (and humans for that matter), it is the front set which is reduced while the rear set becomes the centrum, whereas in Temnospondyls the opposite is normally true, however the late Triassic forms we’ll be discussing feature the more familiar ‘reptilian’ model.

The group emerged during the mid-to-late Carboniferous period, some 310 million years ago, however, since the metoposaurs are main topic of this post, we’ll have to bypass extensively-covering their bretheren. For an overview of the entire gang, do go here.

Metoposaurs have been nicknamed “Crocomanders” due to their superficial resemblance to modern caudates (salamanders, newts, and kin) and their almost-irrefutably crocodilian lifestyle. There is an unusual (and often irritating) trend in fossilized creatures which likely inhabited such a niche: normally, only the skull is preserved. Thus, most of what we know of these animals comes from their cranial anatomy.

The skull is generally sculptured heavily, sporting a labyrinth of grooves and canyons across its surface. In addition to these, however, you may notice some ‘worm-like’ structures on that of the image below (look for the long-ish lines running in front of each eye and curving around the nostrils). Interestingly, these are neither decorations nor muscle-attatchment points. Rather, they contain a sense organ known as the lateral line system. Such an apparatus is most frequently encountered in modern fish, though many present-day amphibian larvae (and a few adults) possess them. These systems detect low-frequency movement through water in a method that is analagous to our sense of hearing. Since these organs only work underwater, we can safely assume that metoposaurs were heavily-aquatic at the very least, as no living animal which isn’t befitting of this description also has a lateral line system.

Mastodonsaurus skull (top view).

Mastodonsaurus skull (top view).

 You may also notice a small opening in the skull just behind the eyes, this is what’s known as a third eye (the ‘parietal’, anatomy fans). Many ‘lower’ vertebrates (fish, amphibians, and reptiles) use them to help identify the darkness of a particular area, as they lead directly to the brain and are light-sensitive (‘photoreceptive’). This is largely responsible for the ‘day-night’ cycle which programs most of their daily routines. It may also be used for hormone production and regulating “bio-rhythm”, however it’s specific function generally varies amongst owners.

If you look very carefully, you may notice that this skull contains four openings towards its tip instead of the traditional two. The posterior pair (that which is closer to the rear of the animal) contains the nostrils, but what of the smaller pair in front of them? That, my friends, carries out a very odd function indeed. You’ve probably noticed that the side-view of the skull displayed some rather nasty-looking enlarged fangs on the upper and lower jaws. How, you may ask, can such a flat skull house such comparatively-enormous teeth? Read on.

When the creature’s mouth was shut, the fangs from the upper jaw rested their tips at the bottom of its mouth. However, the lower fangs, due to the solid nature of the palate in contrast to the fleshy makeup of the tongue and surrounding areas, had no such luxury. To cope with this, most metoposaurs (and indeed, most temnospondyls) began to evolve holes (‘fenestrae’) in the roofs of their maws to house them. In some species (such as the European Mastodonsaurus pictured here), these holes penetrated the top of the skull, meaning that when the jaws were closed, the lower fangs actually slid up through and stuck out of them to resemble a pair of horns! (I’ll bet some of you thought these were cute little harmless critters until I dropped that tidbit. Don’t deny it…:))

Metoposaur dentition isn’t limited to these freakishly-large fangs either. These semiaquatic beasts additionally featured two rows of teeth in their upper jaw. Though it may seem odd, this is actually standard for amphibians.

Postcranially (‘behind the skull’), there isn’t much to cover, for, as I said before, very little of these animals minus their skulls is preserved on a semi-regular basis. However, the animals did sport a shoulder girdle which was sculptured in a fashion similar to that documented in the skull.

Only two genera are known from the late Triassic deposits of the American Southwest, Koskinonodon (aka: ‘Buettneria’) and Apachesaurus (additionally, the non-metoposaur temnospondyls Latiscopus and Rileymillerus are known from the region). It is interesting to note that in the lower parts of the upper Triassic (no oxymoron there…), the representative species (Koskinonodon) is both large and relatively abundant, yet the higher and younger you go, the smaller and rarer the metoposaurs become, along with all amphibians. Temnospondyls went nearly extinct at the close of the Juriassic period (some 145 million years ago), and indeed, most paleontologists believed that they had retired to that great, big swamp in the sky around this time for decades, likely due to competition from crocodylians. However, it was later discovered that at least one species (Koolasuchus, not a metoposaur, but a temnospondyl nonetheless) lasted until the early Cretaceous period in Australia (where the water was likely too cold for crocodylians since it was located much farther south than it is today), some 50 million years later!

I hope that this has proved to be of interest and not eccessively verbose. As always, may the fossil record continue to enchant us all.

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6 04 2009
Flat Frogs 102 « The Theatrical Tanystropheus

[…] year’s WAVP meeting in which he gave a critique of my “Flat Frogs 101″ post. The updated version is now […]

15 12 2009
Week Of Wonders: Anthracosaurus « The Theatrical Tanystropheus

[…] of our synthetic biases, a number of prehistoric amphibians (including everyone’s favorite (‘flat frogs’) were in fact equipped with truly ferocious-looking dental arrangements. But arguably no amphibian of […]

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