The Overhype--It Burns!

Surely you've all heard of Ida the Adapid by now. She has, perhaps, been oversold. She is not the "Holy Grail" of paleontology, nor does she cure cancer, though you wouldn't know that based on the mainstream media, who is going apeshit insane over this relatively vanilla fossil (save for its awesome preservation). No time right now to comment further, but read all those links and this one too for a pretty complete picture of the animal's relevance.

A Teaser with Teeth

Just to let you all know, I'm prepping a big ol' post about saber-toothed mammalian carnivores. It's the next in my "taxonomy lessons" series. Remember the first one? A few posts down? About Puijila? Geeze, I'm focusing on mammals a lot lately. As I admitted to Scott earlier, the more I study mammals, the more interesting they become! I think it's the embolotheres that won me over. Freaky big-nosed brontotheres, man. Anyway, stay tuned for a new taxonomy lesson in the next day or so.

Embolotherium grangeri

Little something I whipped up last night after flipping through Matthew Mihlbachler's epic Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla, available for free at the American Museum Novitates website. It is almost 500 pages of brontothere excellence.

Fun fact: all those restorations you see of many brontotheres (including Embolotherium) with a giant horn sticking out of the nose is wrong--the nasal "ram" has a deep channel on the anteroventral surface for the nasal septum. This means the nasal cavity extended to the peak of the ram. These were big-nosed animals! Of the two species of Embolotherium, E. grangeri's ram is tame by comparison to E. andrewsi!

A Lesson in Taxonomy Starring Puijila darwini

Phylogenetics is kind of like the mythical hydra: for every gap closed by a new fossil, three new gaps appear. This makes paleontology very exciting, but it also makes the science easy to pick on by Creationists and other science-deniers. Why are three gaps opened instead of just two? Well, because we weren't around to see one species branch directly from another, taxonomy assumes that all common ancestors are unknown. All you can really do is say that Velociraptor and Deinonychus share certain features inhereted from a common ancestor. Even if Tyrannosaurus evolved directly from Daspletosaurus, it's just not something we can ever know. So because every animal in the fossil record (and in the modern world) exists on its own "sidebranch" to the main line of faunal evolution, then three gaps appear:

1) The gap between the new taxa and the common ancestor;
2) The gap between the common ancestor and its the next rung down;
3) The gap between the common ancestor and the next rung up.

Here's a simplified Dinosauria cladogram illustrating this point:

When I say "shared characteristics," I mean things like number of fingers or length of cervical ribs, things like that. Some of these relationships are based on very technical measurements and features. At any rate, the tree is designed to show that Sauropodomorpha is closer to Theropoda than either is to Ornithischia. That doesn't mean thatt Dinosauria is polyphyletic, it just means that Saurischia is a more exclusive group than Dinosauria. You could say that the features diagnostic of "Dinosauria" are entirely arbitrary, and you'd be right. The whole point is to show relationships, though. If we decide one day that silesaurids should be dinosaurs, the label "Dinosauria" would be moved one rung down to include Silesauridae, which is currently considered more or less an outgroup to the Dinosauria proper.

So what happens when a new fossil fills a critical gap in an otherwise poorly-known transitional series? Let's use dinosaurs and birds as an example. The relationship was realized back in Sir Richard Owen's time. In fact, he favorably compared the femur of Megalosaurus to that of an ostrich. Anyway, watch what happens when we toss Archaeopteryx between Allosaurus and Gallus:

Because Archaeopteryx shows features unique to it and exclusive to both birds and Allosaurus, it cannot be suitable common ancestor between the two outgroups. However, because it has more features in common with Gallus than Allosaurus, it must be closer to the chicken. But because it has its own unique suite of characters, it is not a suitable ancestor. So it split off from a theoretical ancestor between itself and Gallus. Now, we'll probably never find an actual ancestor-decendant relationship in the fossil record, but we can come darn close and make predications as to what features that ancestor should have. Eoraptor, for instance, is the basalmost saurischian dinosaur known. It's got a few unique characters (apomorphies), but on the whole it's a fine model for the common ancestor between Sauropodomorpha and Theropoda. That is, Eoraptor has lots of features common to both groups.

Here's a simplified theropod cladogram using manual digits as an example of this principle:

So when did theropods lose that fourth digit? More fossils are necessary for that answer. You'd have to find a theropod that, morphologically, falls between Herrerasaurus and Allosaurus (like Carnotaurus) and count its fingers (it has four). Then go farther up the tree to, say, Spinosaurus (who is between Carnotaurus and Allosaurus) and you see that has three fingers. So you've further narrowed the taxa gap for that finger to be lost. Likewise, when did tyrannosaurs lose their third finger? Basal tyrannosaurs like Guanlong and Eotyrannus have three fingers, but later ones like Gorgosaurus and Tyrannosaurus have two. Someday we'll find a tyrannosaur with two full fingers and the third finger has been reduced to a splint of bone (as the fourth one is in Herrerasaurus).

The same process has recently been done with whales. Whales used to be a total mammalian mystery. Aside from the fact that they gave birth to live young and nursed their babies, modern whales were far too derived to pinpoint a fossil ancestor. Because of their bizarre skeletons, shaped by a millenia of marine adaptation, comparisons to other modern mammal groups was virtually useless. The cladogram basically looked like this:

But then, paleontologists discovered Ambulocetus natans, and the whole cladogram changed. It just takes one fossil to demonstrate relationships between any two groups.

Aside from being a vicious shallow-water predator with whale-sized dentition, Ambulocetus had a unique ankle structure shared by it and the Artiodactyla, an enormous group that also includes goats, cows, hippos, pigs, and antelope (and more). But because Ambulocetus also had a very unique inner ear shared only by whales, scientists knew that it was a ridiculously basal whale. More finds would come later further solidifying the artiodactyl link (Rodhocetus) and show that early whales hauled themselves onto land to give birth (Maiacetus). The big mystery now is exactly how far back toward the origin of the Artiodactyla whales go, and also when they lost their hindlimbs (are there fossils that document this? Anybody?).

So now the whale cladogram is a lot clearer:

So with all that information in mind, I present to you Puijila darwini, a new basal pinniped that sheds light on the transition from mustalid-like carnivore ancestor to flipper-finned seal. Pinnipeds are a group of extremely diverse and successful mammalian carnivores. They include seals (eared and otherwise), sea lions, and walruses. Their closest non-marine relatives are bears and mustalids (weasels, otters). The ancestor of modern seals has been very difficult to nail down, though. The oldest known seal is Enaliarctos, which already has well-developed flippers. So there exists a big morphological gap between mustalids and seals. That gap has now been partially...um...sealed!

It's less than a meter in length and was found on Devon Island, one of those horribly cold islands above Canada near Greenland. Puijila has a seal's head, including the osteological correlates indicating large, sensative whisker pads. However, its body and feet are more like an otter in that it has, you know, feet instead of flippers. The fingers and toes are flattened, though, indicating extensive webbing (same correlates are in beavers, otters, and Casterocauda). And check out that bacculum! I would like to point out, however, that the feet and hands of Puijila are longer than any otter or beaver.

Look at that muzzle--I would not want to come toe-to-snout with Puijila, though it's small enough you could potentially kick a field goal with it. At any rate, a few more interesting points about Puijila. First, it was discovered well within the arctic circle, lending some credence to the notion that pinnipeds originated in the colder northern waters of the arctic. What's more, its swimming style was more toward pinnipeds than mustalids: quadrupedal "doggie paddling" rather than pelvic paddling. Puijila's long tail probably did very little in the water, and the limb-centric swimming style of Puijila could have easily developed into pinniped swimming, which is similarly dominated by the limbs. Finally, Puijila was not an oceanic swimmer. It preferred freshwater lakes and streams, which indicates that pinnipeds went through a freshwater "phase" before braving the deep blue sea. This is to be expected given other marine mammal's ancestral records (Ambulocetus, Pezosiren).

So what's the moral of the story? Puijila represents a good model for the common ancestor between it and modern pinnipeds, both morphologically and ecologically. It helps to close the distance between mustalids (and bears) and walruses, and future fossil discoveries may show other transitions, such as the loss of caudal vertebrae, the perpetual retraction of the hindlimbs, or the enlargement of manual digit I. So think of Puijila as a seal-headed, long-footed otter, and a virtually perfect representative of common ancestry. Feel free to put together your own cladogram!Oh, and here's a picture I whipped together from the description.

Tune in next time for: "The Headache Of Convergence," or, "Ornithischians: a Poor Choice!"

Ambulo...sealus!

I'll do a bigger, better post on this later, but for right now, I suggest clicking on over to Not Exactly Rocket Science and reading about an incredible new fossil!

Slowly Learning to Read

I must like this "reading" thing, because I just bought another book online. The Big Cats and Their Fossil Relatives looks to be a sister tome to the dog book I bought a few weeks ago (which is awesome, by the way). Cat evolution is something I've always wondered about, moreso than dogs. I mean, dogs are great, but cats can drag carcasses up trees! And there were never any saber-toothed dogs. Also, after reading this book, I'm sure I'll be able to give Catsby a hard time for being a very, very lazy cat, and thus, an embarrassment to her wild ancestors.

Taxonomy is, on Occasion, Frustrating

Here's a question for you: See the picture on the left? That's the famous marine ground-sloth, Thalassocnus. As you can see, the genus is split into five distinct species, T. antiquus, T. natans, T. littoralis, T. carolomartini, and T. yaucensis. If they were dinosaurs, most of these species would probably be given distinct generic names. In paleomammology, it seems that authors are far more reluctant to separate animals at the genus level, even when the various species within that genus are irritatingly disparate. This may be because of another hallmark of mammalian paleontology that I find infuriating: In one of the three or four papers I have on aquatic sloths, the author(s) toy with the idea of separating some of the sloths at the generic level, but decide against it, concluding that the five creatures form a linear progression from semi-aquatic sloth to more fully-aquatic sloth. As in one population simply replaces another.

That thinking is carried over dog evolution, too. I'm reading a great book right now called Dogs: Their Fossils Relatives and Evolutionary History. My only knock against it is that the author continually posits that a known form of ancient dog (like Hesperocyon) simply "gave rise" to another dog, which in turn evolved into yet another form. In the discussion on borophagines in particular, the idea of succession is hammered into the reader's head. Paleoanthropologists used to have this idea about hominid evolution. Remember when there were only two hominid genera, Australopithecus and Homo? Now things have gotten a little better, with Paranthropus, and Ardipithecus added to the mix. Each distinct genus, however, is packed with several species. Homo consists of at least five, Australopithecus includes four or five, and even Ardipithecus, which is known from scrappy remains, consists of two species.

In mammal taxonomy, it seems like generic distinctions are tied either to chronology or some debatable "breakthrough" feature within a particular lineage. For example, very little differentiates H. habilus from the later australopithecines except for circumstantial evidence of tool usage. And even tool usage is shakey ground for a new genus name--chimps, gorillas, and orangs all use tools to some degree, and I'm sure australopithecines did, too. I read back in college about an anthropologist who wanted humans to share the same genus as chimps. We'd just be called Pan sapiens. Seriously? There are enormous morphological differences, both skeletal and otherwise, between me and Cheetah. Luckily, this thinking has not flown with the scientific community.

Things get even more bizarre when you think about modern mammals. The Indian and African elephants are generically separate (Elephas and Loxodonta, respectively). In a few million years, when future paleontologists are digging up their bones, will they be considered similar enough to warrant unification under one genus? Better yet, rather than dividing up the numerous populations of Indian and African elephants into distinct species, they are merely given subspecies variations. The savanna African elephant is L. africana africana while the forest elephant is E. africana cyclotis. Even better? One of the closest extinct relatives of the modern elephant, the mammoth, comprises of one genus and up to eleven species, based mainly on where they lived.

None of this would be tolerated in dinosaur taxonomy. Virtually every bone that comes out of the ground is given, at the very least, a new species designation. More often, you get a new genus, but this is often based on how complete the material is. While mammal taxonomy might be a case of under-splitting, dinosaur taxonomy seems over-split. Arguments about as to whether the European species of Allosaurus is a new species or an old species. If Allosaurus was a mammal, it would probably be considered a different subspecies. The mind boggles. I was blown away when Microraptor gui, Microraptor zhaoianus, and Cryptovolans pauli were theorized as being synonymous (Senter, et al. 2004), an idea that has continued to hold sway. "Dave," a juvenile sinornithosaurine dromaeosaur from China, has not yet been officially given a taxonomic name, even though it's pretty clear that the little bugger is a juvenile Sinornithosaurus. But what species is it? Liu, et al., in 2004, named a new species (S. haoiana) based on differences in the skull and pelvis. YARG!

How would mammalian taxonomy deal with a group like tyrannosaurs, where each animal seems to represent its own distinct place in the family? There isn't really a lot of "progression" with tyrannosaurs. You've got the basal forms, like Dilong and Guanlong, sure, but after that things tend to go to hell. Asian alectrosaurs are wholly different than, say, Alioramus or Appalachiosaurus. And those basal forms are, themselves, quite distinct from the albertosaurines and tyrannosaurines. And where does Eotyrannus fit in? Heck, what about Tarbosaurus, which could either be a sister taxa to Tyrannosaurus or the ultimate Asian tyrannosaurine that evolved in parallel to the North American tyrannosaurini from some Alioramus-like ancestor?

Is the mammalian fossil record that much more complete that we can point to Hesperocyon and say, "Okay, this was the direct ancestor of Osbornodon?" That just seems ridiculous! And even within Osbornodon, there are different species, including O. fricki, O. iamonensis, and O. sesnoni. And the ones in the middle are considered transitional between the earliest and the latest, as though a smooth progression from one end of Osbornodon to the other occurred! It's maddenning!

As Scott Elyard has reminded me on many occasions, fossil ancestry can never actually be known. The best we can hope for is to find a sister-group relationship between any two animals. You cannot point to Daspletosaurus and call it the ancestor of Tyrannosaurus. Instead, the two are sister groups, both born from some unknown common ancestor.

And that brings me back to Thalassocnus, the marine ground sloth. Maybe those different species occur in a fairly linear progression through time, one seemingly replacing the other, but that can never be known. Look, let's pretend that Dromaeosaurus, Velociraptor, Deinonychus, and Utahraptor are all called Velociraptor. For fun. V. albertensis, V. mongolensis, V. antirrhoppus, and V. ostrommaysi. Much as we'd like to, we can't say that one just evolved into the next, which evolved into the next, which evolved into the "pinnacle" of Velociraptor.

The best we can do is create a long tree, like this (I don't have a scanner at my side):

V. albertensis + (V. mongolensis + (V. antirrhoppus + V. ostrommaysi)

And each new step would require its own specific name. The most inclusive group might be Velociraptoria, the next step up might be Velociraptoridae, and the most inclusive group, comprising of V. antirrhoppus and V. ostrommaysi, might be the Velociraptorinae. But it's not a "line of descent," it's a branching bush with unknown common ancestors.

So that's what I don't understand about mammal taxonomy. And that's my rant for the day. :-)

Glyptodon crivapes

I was browsing through the Tetrapod Zoology archives when I rediscovered this brilliant photo of Glyptodon crivapes' skull. Glyptodonts are big armadillo-like xenarthans, although how closely related they are to modern armadillos is a bit hazy. Personally, I think the skull looks a bit more like the skull of, say, Megatherium (giant ground sloth) than a nine-banded armadillo, but of course I know very little about mammalian taxonomy.

Anyway, look at how beautiful that skull is! It looks like it died a few months ago, not millions of years ago! The preservation is astounding. I'd love to see a real "live" glypodont skeleton in a museum.

One more thing, dear readers. Given their bizarrre skulls (look at that jugal flange!), it's incredibly difficult for me to envision their muscular anatomy. Are there any papers, books, etc. which investigates xenarthan cranial musculature? In the meantime, check out this post at Tet Zoo for more information on the Cenozoic's take on ankylosaurs!

Indohyus: Awesome, but not a Whale

A note on this restoration: It is tentative. Until I get the damn Wacom tablet working again, you'll have to do with this hairless, B&W version of Indohyus. When my technology decides to work for me, I will replace this shoddy line drawing with a much improved, fuzzy, colorful animal. But I'm going into blogging withdrawl, having not done so intently since Christmas. I apologize to all six of my readers, because I've been promising a post about Indohyus for awhile.

For those paleo-bloggers among you who have been living under a rock for the past few weeks, there's a new ancestral whale in town. You didn't think it could get much more basal than Pakicetus, did you? Honestly, it can't, but Indohyus (Thewissen, et al. 2007)* comes darn close. New fossil material from the quite old genus shows that the Raoellidae, the family to which Indohyus belongs, shares quite a few features in common with early and modern whales. As I will illucidate further on, this does not mean that Indohyus is actually a basal whale.

About the size of a racoon, Indohyus a lithe, gracile little animal. At first (or second, or third) glance, this raoellid looks absolutely nothing like the superficially crocodilian Ambulocetus or foxy Pakicetus. Truthfully, Indohyus has no modern analogue, which may be why the media has been so hard-pressed to compare it some extant animal for their readers. In a now infamous flub, Seth Borenstein of the Associated Press, in a stumbling attempt to explain what Indohyus looked like for those of us who couldn't just look at the accompanying image (by the excellent Carl Buell), suggested that it resembled "a long-tailed deer without antlers or an overgrown long-legged rat." Deer don't look much like rats, Seth.

Comical analogous snafus aside, the point is that Indohyus is suspiciously devoid, on the surface, of any features that would identify it as a potential cetacean ancestor. Probing deeper, however, Thewissen, et al. have found several subtle clues to its common ancestry.

First and foremost, little Indohyus' middle ear features an extra "wall" of bone called the involucrum, which helps whales hear underwater. Mammals are able to hear underwater thanks only to the vibrations of the skull bones (so our underwater hearing is severely dampened). Sound waves hit the whale's involucrum, and the waves are amplified and directed toward the ear canal. Modern cetaceans have managed to evolve melons and echolocation which further amplify sound waves via both the melon and the lower jaw. The involucrum is merely the first step in what will eventually be a very complicated method of hearing underwater. In a way, it's not a whole lot better than the basal mammalian condition--the involucrum merely amplifies the skull vibrations brought on by underwater sound waves.

In fact, Thewissen et al. aren't even that excited about the involucrum. Rather, they focus on the fact that Indohyus was an aquatic wader or bottom-walker. The bugger displays osteosclerosis, which is a fancy word for "thick limb bones." Osteosclerosis is not a unique feature to whales. Virtually all aquatic vertebrates evolved it due to the rigors of a marine lifestyle. In terms of bone thickness, Indohyus falls somewhere between hippos (which are bottom-walkers) and Ambulocetus (a swimmer). Because Indohyus' gracile limbs were not well adapted to a swimming lifestyle, Thewissen et al. interpret Indohyus as a bottom-walker.

The chemistry of Indohyus' enamel was studied to try and figure out its diet, but the authors could only conclude that Indohyus' diet must have been quite different from Pakicetus and Ambulocetus. Omnivory or herbivory are suggested, although the authors lean away from aquatic foraging toward the end. Thewissen, et al. suggest that Indohyus lived a lifestyle comparable to the muskrat, who spends most of its day in the water, coming ashore to sleep and eat its leafy greens (although the muskrats here in Alaska are strictly carnivorous).

Now, on to the phylogeny. I'm stealing Carl Zimmer's excellent cladogram, which shows exactly how Indohyus relates to whales (as far as we know) and why it's NOT on the direct cetacean lineage. Sorry, Carl!

What Indohyus and its raoellid cousins represent, kids, is an outgroup. Indohyus : Cetacea :: Silesaurus : Dinosauria. Need another example? Okay, I can do this all day. Herrerasaurus : Saurischia :: Indohyus : Cetacea. Want a non-dinosaurian comparison? Well, too bad. At any rate, Indohyus is too specialized to be a proper ancestral whale. Also, raoellids were around at the same time as the first cetaceans. Indohyus was not some ancestral whale. Rather, the Raoellidea shares a common ancestor with the Cetacea, and that common ancestor had an involucrum and was probably semi-aquatic by way of osteosclerosis.

The "Y" node on Carl's cladogram with Indohyus on one end and the branching Cetacea on the other is an unnamed clade. I humbly submit "Cetaceaformes" or some similar term.

The realization of Indohyus' cetacean features further calls into question the genetic evidence favoring a hippo/whale grouping. It may be that hippos still are related to whales, but in a (hippos + Cetaceaformes) way. We really need some basal hippo fossils. It could be that an aquatic nature was basal among some hypothetical hippo/Cetaceaformes ancestor (I don't even want to think about what that clade would be called), and that the invoculum is a novelty for Cetaceaformes. Now I'm rambling.

But that's Indohyus. An awesome little animal, to be sure, but I don't want people getting the wrong idea about its relationship to whales. It is NOT a whale, but an outgroup to whales.

* Was anyone else annoyed that Thewissen, et al. did not reference the species anywhere in the paper? According to the always-reliable Wikipedia, Indohyus consists of two species, I. major and I. indirae. It's possible, if not probable, that the two species are synonyms of each other, but I would have liked some explaination of Indohyus' taxonomy in the paper. It wasn't clear whether Thewissen, et al. was describing a brand-new creature (as the news items would lead one to believe), new material from an existing taxon, or a new taxon from somebody's basement.

Pakicetus' Vegetarian Ancestry

Well, it turns out that pakicetids, known to you and I as the earliest known whale ancestors, had vegetarian ancestors of their own. This week's Nature has an article about Indohyus, a new deer-like terrestrial whale from India. The creature can be identified as a whale based on the structure of its middle ear. Surprisingly, it is herbivorous. Pakicetus, Ambulocetus, and all the other whales are carnivorous, so one would expect that whales came from a carnivorous ancestor. Such, it seems, is not the case.

More surprisingly, this is the first time in MY experience that an herbivorous animal has gone back to being carnivorous. The evolution of herbivory requires several changes in the body, including the gut, dentition, behavior, and metabolism. To go BACK to carnivory seems like a waste of all that effort. Indohyus supposedly fed on water-based vegetation. Why did whales so quickly turn to meat? Are there not enough plants underwater?

And then, of course, there's some exciting marsupial news: It turns out that kangaroos didn't always hop. Early versions galloped. Nambaroo gillespieae is a virtually complete early kangaroo that ran on all fours, may have had an arboreal streak, and had "fangs." Despite the fearsome dentition, Nambaroo was still a vegetarian at heart.

Indohyus is in this week's Nature, and Nambaroo is in the Journal of Paleontology. If any of you out there in Readerland have access to these journals, I would greatly appreciate the PDF.