'Donts and 'Apsids: Ancestral Dinos and Mammals of the Mid-Triassic

When it comes to dinosaurs and mammals, neither had quite yet evolved yet.  Most people consider animals like Herrerasaurus and Eoraptor to be among the oldest known dinosaurs, but others now consider Nyasasaurus to be the oldest, originating from 240 MY old rocks from Tanzania.  Many dinosaurs looked very similar to other, closely related archosaurs, and only extensive research and more specimens will be able to shed light on these ancient critters.

Mammalian ancestors took the form of the now-extinct dicynodonts and the cynodonts, the latter of which include modern mammals, as well.  In modern mammals, you can see how the skull only has a single hole behind the eye (the space where the coronoid process sneaks in between the main part of the skull and the extruding zygomatic arch), making it a synapsid, or "one-holer."  In previous posts, we've talked about primitive, mammal-like animals such as Dimetrodon and Cotylorhynchus.  Both of these critters are synapsids.  Diapsids, or "two-holers" (remember from our recent Latin/Greek Roots post that the root "di" means "two" [click HERE to read that post]), is another large group, and includes everything from crocodiles to dinosaurs, lizards to snakes, and tuataras to birds.  It also includes the archosaurs, a subgrouping of diapsids that are characterized by an additional hole in the skull, bringing the total number of skull holes up to three.  So some diapsids are also archosaurs, such as birds, dinosaurs, and crocodiles.  There's also the anapsids, which are animals with no holes in the skull, such as amphibians and turtles.  Taxonomically, this can get a bit confusing (especially since sometimes an animals classification doesn't correspond to the number of holes that it has at that point in its evolutionary history), and maybe later we can go into greater detail about these different 'apsids, but below we have a nice picture that should help clear things up a little.

Holes in the skull.  On the top left, we have the prehistoric sea turtle Protostega, an anapsid, with no extra holes behind the eye socket.  Below Protostega, we have Prestosuchus, a type of archosaur.  Not only does Prestosuchus have the two holes in the skull behind the eye socket that characterize older diapsids, but it also has a third hole, in front of the eye socket, but behind the nose openings.  On the bottom right, we have Edaphosaurus, a primitive synapsid.  The largest hole in the skull, furthest on the right, is what will one day become the hole that the coronoid process sneaks through, between the zygomatic arch and the rest of the skull.  In the picture above Edaphosaurus, you can see what I'm talking about, with the extinct mammalian synapsid Hyaenodon.  Here, you can see the little nub of the coronoid process between the zygomatic arch and the skull.

As we talked about in that Latin/Greek post that I mentioned above, the name of the primitive, fin-backed synapsid Dimetrodon means "two measures of teeth," referring to the two different types of teeth this animal possesses.  This is a feature known as "heterodonty," a term that means "different teeth."  Most mammals are heterodonts, and most other animals like reptiles are not, but it doesn't always work that way.  Modern cetaceans such as the sperm whale, as well as orcas and dolphins, are homodonts, meaning that they only have one type of tooth in their mouth.  If you look at ancient ancestors of whales, such as Basilosaurus or Zygorhiza, you can see that they have different types of teeth in their mouth.  This condition can be traced all the way back to 50 MY old Pakicetus.

A trio of cetacean skulls.  On the top left, we have Pakicetus, a terrestrial ancestor of the cetaceans, that lived in Pakistan approximately 50 MYA.  Below Pakicetus, we have Zygorhiza, a more derived and fully aquatic cetacean.  In both Pakicetus and Zygorhiza, you can see how the front teeth and back teeth are different, with the front teeth more for gripping prey, and the back teeth perfect for slicing.  On the right, you can see the skull of the modern killer whale, or orca, which has only one type of tooth in their mouth, the conical, gripping teeth.

Then, of course, there are the heterodont reptiles and dinosaurs such as the Cretaceous crocodilian Malawisuchus, and the dinosaurs Heterodontosaurus (literally meaning "different-toothed lizard") and the oviraptorosaur Incisivosaurus.  We also talked about the primitive pterosaur Dimorphodon (two morphs of teeth) in the Latin/Greek post as well.  If you look at the skulls of any of these animals, you can clearly see the different types of teeth in their mouth.  Cynodonts were not merely an aberrant heterodont form amongst a vast sea of closely related homodonts, but instead were precursors to the default heterodont condition seen in mammals.

The skull of Heterodontosaurus, on display at the American Museum of Natural History in New York.  You can see the two different types of teeth in the skull, especially in the lower jaw.

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Moenkopi and Lykins: The Mid-Triassic in Western North America

In this post, we continue with out exploration of the Mid-Triassic, 240 MYA.  In this post, we look at the Moenkopi and Lykins Formations of North America, and see what they can tell us about this ancient time.  We also delve a bit into the evolution of fin-backs and sails as display structures, like you can see in the picture below.

In the Moenkopi Formation of North America, we can gain a more complete understanding of the terrestrial fauna of this time period.  We already discussed the rauisuchian Ticinosuchus and the protorosaur Macrocnemus from Monte San Giorgio, as well as the large temnospondyl amphibian Eocyclotosaurus from Grès à Voltzia, but there were lots of other very exciting animals alive back then as well.  Arizonasaurus, a poposaurid archosaur, was likely one of the top predators, and had a back adorned by a Spinosaurus-like sail, similar to the sail seen in the more primitive German archosaur Ctenosauriscus.

We talked about the poposaurs in a recent post which you can read by clicking HERE.  The poposaurs, as well as the rauisuchians, were both members of a large group that many scientists refer to as "pseudosuchians."  These animals were crocodilian in nature, and fairly closely related to them.  But some pseudosuchians, including some rauisuchians and poposaurs, actually evolved a body design similar to some types of dinosaurs, where they could walk on either two or four feet.

Let's jump back to the sails on the back of Arizonasaurus, Ctenosauriscus, and Spinosaurus.  The first two are fairly closely related to each other, but Spinosaurus is not closely related at all, separated by around 150 MY of geologic time.  The question is, why would these animals have convergently evolved these sails on their backs?  The orthodox answer is that the sails help the animal thermoregulate, that by turning the sail towards or away from the sun, it would help the animal warm up or cool off.  Similar ideas have been proposed for other animals that feature similar anatomical structures, such as Stegosaurus with its double row of plates down its back, or the primitive synapsids Dimetrodon and Edaphosaurus.

A model of a juvenile Stegosaurus from the Morrison Natural History Museum.  Other stegosaurs that are very closely related, such as Kentrosaurus and Wuerhosaurus, have very different shaped plates, and a different amount of plates, as well.

This idea has some major flaws, however, as argued by paleontologist Dr. Robert Bakker in his excellent and influential book "The Dinosaur Heresies."  In the book, Bakker points out that very close relatives of these sail-animals don't have these strange fins on their back.  If the thermoregulation theory is accepted, then that would suggest that these very closely related animals had very different thermoregulatory needs.  For example, Bakker points out that the primitive synapsids Dimetrodon and Sphenacodon are very closely related to each other, and most of their anatomy is very similar, other than the fact that Dimetrodon has that enormous sail on its back, and Sphenacodon has only a very slight elongation of its vertebrae.  If we accept the thermoregulatory hypothesis at face value, it would imply that, despite being very similar in anatomy and lifestyle, for whatever reason Dimetrodon and Sphenacodon had drastically different thermoregulatory needs.  Below, we have a chart showing a sail-back on the left with a closely related animal on the right, this one lacking a sail.

So what do we propose instead?  Most likely a means of attracting a mate.  In animals today, it is display structures and behavior pertaining to courtship that changes the most.  An excellent example of this is the birds of paradise from New Guinea, which we discussed in greater depth in a post with a similar focus, in regards to the plates of Stegosaurus, which you can read HERE.

Sharks, such as the very strange-looking Hybodus, have also been discovered in the Moenkopi Formation.

Where I live in Colorado, the Lykins Formation is approximately contemporaneous with the Moenkopi Formation.  The Lykins Formation isn't the most exciting of Colorado's geologic formations (at least not for people interested in fossils or excitement), but stromatolites can be found in some areas of the formation.  Stromatolites are layers of wavy and convoluted cyanobacteria that sometimes form in areas of shallow water.  Cyanobacteria by themselves aren't very big, as they are simply single-celled photosynthetic bacteria.  However, together, the gelatinous secretions they produce is enough to trap the sediment that settles out of the water, forming visible laminations that sometimes fossilize.

Stromatolites were much more common prior to the Cambrian Explosion approximately 500 MYA, as back then there wasn't really anything that could eat it.  Believe it or not, layers of cyanobacteria are notoriously bad at running away from herbivores, even something as slow as a snail or a slug.  Today, stromatolites are relatively rare, especially considering their past abundance, but you can still find them in isolated areas like Shark Bay, Australia, and Lake Salda in Turkey.  Most stromatolites form in areas that discourage herbivore grazing.  Shark Bay and Lake Salda are both hypersaline areas, places where most herbivores simply don't want to go (especially slugs and snails).  More recently, stromatolite-like growths were found living in an abandoned asbestos mine in Yukon, Canada.  This indicates to us that the parts of the Lykins Formation in which the stromatolites are found were likely not conducive to supporting herbivores, perhaps also due to hypersaline conditions.

Join us soon for our next post, in which we look at ancestors of both dinosaurs and mammals that were alive during this time!  We will also do a little investigating into different types of dentition, so stay tuned!

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