Wednesday, March 26, 2008

Once Upon the Permian: Beaked Bites of a Lost Lineage

Imagine you were standing out in the field and there grazing was a herd. It wouldn't be a herd of cattle. It couldn't be. Cows didn't exist. Mammals didn't exist. You...well... you must have been borrowing the TARDIS or hitched a little with HG Wells or Marty McFly, because there are no people. In fact, even if there were cattle, they would probably starve: there was nothing even closely related to grass or any other flowering plant rooted anywhere in the world. No, in the days of the end Permian, when Gorgons ruled the world, the thing that looked up at you in the field as a member of the heard didn't even look like a cow. With its semi-sprawled posture and beaked face, it was no cow. It and the other members of its herd were something different. It was something that survived far better than the Gorgons, its original predators. Its descendants would outlast the sabre fanged menaces to the early Cretaceous at least. Alas, they didn't make it to the present or even into the Cenozoic as far as we can tell. They're your cousins umpty million years removed. Those creatures with their bleary eyed gaze, beaked mouth, and - possibly - hairy bodies that oggle you in their semi-sprawled state are dicynodonts.

The dicynodonts are moderately more famous than the Gorgons. They are one of the major actors in the Late Permian Ecosystem and were probably one of the main sources of prey for the gorgons. What they are most famous for though is not the Permian Era, but just afterwards, on the other side of the Permian Mass Extinction, in the Early Triassic. That would be because Lystrosaurus would become the most successful and wide spread critter until the advent of Humanity.

They were found on every continent of the modern world: in truth, back then, it was all one giant supercontinent called Pangaea. Even so, they ranged from pole to pole, across virtually all terrains and environments. Lystrosaurus was what we call a disaster taxon: a species or genus of critter or plant that exploits the clearing of niches by some sort of extinction event. In this case, the ultimate of extinctions (to date) is what cleared the actors so the Lystrosaurus' time strutting and fretting on the geological stage would be noticeable at all. Unfortunately, multiple times, this time period has been called "When Pigs Ruled the Earth." Alas. Partially that's because they were approximately pig sized and generalist herbivores of a sort. I believe that the origin of the phrase can be traced back to Dr Micheal Benton, since I encountered something similar to it when reading his book on the subject. However, let's back up a minute, or perhaps better yet, several million years and discuss the chronology of the dicynodonts before - and after! - Lystrosaurus.

The dicynodonts appeared during the great radiation of synapsids in the Middle Permian, around 230 million years ago, alongside the Gorgons and numerous others. The dicynodonts ranged in size from moles and gophers to as large as hippos. They were exclusively herbivores. It was thought for a very long time that they were largely wiped out by the Triassic-Jurassic Mass Extinction with a mere handful surviin into the early Jurassic and petering out there as the dinosaurs took over. Remarkably, there was a singular fossil found in Australia from the early Cretaceous from an unknown dicynodont. It came from a region that contained exclusively Cretaceous strata and thus precluded this being a reworked fossil. This is a good example of a Lazarus Taxon or Ghost Lineage. It's exciting that the megafauna therapsids held out so long, but in some ways given the fauna of what is now Australia and then a part of Gondwanaland, it shouldn't be was much of a surprise.

Australia has been something of a refugium for animals for a very, very long time. Obviously, the fauna of Australia is pretty unique now. However, even back in the Cretaceous this is true. For a moment, this bares discusses, because its something of an enigma. While our late surviving dicynodont is one good example, there are others as well. Another excellent example is Koolasuchus. This is a Temnospondyl that seems, as far as we can tell, to have been the last of its kind. It lived in rift valleys that were too cold in winter for crocodiles to survive in and presumably lived a similar life style. Another good example of Australia's odd and unique refugium status is that of the dominance of the Hypsilophodonts long after they were supplanted elsewhere. The most famous of the Hypsilophodonts from Australia is the odd, but interestingly dark adapted Leaellynasaura amicagraphica. As to why this is the case, since Australia was a part of the greater Gondwana, we're not sure. The only guess is that Australia and Antarctica might have been more isolated because of the rising Cretaceous continental seaway transgressions. However, we're not sure, because it would be thought that Australia should have something similar in fauna to at least the late Jurassic doesn't. It truly is odd. At any rate, let's return to our subject at hand, the dicynodonts.

So what makes a dicynodont a dicynodont? The name dicynodont literally means "Two" (di) "dog" (cyn) "tooth" (dont). The dicynodonts lost most of their teeth, including all of their incisors, through their own convoluted evolution except those two tusks that reminded their early discoverers of dog's teeth, especially the canines. Even so, some of the dicynodonts lost even those. So what did they eat with? Something that was unique among the synapsids: they had a beak. Instead of dicynodont, perhaps they ought to have been called psittacostiosaurs (bird mouth reptiles/lizards) or psittacoaazosaurs (pretty much the same thing) rather than refer to the tusks. Their jaws were something extraordinary though. They had a shearing action that, as far as I have found to date, is unique among the synapsids. To show it, I am going to swipe a picture from what is probably one of the best websites on the therapsids online:

In some ways, it reminds of how the Gorgons would thrust their jaws forward to get around their sabre teeth. The closest analogs to their seem to have been either the ceratopsians or the rhynchosaurs. Their beaks filled the role of incisors and their own bony plates or dental batteries would sheer the plant matter into smaller bits to be digested. Our own molars crush and grind. Their equivalents were closer in function to our incisors....which seems a little odd to us with the generalized mammalian set of teeth. It is interesting that this chewing arrangement arose three different times in three different very unrelated lineages. This tells us that the arrangement of teeth and chewing as such was useful for some reason. Also interestingly when this arrangement arose in the dicynodonts, it was the first complex chewing mechanism to have evolved.

There are other characteristics that characterize up the dicynodonts, one that was important was the arrangement of their legs. We have a pretty good picture of how the dicynodont legs evolved over time. Originally, soon after their appearance in the fossil record, they were sprawlers, that is to say that they had their legs jutted out not unlike a crocodilian does, looking as though they are in the start position to do push-ups. At the start, botht he front and back legs were like this. Over time though, and especially in the larger specimens, the hind legs were pulled in under the body to support its larger weight and possibly deal with the the improved respiratory system that may have accompanied it. Based on fossil footprints from South Africa's Karoo Basin, the locomotion of the dicynodonts was something more akin to a slow waddle than anything else: these guys were not going to outrun anyone. On the other hand, based on their bite, they might have given their predators quite a fight.

Their senses were not as developed our mammalian line. Their hearing was poor and limited to lower frequencies rather like the Gorgons. The acute sense of hearing that we take advantage of to listen to Mozart or try to remove as much as possible by playing the ipod too loud is a mammalian derived characteristic: our earbones as call them are derived from our ancestors' jaw bones. Interestingly, it seems at though the sense of smell for a dicynodont was not very developed either. Unlike the Gorgons, they had no reason to track anything by smell, but on the other hand, they couldn't get a whiff of a stinky Gorgon until it was too late. Their eye sight seems to have been pretty good and they would have relied on that as their means of detecting danger and getting around.

There are a few other distinguishing characteristics as well. Finishing up with the head, their's was heavy. Really heavy. They had extensive musculature across their back to support it. They would have had thick, meaty necks if we were to see them. In fact, several vertebrae were fused for attachment purposes if I have understood correctly. Dicynodont bodies were also tubular. They were eating machines and their bodies were built for it. They had a short tail for a therapsid. It probably didn't look like a mammalian tail though.

One thing that we seem to know is that the dicynodonts were definitely social. The Placerias Quarry has what seems to have been a herd of Placerias that died there, possibly due to a drought.

What don't we know? Simply. A lot. We don't know if the dicynodonts laid eggs or had life birth or something in between. Based on recent work though for the genes related to producing egg yolks, there's a good chance that laying eggs is the basal position for the synapsids in general. There's also the possibility that the dicynodonts gave milk. However, we really don't know and it depends on when lactating evolved. Was it before or after the split off of our ancestors and our subjects here? we don't know. There's working being done though. Likewise, we don't know if they were hairy or not. No dicynodont mummies have been found. There was some fossilized skin of a therapsid (or possibly two), but neither was a dicynodont; however, what those skin samples showed, despite the drawings above that show hair, was that there was none. There is no mention in the literature that I have perused that dicynodonts had any identifying marks on the skulls that suggest that they had nerve endings for whiskers. Though that would have made them even more...odd...looking. Finally, no one really seems to know whether or not these creatures were warm blooded or not.

Most likely the original main carnivores of the dicynodonts were the Gorgons. However, the therocephalians and the large croc-like amphibians probably munched on them as well. As time went by the archosaurs took over that role after the Permian Extinction wiped out the previous fillers of that role. In the end, you can imagine that theropods were doing the dirty work in what is now Gondwana at the twilight of the dicynodonts. Just imagine, a big allosaur ripping apart a carcass instead of that Euchambersia. Talk about an odd, odd visual. It's almost like one of those B movies where they mix up all the different dinosaurs from different periods. was real.

That said, it is very possible that the dicynodonts made it to the end of the Cretaceous. What a bitter irony it would be that they made it through all those millions of years of dinosaur dominance and two of the five major mass extinctions only to get munged by a rock from the sky. Such a pity. After all, they might have had a chance to do the Lystrosaurus dance again. Then again, what would have happened had they not had most of their families wipe out by the Permian Extinction? What would they have done had they survived the KT impact? Assuming that they made it to the KT, of course.

However, just because I am me and can't help myself: let's play a little what-if here. By luck, there's a microsized dicynodont that lived off shore of Gondwana. It's suffered island dwarfing and is about a half meter long. The Great Rock in the Sky pours itself into the Caribbean and the dinos dies. Well, the non avians ones. By a fluke that didn't happen OTL, the small dicynodonts get rafted off their island before they die out as they did in OTL. ATL they end up on Australia and are able to find just enough food to survive the months of darkness that crashed the Mesozoic ecosystems. Then in Australia, because they have the size edge on the mammals, they grow large and in charge...for the herbivore role. At least for a time. I could easily see a Paleogene in Australia and Antarctica with large sized dicynodonts. Past that? Whose to say...

To wrap up, I hope that you all have enjoyed our little exploration of the dicynodonts. My apologies that this is not as long or nearly as high of quality as the post on the Gorgons. I am under a lot more of a time crunch these days and I have been using this as a means to relax as I dig through articles and try to write this. I'll be revisiting the dicynodonts when Zach and I finally get the No Permian Extinction Post & Art done. It'll be a while though. Too much to do with the mischief and not enough time.

Hey, if there's anyone out there with $30k to burn on a sponsorship lemme know. I am getting tons of expensive goodies for my side project but no cash. Need cash. Machinists don't get paid on software, hardware, or other goodies. Sheesh.


Zach said...

Wonderful overview, Will. And yes, I'm still working on the walrodont. I've actually got a fairly good working model, but I just don't know what to do with the hindlimbs.

I'm doubting that dicynodonts were warm-blooded, just because of their inefficient chewing mechanism and sprawling gait. They also weren't very large (I think the largest was the size of a cow, actually).

There is a lot more I'd like to know about the group though (I'm sure we all would). Very strange critters, with some of the wierdest skulls around!

Will Baird said...

They also weren't very large

uh. Placerias was as big as a hippo.

mythusmage said...

Thanks for the idea.

You see, I came up with a variation on the unicorn once, in which the animal is an advanced therapsid. I was originally thinking something like one of the standard mammaliforms, but your essay persuaded me to go with an advanced dicynodont.

Scenario: A dicynodont genus survives up to and a bit beyond the KT boundary. It's a small animal, herbivorous, with an elongated muzzle, prominent ears, and a diastema between beak and molars. A few species are horned, though those are not ancestral to the modern unicorn.

The rise of the grasses means the end of the last of the true ceratopsians (no asteroid impact, the K/T boundary is set thanks to the Deccan Traps, and so the extinction of the non-avian dinosaurs is more drawn out and certain groups survive to the present day). In any case, these late dicynodonts are able to take advantage of the new grasslands, much as a group of mammals is, and in due time a one-horned, one-toed, horse-sized mammaliform animal appears, and is domesticated and comes to serve much as the horse does.

But not entirely as the horse does, having a few traits that means it can't be treated or used exactly like a horse. Unicorns not being as efficient in drayage, somewhat wilder in temperament, and requiring special treatment during pregnancy and a more restrictive diet.

And my decision to make them highly evolved dicynodonts is all because of you. :)

Andreas Johansson said...

The dicynodonts appeared during the great radiation of synapsids in the Middle Permian, around 230 million years ago

Nitpick, but the Middle Permian would be more like 270 million years ago.

Anonymous said...

Found your blog by accident, and all the better for it :) I live in the Karoo desert region of Central South Africa and we have a number of dicynodont fossils that have been collected in the area and a few other additional smaller fossils on our farm. Fascinating blogging and very informative. Shannon Fletcher, Karoo, RSA - some links on the gorgons found in the karoo :

kathleen sisco said...

Wonderful info, Thanks.

Glad to see more info out on the Triassic. I hope more attention is paid to S W Carey's two expansion events 50 million years apart. That fits the two great extinction events.
Even NASA now admits we are under compression but identifies it with an unfound gravity wave: flat and long.
If these compression events are regular and periodic (perhaps from a local like Proxima Centauri) the extra energy that produce compression events which move the heliosphere in close to Mercury can be from the galactic center?
Just read from a previous blog that the Gobi is green in the spring from chives. Interesting.