Friday, June 27, 2014

Species That Don't Get Enough Publicity #7: Heterodontosaurus

Today’s animal we’re looking at you’ve probably heard of, or perhaps not. Most dinosaur encylopedias and other comprehensive works mention it, but it’s not in any museums outside its home, and it hasn’t made a single appearance on big or TV screens.  No toys, no dedicated books, only a small bit of art for it. It seems that while the large weird dinosaurs attract a great deal of attention, the smaller ones, even more bizarre, do not. There’s a whole plethora of small but striking animals, but we’re looking at one in particular from an obscure location that is absolutely unique.

Perhaps besides its size (only 3 feet, the size of a small dog) and its location (South Africa), its very name makes it so obscure. While Triceratops, Apatosaurus, and Tyrannosaurus are 4-5 syllable monstrosities, Heterodontosaurus has a whopping 7 syllables and 18 letters, not counting the rather small and bland species name tucki.  The name, however, says a great deal on why this dinosaur is so significant. It is in Greek, of course, meaning roughly “Lizard with different types of teeth”

Teeth are the key here. There are two conditions in all animals with teeth-heterodonty and homodonty. Extant reptiles, amphibians, and fish have homodont dentition, meaning their teeth are identical in every way but in stages of growth. A mature tooth of a snake, crocodile, or salamander will look the same no matter where in the mouth it is. Most extant reptiles are diapsids, meaning they also have two pairs of holes in their skulls above and behind the eye sockets. All diapsids are homodonts, including dinosaurs. All dinosaurs have one type of tooth; check this out next time you’re at a museum.

In contrast, almost synapsids (only one pair of holes behind the eyes and ears) are heterodonts. Remember Dimetrodon? It gets its name from the two different kinds of teeth-large, long incisors for cutting off chucks of meat, and shorter, broader carnassials for chopping the meat into small pieces.  More advanced synapsids grew canines for killing or fighting, and we retain these canines despite the fact that they have lost their specialized function. 

Heterodontosaurus is a dinosaur with three kinds of teeth.  In front of the skull, there is a beak like later dinosaurs, but on the maxilla (upper jaw) of the beak there are some small shearing teeth. Then we have tusks. This dinosaur had tusks: one pair of long teeth, like that of a pig, on the lower mandible behind the beak

When A.J. Charig and A.W. Crompton discovered the skull in 1962, the tusks were the feature that stood out, of course. Since then, numerous suggestions have made on how they were used. The beak, along with the shape of the skull, places this animal at the base of the Ornithiscia, plant-eaters that include Hadrosaurs, Ceratopsians, Stegosaurs and ankylosaurus. Most herbivores with tusks or large canines like elephants, hippopotamus, musk deer, muntjac deer, gibbons, monkeys, pigs and gorillas use them for social purposes: they are used in duels between males for social standing and mating rights. As well, they are deadly weapons used to inflict horrific slashing wounds on predators with vicious bites.

Pandas and gorillas also use their large canines for tearing open plant stems  or wood, and elephants use their tusks to uproot trees, bringing down branches, and digging for water and roots. No doubt Heterodontosaurus used its tusks in a variety of ways.

Finally, the cheek teeth indicate this animal was already a specialized browser. The teeth are shaped in compact batteries, an adaptation found in animals that chew their food. So while the sauropods and their ancestors had to use their giant guts and gizzards filled with powerful muscles, acid, and pebbles, ornithiscians were inventing chewing.

In 1976, a complete specimen was found, revealing another unique anatomical trait. While it had three long fingers on each hand like most dinosaurs, fingers tipped with long curved claws probably used for browsing, digging, and fighting, it also had two small, vestigial fingers right next to them.  These tiny fingers, according to their articulation, were opposable, much like the opposable pseudo-thumb of a panda. So while the clawed fingers pulled branches towards the mouth, the smaller fingers could grip them and articulate them.

The body is small and slender, something that would lend to swift, nimble movement.  It probably then lived like the aforementioned musk deer and pigs. Like pigs, it might even have been omnivorous, using the canines to grip meat or tear open termite mounds. As in a previous article, strict herbivory is rare.

There’s one final twist to this animal; relatives of Heterodontosaurus have been found in the same region and period-the more primitive Abrictosaurus, distant relative Lesothosaurus, and the obscure sister species Lychorhinus (attempts to synonymize it with Heterodontosaurus have never been conclusive), and smaller Pegomastax, but they’re poorly defined  and represented. Other relatives include Echinodon from Britain and Fruitadens from USA, but it’s the Chinese Tianyulong that has been the best preserved and provided another revelation.

Tianyulong was found to have been covered in filaments, long, hollow, quill-like scales, something like the earliest feathers.  This had led to the suggestion that all dinosaurs had integument of some kind, be it fuzz or feathers. I think this is a bit of a leap in logic and that it’s unsupported by evidence. After all, the animal had canines, something that would imply all later dinosaurs had them as well, which is false.

However, it’s likely that other members of the Heterodontosaur family had these quills, and so Heterodontosaurus and all its kin would be absolutely-bizarre looking. Imagine, if you will, the Upper Elliot Formation, South Africa, in the Sinemurian stage of the early Jurassic.  The land is covered with vegetation; food for huge prosauropodmorphs: Massospondylus, Aardonyx, Plateosauravus, Gryponyx, tiny Igavusaurus,  giant Melanorosaurus. Predators would follow the herds, picking off the juveniles and smaller prosauropods: the small Megapnosaurus and the much, much larger Dracovenator.  Megapnosaurus would also hunt the small animals of the underbrush-the first mammals, lizards, insects, and small dinosaurs. These small dinosaurs would run like deer, but with the quills of a porcupine, the claws of anteaters and the teeth of boars; even if Megapnosaurus was small and young enough to catch them, they would be in for a horrible fight with their teeth and claws match by their prey’s.

In that land, ancestors of the great Triceratops and Anatosaurus would live in the shadow of the sauropodmorphs; but overshadow them in terms of their appearance-tiny, fuzzy, clawed, thumbed, and tusked.  No other dinosaur can claim to have that, no matter what their size. And that is what makes Heterodontosaurus so special.

So anyway, Hollywood, give this guy a break. Museums, put him on display. Toy companies, here’s a new fresh face. Authors, think about the crap you’re writing and how this genus could perk up the place. And remember, shop Heterodontosaurus where you work or play. Or at least learn how to pronounce the name.

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