Leviathan = Livyatan

I'm very late to the party here, but in case you weren't aware, that big raptorial whale that was discovered two months ago, originally named Leviathan, has been renamed Livyatan because the original name was found to be a junior synonym of Mammut, which sucks. To be fair, the name Livyatan conjured up much the same image: it's the Hebrew name applied to large marine monsters. Doesn't quite have the same ring to it, but hey.

Gigantic Diversification

2010 has been the Year of the Ceratopsian, certainly, but the lion's share of new named taxa have belonged to the Ceratopsidae--the large-bodied horned dinosaurs. Just for funsies, I'm going to list all the species known in 1995, then list all the species named since then. I think you'll see a massive increase in ceratopsid diversity. Ready? HERE WE GO! I've marked species of uncertain validity with a question-mark.

As of 1995:

Chasmosaurinae

Chasmosaurus belli
Chasmosaurus russelli
"Chasmosaurus mariscalensis"
Pentaceratops sternbergi
Anchiceratops ornatus
Arrhinoceratops brachyops
"Diceratops" hatcheri (?)
Torosaurus latus
Torosaurus utahensis
Triceratops horridus
Triceratops prorsus

Centrosaurinae

Centrosaurus apertus
Styracosaurus albertensis
"Styracosaurus" ovatus
Einiosaurus procurvicornis
Achelousaurus horneri
"Monoclonius flexus"
"Monoclonius nasicornis"
Pachyrhinosaurus canadensis
Avaceratops lammersi
"Brachyceratops montanensis"

Since 1995, as of this date:

Chasmosaurinae
Agujaceratops mariscalensis
Chasmosaurus irvinensis
Coahuilaceratops magnacuerna
Eotriceratops xerinsularis
Medusaceratops lokki
Mojoceratops perifania
Ojoceratops fowleri
Tatankaceratops sacrisonorum

Centrosaurinae
Pachyrhinosaurus lakusai
Albertaceratops nesmoi
Centrosaurus brinkmani
Diabloceratops eatoni
Rubeosaurus ovatus
Sinoceratops zhuchengensis

Am I forgetting anyone? Even though we've actually lost a few species, and sometimes even genera, since 1995 (Monoclonius, Brachyceratops), and a few have been renamed (Agujaceratops, Rubeosaurus), the Ceratopsidae has, overall, exploded in diversity. Currently, there are 24 more-or-less valid genera (I'm lookin' at you, Nedoceratops) containing a whopping 30 species, unless I'm missing anybody, in which case there are actually MORE. And we're not done yet. There are several yet-to-be-published taxa that I'm personally aware of, and I'm sure my more informed colleagues know of even more. And the year is only half over. A lot more publishing can happen between now and 2011. Let's hope that 2010 continues to deliver more delicious horned dinosaur goodness!

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.

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!"

How the Turtle Got Its Shell: Addendum

eBlogger does not like it when you try to screw with a long, image-heavy post. When you try to edit said post by, say, adding a new picture, the rest of the post goes to hell. Anyway, here is another fantastic picture by Matt Celeskey of Chinlechelys, showing the known shell fragments and what they represent. There's a little more to the skeleton, including some vertebrae, a bit of plastron, and cervical spines (like Proganochelys has). But the carpacial elements are the most telling bits.

How the Turtle Got its Shell

Testudo hermanni in cross-section.

Turtles are weird. These are sauropsids with an armored shell enclosing virtually their entire body. The two halves of this shell—carpace on top, plastron on bottom—seem to have separate origins, and come together at a “seam” formed by osteoderms. Their elbows are backwards. Their vertebrae, having no mobility, have become elongated tubes connected to the underside of the carpace. The shoulder girdle sits vertically in the shell, seemingly within the ribcage, and in many turtles, the serpentine neck is able to retract into the shell. Other turtles are able to move their limbs into the shell as well. Skull shape varies wildly among turtles, but in all cases is an anapsid structure: like many basal sauropsids, the only fenestrae in the skull are for the eyes and the nose. While most modern turtles are semi-aquatic, at least one large group (the tortoises) are fully terrestrial and seem to like hot, arid climates. Other turtles are almost entirely aquatic—the sea turtles—who only come onto the beaches (with great difficulty) to lay their eggs. The retention of egg-laying is strange given that so many other sauropsid groups who returned to the sea gave birth to live young, including ichthyosaurs, nothosaurs, plesiosaurs, and mosasaurs. Turtles have outlasted all of those groups, having emerged during the Late Triassic and surviving well into the present day, yet the sea turtles never took a lesson from their live-bearing cousins. For over one hundred years, Baur’s Proganochelys (= Triassochelys) was the most basal turtle known. This Late Triassic turtle, about a meter long, was adapted to terrestrial life with a heavily armored shell, spike-covered tail, cranial spines, and smaller spikes coming off the neck. Unfortunately, Proganochelys told paleontologists little about the phylogenetic origins of Testudines.

The problem with turtles is that they have a ridiculously derived body plan, totally different from any other animal, either living or extinct. Plenty of other amniotes have armor, but none have a body enclosed within a bony, armored shell. Glyptodonts came close to turtles, as their dorsal vertebrae and pelves fused to the underside of their massive patchwork shell. Placodonts, early shellfish-eating sauropterygians, came very close to the turtles in terms of shell design. Within Placodontia, two groups evolved different shell types. The smaller cyamodontoids developed a two-part carpace, with a broad shell covering the shoulders and most of the body, and a second smaller carpace covering the pelvis. Cymodontoids did not have plastrons. Another placodont, however, Henodus, had an incredibly broad carpace and plastron. Both halves were wider than the animal’s skeleton, and would have stuck out laterally from the body. Unlike cymnodontoids, however, the carpace of Henodus was a single broad structure that ran the length of the animal from the shoulders to the base of the tail.

A trio of placodonts by Darren Naish. From left to right: Placodus, Placochelys, and Henodus.

Placodonts, however, developed their shells in a different way than turtles. While turtles build their shells from a combination of expanded ribs and a small number of interlocking armor plates, placodonts built their shells with hundreds of very tiny osteoderms, much like the mammalian glyptodonts. Still more problematic is the fact that placodonts are unquestionably diapsids—their skulls have eye, nose, ear, and temporal fenestrae. Remember that turtles do not have any holes in their skulls behind the eye.

So what are turtles? These shelled creatures have a wonderful fossil record post-origin point, much like pterosaurs and bats. However, their bodies are now so ridiculously derived that figuring out their relationships based purely on morphological comparison to other living groups of animals is an exercise in futility. Let’s use a more familiar example. Take whales, for instance. Let’s say that you were given the task of figuring out which living group of animals is most closely allied to Balaenoptera musculus (the blue whale) based solely on that whale. It would be easy to mark the whale as a placental mammal—it gives birth to live, developed young and nurses its babies. It’s endothermic, grows quickly, and moves its spine up and down instead of side to side. But after that, where would you position Balaenoptera among terrestrial mammals? Is it closer to lions than walruses, or perhaps cows and antelope? Hippos? Horses?

The skeleton of Balaenoptera musculus. Have fun with that.

The fact is, you need more basal forms. As any group of animals evolves, many of the features it once shared in common with other animals disappear. Balaenoptera lost its hind legs and teeth, retraced its nose to the top of its head, repositioned its eyes close to the jaw articulation, grew baleen along the gumline, and turned its forelimbs into powerful flippers. However, when you look at other whales, you quickly see that Balaenoptera (and other baleen whales) form their own cohesive group to the exclusion of others. Physeter, the sperm whale, does have teeth, as do belugas, narwhales, dolphins and porpoises. So those groups are more closely related to each other than to baleen whales. The more features a group has in common, the more closely related they are.

And now that we have a good understanding of whale evolution thanks to wonderful fossil evidence, we can see how the cetacean body plan evolved, from terrestrial carnivore (Pakicetus) to semi-aquatic opportunist (Ambulocetus) to four-flippered doggie-paddler (Rodhocetus) to vestigal-hindlimbed giant (Basilosaurus) to more-or-less modern toothed whale (Squalodon). We can even see now the transformation between toothed, predatory whale (Janjucetus) to a whale with both teeth and baleen (Aetiocetus) to modern baleen whales.

So what does that have to do with turtles? Well, modern turtles come in a wide variety of forms, from freshwater turtles to sea turtles to tortoises and enormous snapping turtles. Turtles are a monophyletic group—that is, all turtles share certain characters that indicate they all came from a single common ancestor. So even though snapping turtles look a lot different from matamatas, you can bet they both belong to Testudines. However, up until just this year, the earliest fossil turtle was Proganochelys, and it was already a more-or-less “modern” turtle. It already had the particular shell plate pattern you see in modern turtles and it already had a fully enclosed body with a large carpace and plastron fused in the middle. In some respects, especially its numerous spikes, Proganochelys did differ from modern turtles, but those features were probably unique to Proganochelys itself (or its family). So Proganochelys tells us very little about where turtles come from.

Gamera...I mean, Proganochelys!

That didn’t stop some authors, however, from trying to piece together turtle relationships based on Proganochelys. Baur himself compared sea turtles to ancient plesiosaurs, a view criticized by Williston as being based entirely on similarities in how the two groups swam. He wrote that neither could possibly be derived from the other, as they are both woefully disparate. Cope believed that turtles came from a group of extremely basal sauropsids (like Diadectes) which had small armor scutes along the back. The beast is now seen as an unspecialized “stem reptile” helping to span the gap between non-amniote tetrapods and sauropsids. In 1902, Jaekel suggested that turtles were related to the placodonts based on the fact that both groups have shells. The first major discussion on turtle origins, however, was written by Gregory in 1946, in which he turns his attention to the Permian pareiasaurs, a group of large, heavy-set anapsid sauropsids. He writes:

“When I turned my attention to the pareiasaurs, I was surprised to find that apart from their gigantic size they seemed on the whole to afford an excellent starting point for the chelonian line, both in their general construction and in many features of the skull, vertebrae, ribs, girdles, limb bones, hands, and feet.”

In addition to supplying a relationship between pareiasaurs and chelonians, Gregory dismisses captorhinid, seymouriamorphs, and placodonts as turtle relatives. He proposes that turtles started out as a “pug-like” branch of the pareiasaurs, perhaps related to Elginia. In 1947, Olson proposed a reorganization of Reptilia into two branches, the Parareptilia (basically anapsids) and Eureptilia (diapsids), and that turtles rested on the former branch. In 1969, Carroll allied turtles with the captorhinids. 1997 saw two opposing papers in the same issue of Zoological Journal of the Linnean Society. In one, Lee proposed that turtles were pareiasaurs. DeBraga & Rieppel, however, used a broad array of amniote taxa in a large cladistic analysis to find that turtles formed a sister group to Sauropterygia, just like Jaekel had suggested 95 years previously. This proved to be a very contentious idea, but support mounted. In 2005, Hill published a large phylogenetic analysis that included representatives from a very large number of amniotes, and found turtles to be a sister group to the Lepidosauria. Hill used integumentary characters in addition to skeletal elements, and he emphasized their importance.

But fossil evidence is still scant. Luckily, 2008 saw the discovery of who wonderfully important new turtles, one complete, one not so much, and new embryological evidence for how the shell forms in modern forms.

First came the discovery of Chinlechelys tenertesta, a very fragmentary turtle which, happily, preserved a cross-section of the shell. The shell is not fully fused to the underlying ribs, which are, themselves, still individualized. The shell is extremely thin (discounting the ribs), only 1 to 3 millimeters thick. Chilechelys shows that the overlying osteoderms on the back contributed to the shell separately from the expansion of the ribs. In modern turtle embryology studies, the ribs expand at the same time as the armor fuses to them. So it would appear that, according to Chinlechelys, the ribs expanded under the skin of an armored animal, and the armor later integrated into the bony ribs. The authors suggest a progression like this:

Chinlechelys suggests that armor was present in the terrapin ancestor.

Related to the question of rib expansion is scapular position. The turtle shoulderblade seems to sit within the ribcage, while in all other vertebrates, it is positioned dorsal to the thoracic ribs. At this year’s SVP, Lyson & Joyce demonstrated that once your remove the bony shell elements, which are not homologous to other tetrapods, and reduce the expansion of the ribs, it turns out that the scapular blades, while still vertically oriented, frame the neck and sit anterior to the thoracic ribs. Lyson & Joyce implied a relationship with basal amniotes due to the position and orientation of the scapulae. What’s important, though, is that the authors showed another important feature of how the turtle shell develops—the ribs expand, and those expansions are filled in and covered by unique ossifications. As Chinlechelys has shown, those ossifications are then fused with overlying dermal armor.

But wait—there’s more! Just a few weeks ago, word came down the pipe of a turtle older even than Proganochelys, a turtle known from a virtually complete, well-preserved skeleton! This turtle, Odontochelys semitestacea, an aquatic form that seemingly lacks a carpace. You read that right—Odontochelys doesn’t seem to have a carpace. Matt Celeskey, from the Hairy Museum of Natural History, was kind enough to let me use two of his beautiful illustrations of the critter’s skeleton. The first is a dorsal (top) view:

Dorsal view of Odontochelys

Let’s talk about that. Notice how the ribs are expanded, but not to the extend you see in modern turtles. Additionally, there are no costal plates or shields. The animal does preserve a row of small neural plates running down the back, a feature present in modern turtles. The skull is long but still anapsid, with a long neck and tail. The plastron is complete and remarkably large, with flared corners which seem to overgrow the width of the ribcage. But what about that plastron?

Ventral view of Odontochelys

There’s your fully-developed plastron! Aside from its flared, “combed” edges, the plastron is modern and surprisingly large. A ventral view of the skull gives a look at the dentition, with teeth in both the upper and lower jaws, premaxilla and maxilla. The authors of Odontochelys propose that this new turtle simply doesn’t have a carpace, that the plastron developed first, and that because Odontochelys was probably a semi-aquatic turtle, that turtles evolved in a marine environment. Furthermore, their phylogenetic analysis places Testudines firmly within Diapsida, and again supports a sister group relationship between turtles and Sauropterygia. In the “News & Views” section of Odontochelys’ issue of Nature, Reisz & Head suggest that Odontochelys lost its carpace due to its semi-aquatic lifestyle. They point to the fact that it has neural plates, so the carpace isn’t entirely absent. Indeed, what if Odontochelys had osteoderms that were not preserved? Actually, the modern soft-shelled turtle has both reduced its bony carpace and lost its dermal armor entirely. Leatherback sea turtles reduce the costal plates and shields, and ancient Archelon reduced its carpace to almost nothing but neural plates and ribs!

Notice also the flared "combed" plastron edges in the leatherback (middle) and Archelon (bottom). Odontochelys gives us competing hypotheses about the turtle shell. Either Odontochelys descended from a shelled ancestor, or it simply did not have a carpace, but its neural plates form the first part of the structure. Chinlechelys further muddies the picture, as it suggests that the ancestral turtle did have thin dermal armor--which is either absent or unpreserved in Odontochelys. What is exciting, though, is that Odontochelys, as the most basal turtle known, gives paleontologists a better opportunity to figure out the taxonomic position of Testudines among other sauropsids. Just like with whales, the most basal your taxon, the better your chances of figuring out who it is related to. A string of evidence from the last decade suggests that turtles are, in fact, diapsid reptiles related to Sauropterygia. So turtles really are close to placodonts, despite their differences in shell construction! Interestingly, if turtles are removed from Anapsida (or "Parareptilia"), that would mean that the only living sauropsids are diapsids! If turtles are pareiasaurs, they would be the sole representatives of a very ancient lineage, but it's looking worse and worse for the parareptiles.

We still don't have a Testudine Archaeopteryx, but it's been a good year for basal turtles. I can only hope that a shell-backed urvogal remains buried somewhere, awaiting discovery, and I look forward to reading about it! A special thanks goes out to, who provided those wonderful skeletal drawings. I tell ya--you kids and your digital media. You're zooming past dinosaurs like me who use traditional tools. This old dog's got to learn a new trick or two!

The most bizarre restoration of Odontochelys I've come across, from Wikipedia.

EDIT: I do plan on adding a reference list, but perhaps tomorrow. This post already took three hours to write (most of that is research). I'm also going to add another awesome drawing from Matt, so keep an eye out!

Wherefore art thou, Pelycosauria?

I love pelycosaurs. They're probably the most famous non-dinosaurian prehistoric critters. They're not even reptiles, though--they're basal synapsids. They're more closely related to you and I than to lizards and crocodiles. Pelycosauria used to be a coherent group featuring a variety of Permian forms that all looked fairly similar. So similar, in fact, that in at least one case, one distinct genus was considered the female of another genus! Pelycosaurs were largish, lizard-like critters with large skulls and mean, bladed teeth. There was at least one herbivorous pelycosaur, Edaphosaurus, a small-headed form that lived alongside uber-carnivore Dimetrodon.


Some pelycosaurs developed sails on their backs. In Dimetrodon, the sail was tall and roughly symmetrical from front to back, as its tallest point was in the middle. In Edaphosaurus, though, the sail was much shorter on the neck, and the neural spines were swept back, giving the sail a ramp-like profile. Additionally, the neural spines had cross-pieces of bone along its length. The sail would have looked spikey! But most pelyosaurs were sail-less, including basal form Ophiacodon.



Lately I've been hearing that pelycosaurs are not a coherent group. That is, Dimetrodon is closer to stem mammals than Edaphosaurus is. Instead of forming their own family, it would seem that pelycosaurs form a stepwise progression toward stem-mammals, just like rhamphorhynchoids form a stepwise progression toward pterodactyloids. I'm perfectly willing to accept that, but I'd like to know where this idea came from. Additionally, are there any taxa within the traditional "Pelycosauria" that DO form monophyletic groups? There are such minor groupings among rhamphorhynchoids, after all. So while "Rhamphorhynchoidea" is not a monophyletic group, Anurognathidae IS, and so is Rhamphorhychidae (Rhamphorhynchus, Scaphognathus, a few others). So are there any pelycosaurs that form a monophyletic relationship?

Special Shout-Out to Nick Gardner

I have to thank Nick Gardner, who as you may remember, hates theropods. He (and Will Baird, actually) has been hounding me about my mispeakings regarding Odontochelys and turtle origins. I have never understood the morphological basis for a diapsid origin for turtles, and grew up with the dogma that Testudines are the only living group of anapsids whose closest (extinct) relatives were the pareiasaurs like Scutosaurus.

Well, Nick pointed me in the direction of an important paper by Reippel & Reisz which gives said morphological basis. I would be remiss were I not to pass this information along:

Rieppel & Reisz (1999). The origin and early evolution of turtles. Annu. Rev. Ecol. Syst. 30: 1-22.

And I found an alternate, albeit much older opinion by William Gregory comparing turtles to pareiasaurs and placodonts. Although probably outdated, this paper contains absolutely wonderful illustrations. You don't see this anymore.

Gregory, W. K. (1930). Pareiasaurs versus placodonts as near ancestors of turtles. Bull. Am. Mus. Nat. Hist. 86: 275-326.

I will have more to say on this topic after fully digesting both papers. Thank again, Nick!

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. :-)

Linnaeus' Legacy Part the Eighth!

Good evening, ladies and gentlemen! Welcome, welcome, to the latest edition of Linnaeus' Legacy, the premier taxonomic blog carnival of the Interwebs. Christopher Taylor, that handsome devil, was gracious enough to permit me to host the carnival at my humble blog. A round of applause, ladies and germs! Now, step right up and get your tickets, and behold the wonder that is...taxonomy.

Behold, behold! In this tent, marked "Biological Ramblings," you will find to your horror questions regarding Toucan Systematics. Those sly birds do get around!

Step this way, please, no crowding. There's room for everyone! Gaze, and be amazed at this unique Podblack, and its discussion on the natural historian in Douglas Adams. He may have written a certain guide to the galaxy, but I'm betting the man never hitchiked to the Galapagos! Oh, I amuse even myself, folks!

You may have thought it merely mythical, but ladies and gentlemen, I assure you, The Barcoded Ant really does exist! Lest you forget, horrible mutated ants once reigned supreme across the New Mexican desert, and barcoded insects would only terrorize our population further!

Watch in wonder as Chris Taylor himself takes aim at problematic taxa, including microbes with names like Rhenocystis latipendunclata and poor taxonomic practices!

Darren Naish (and many others) regales us with tales from the front lines of Aetogate, and Chris Taylor muses on who actually owns the data in question. Fascinating stuff, and very important questions are raised! Read on, and be enlightened!

Look--up in the sky! It's a bird, it's a plane, it's...a flying trilobite? Alright, who let them out of their cages? I was Glendon, wasn't it?

The International Institute for Species Exploration has knocked out a list of the Top Ten New Species of 2007. Although, between you and me, folks, I think there are probably plenty of fossil animals a lot cooler than Gryptosaurus up for grabs in 2007. Maybe that's just my ornithopod bias talking, but hey!

And before you go, folks, gaze--if you dare--into this aquarium of Mesozoic marine monsters! They will terrify and potentially eat children--which is why you had to sign that release before coming in--so keep back, and for God's sake, don't tap the glass!

Thank you, thank you, ladies and germs, for attending this edition of Linnaeus' Legacy! Are there stragglers among you? That's just fine--we'll be updating this list until Sunday, so if you weren't able to make it out here today, just let me know, and I'll add you to the list! Until next time!

Draconian Systematics

Draconia was diagnosed in 1978 by Irwin & Jones as "medium to large diapsid reptiles with three functional pairs of limbs, one of which has been modified into bat-like wings." Mooney (1962) noted that dragons have a strong tendancy toward secondary flightlessness. Extent Draconia is divided into several families: Eudracocidae, Palusodraconidae, Argosidae, Taurodracocidae, Harenadracocidae, Rugodracocidae, Chasmodracocidae, Dracolympidae, Cryodracocidae, and Felimimidae. As Irwin (1996) pointed out, virtually every genus of extent dragon is placed in its own family aside from members of the well-supported Eudracocidae. "The various extent species of dragons," he wrote, "come in a wide variety of disparate forms and are difficult to resolve in relation to one another."

The difficulties in resolving living dragon relationships are further impeded by the terrible fossil record for dragons. The oldest dragon fossils date back to the Early Cretaceous in the form of a partial skull from India. Protodracos rex (Hutchings, 1984) was a small dragon, estimated to be less than eight feet long, and was carnivorous. The skull is well preserved but fragmentary--only the portion in front of the orbits is known. However, it shows characteristic dragon features like a large recessed naris for fleshy outer nostrils, a short, squared-off snout, and evidence of external ornamentation. Strangely, Protodracos has heterodont dentition, with large fang-like teeth at the front of the jaw, and smaller alligator-like teeth behind the premaxilla. One of the best-known fossil forms is Ambulodracos franco (Crank, 2000), an enormous Paleocene form from France. Known from a partial, but skull-less, skeleton, Ambulodracos was thirty feet long and heavyset. It has small wings, and demonstrates that dragons grew very large and flightless early in their evolution. Dragons do not turn up again until the Miocene, where a potential ancestor of Harenadracos (Wilder, 2004) is known, as well as a Central African ancestor of Megalodracos (Bosley, 1923). There are other bits and pieces known from the Oligocene and Pliocene, but they are not worth mentioning here.

Irwin (1996) suggested that, among extent Draconia, the Taurodracocidae was the basalmost member, but that "it is far removed from its ancestral stock." Irwin surmised that among all flightless or semi-flightless forms, Tauropesa had the most atrophied flight aparatus. He also noted its long, sinuous tail, which he considered a primitive feature. The Cryodracocidae was regaled to a basal, but unresolved, position given its enormosity, extreme wing atrophy, and long tail. Similarly, Argos argos was placed in its own family and placed closer to other dragons than to Tauropesa or Cryodracos. Wing retention, a shorter caudal series, dorsal spines, and extreme cranial ornamentation were all reasons for this more inclusive grouping.

He also united Eudracos, Megalodracos, Feradracos, and Sinospondylus in a clade called Neodraconia, of which Sinuospondylus was the basalmost member. The other three are united in a monophyletic Eudracocidae consisting of (Eudracos + (Megalodracos + Feradracos). A fossil form from Spain named Protopaluso (Arnold, 1988) links the Palusdraconidae to the Neodraconia. Irwin & Jones (1978) believed that Chasmodracos fell close to, but not within, the Eudracocidae. Irwin (1996) felt that Chasmodracos was probably closer to the Neodraconia than Palusodracos, but was unsure of which taxon should be considered the outgroup. Irwin named a new group, Europadracoidea, to include an unsresolved triochotomy of Palusodracos, Chasmodracos, and the Neodraconia.

Irwin also considered the possibility that Harenadracos and Rugodracos shared a common ancestry, based prominantly on the presence in both of a rostral bone and triangular skulls in dorsal view. He named this unranked clade Rostrodraconia. Irwin had trouble finding places for Felimimidae and Spinodracos. He wrote that Felimimidae was "not well understood" (from an anatomical perspective) to warrant placing it among other dragons yet. Irwin also did not include the creature in his phylogeny but remarked that it shows some superficial similarities to Argos, including a noticable underbite and pattern of spines. Upon the discovery of Dracospartus in 2003, Krause created the Dracolypmidae to include it and Spinodracos. Fletch (2004) suggested that Spinodracos was, in fact, the most derived member of the Eudracocidae. This hypothesis may still be correct, although further serious investigations into dragon systematics have not been undertaken.

A larger problem than how extent dragons relate to one another may be how Draconia as a whole relates to the rest of the Diapsida. Although unquestionably diapsids reptiles, the Draconia is notoriously difficult to pin down from there. Dragons are clearly not archosaurs, as they lack antorbital and mandibular fenestrae, but they may be archosauriformes. The presence of a third set of limbs complicates the issue, in that the pectoral girdle has been modified far beyond its ancestral condition in order to facilitate the wings. Still, Jennings (1989) noted similiarities between the pectoral girdle of Drepanosaurus and Megalodracos. Irwin & Jones supported this view, cautiously suggested a sister-group relationship between the Drepanosauridae and Draconia. This view is bolstered by the idea that dragons originated in an arboreal environment, and must have had adaptations early on to move them through such a home. Irwin (1996), however, suggested a more general relationship among the prolacertiformes, noting a tendancy among that group toward arboreality, yet not showing the specialities of drepanosaurs.

References:

Irwin, B. E. & Jones, D. (1978). Monophyly of the Draconia. Draconium 19(1): 25-59.

Irwin, B. (1996). A revised phylogeny of the extent Draconia. In A Brief History of Draconology (Suet & Svenson, eds.). Prince Rupert Press: 56-73.

Mooney, B. D. (1962). Does wing structure simplification lead to flightlessness? European Journal of Draconology 52(3): 368-381.

Hutchings, W. (1984). A Cretaceous origin for Draconia. Draconium 51(4): 487-495.

Crank, E. R. (2000). A large new fossil dragon from the Paleocene of France. European Journal of Draconology 101(4): 512-518.

Wilder, J. (2004). A potential dragon wing finger from the Gobi Desert. Natura Historia 411: 349-453.

Bosley, G. (1923). Megalodracos in Central Africa. European Journal of Draconology 13(2): 244-249.

Arnold, S. W. (1988). A Spanish relative of Palusodracos. Brevia (August): 45-51.

Krause, P. (2003). A heavily-armored Greek dragon. European Journal of Draconology 104(2): 308-329.

Fletch, F. R. (2004). A re-evaluation of the Eudracocidae. Draconium 45(4): 455-461.

Here (shortly), There (will) Be Dragons

I like dragons. I also like taxonomy. I also have a love/hate relationship with the McFarlane Dragon figure line. I've very selective about which ones I buy. Over the last two years or so, I've accrued some 30+ dragons, and as I understand it, a few more will be released this year. I have given them all scientific names. I have also, and I'm totally not kidding, built a phylogeny for these dragons. What's more, I have differentiated between dragons and wyverns. Whether dragons are wyvern-mimics or vice versa is not clear--the fossil record is ambiguous as to which group originated first. Dragons themselves have a poor fossil record, and their origins among Reptilia are unclear. Wyverns, however, are unquestionably archosaurs, although exactly which branch of the Archosauria they came from is similarly murky.

See? This is going to be so much fun. Expect the first entry in an overly-long series on dragon and wyvern taxonomy tonight! The maiden taxon: Eudracos magnificentissimus, otherwise known as the greater European dragon!

Everybody gets sunk into Edmontosaurus

A new paper out this week in Advances in Paleobiology suggests that the Hadrosaurinae, currently comprised of what's thought to be a paraphyletic group of largely crestless duckbill dinosaurs, may have to be renamed as "Edmontosaurinae." To make a long story short, the authors, Francis Boondoggle and Edward Hootenany, have sunk the majority of crestless forms into the already-sizeable Edmontosaurus genus. Such famous forms as Anatotitan, Maiasaura, and Shantungosaurus are now regarded as various species of the widespread North American genus.

"When you look at these creatures, and I mean really look at them, they all basically look the same," said Boondoggle at a press conference. "They all have toothless beaks with grinding teeth and long snouts. To us, that suggests they all fall under the same general category, or in this case, genus."

It's not all Edmontosaurus, though. Gryposaurus, Kritosaurus and Brachylophosaurus will be united under Kritosaurus, as it was the first genus named, in 1910. The three are united by general cranial characters including a shortened face (compared to Edmontosaurus) and a nasal arch. Unlike his colleague, Hootenany believes that, despite the obvious differences between their skulls, Kritosaurus may, too, fall under Edmontosaurus. "You don't call a Bichon Frise something different from an Australian Shepard. They're both Canis lupis familiaris, but most paleontologists would give them different genus names if they were dug out of the ground."

The authors both agree that Saurolophus should remain a separate genus based on its small nasal crest. "In fact," mentions Boondoggle, "Saurolophus may be the genus connecting edmontosaurines with lambeosaurines based on that little crest." But what of Prosaurolophus? "We think that that smaller-crested form is either a subadult or female Saurolophus. It just makes the most sense." Although Edmontosaurus is not the most inclusive genus in the Dinosauria (that would be Psittacosaurus, with about a dozen species), it is now the largest genus to include other taxa previously separated at the genus level. Instead of having four distinct taxa, we now have one big ol' taxon," Boondoggle said with pride.

The new paper recognizes three genera within the Edmontosaurinae: Edmontosaurus, Saurolophus, and Kritosaurus. The authors are now looking at the Lambeosaurinae, which includes the famous crested duckbills like Parasaurolophus and Corythosaurus. "Although our next study is only in the preliminary phase, the lambeosaurines all seem to be the same animal except for crest shape." The duo has plans to refurbish other branches of the Dinosauria as well. "Specifically ceratopsians and early ornithopods," says Hootenany. "Especially in regards to the latter, they all look the same!"

There's No Such Thing as a "Mammal-Like Reptile"

Remember how people used to throw the word "thecodont" around to describe some generalized archosaurian ancestor of dinosaurs? The problem with the term was that it had no formal definition--it was just a loose amalgamation of archosaur taxa which had vaguely dinosaurian bauplans. The term "Ornithosuchians" was used for a little while until it was decided that ornithosuchids actually fell to the crurotarsian side of the archosaur family tree. Eventually, thanks to cladistics and common freaking sense, the word "thecodont" went out of style and was replaced by the much more accurate and exclusive name, "ornithodiran." "Ornithodira" has a formal diagnosis. You can tell if something is an ornithodir or not, but that was not the case with "thecodont." This is one of the reasons I love cladistics--it has really put the taxonomy of extinct animals into the limelight. Sure, there are times when you have an animal like Silesaurus or Dracorex where cladistics actually impedes forward momentum, but overall it's quite the system.

Anyway, there's another outdated term floating around the discourse right now: "Mammal-like reptile." The term is used to loosely describe the critters that aren't snakes, lizards, crocodiles, or Petrolacosaurus that eventually gave rise to mammals. Dimetrodon grandis (above) is an oft-cited "mammal-like reptile" that was, in truth, neither a mammal nor a reptile.

"Reptilia" is a formal clade name that includes anapsids and diapsids. The group is defined by features of the skull, mainly, and exactly how many holes are in said skull, and how those holes are arranged. After a long evolutionary history, anapsids are only now represented by turtles, which are themselves a very ancient group of anapsids. Diapsids include lizards, snakes, tuataras, crocodiles, plesiosaurs, non-avian dinosaurs, pterosaurs, Arizonasaurus, birds, and a bunch of other groups. The point is that both the small anapsid group and the huge diapsid group are reptiles. "Mammal-like reptiles" have nothing to do with reptiles.

You see, Timmy, when tetrapods conquered the land and abandoned the water, they became amniotes. Amniotes then split into two main factions--the reptiles, which focused on adaptability, and the synapsids, which focused on...um...their teeth. Both groups layed leathery eggs, and both groups had scaly skin. So why wouldn't they both be called reptiles? Because true reptiles modified their skulls in one way, and synapsids modified their skulls in a different way. So we can diagnose a fossil animal as either synapsid or reptile by looking at its skull. It's an ancient division, and it's the whole reason those labels exist in the first place.

So when people say "mammal-like reptile," what they really mean is "lower-tier synapsid." I'm not an expert on synapsid taxonomy, but I know that "reptile" never shows up in the formal clade definitions. Think about it this way: the word "synapsid" is the equivalent of saying "reptile," but on the mammal side of things. Mammals are synapsids, just like archosaurs are reptiles. It's not hard.

Now stop saying "mammal-like reptile."