Sunday, March 24, 2019

Your Friends The Titanosaurs, part 10: Diamantinasaurus, Dongyangosaurus, and Dreadnoughtus

For this week's post, we have two of the best represented titanosaur species, plus Dongyangosaurus sinensis, which isn't quite so well-represented but has some unusual features going for it. Our other guests are Diamantinasaurus matildae, one of a small number of Australian titanosaurs and titanosaur-like sauropods which lived close in time to the Early–Late Cretaceous boundary, and Dreadnoughtus schrani, controversial contender for the heavyweight crown.

Sunday, March 17, 2019

The "Proctor Lake hypsilophodont": Convolosaurus marri

This week, the long-rumored "Proctor Lake hypsilophodont" was published as Convolosaurus marri (Andrzejewski et al. 2019). I could hardly turn down profiling a "hypsilophodont" of such long-standing mystery, so let's get started.

Sunday, March 10, 2019

Titanosaur osteoderms: introduction and history of study

One of the most notable aspects of titanosaurs is the presence of bony armor in at least some species. It's where we get Lithostrotia ("inlaid with stones"), after all. Armored titanosaurs have been hinted at since the 1890s and accepted since the 1980s, but despite forty years of publications and many finds across the world, there are still many things we do not know about titanosaur armor. Even the position on the body is mostly a matter of inference.

I'd had the idea of looking at titanosaur osteoderms and ossicles (the technical terms) in more detail since poster Ornithopsis commented on them following the very first "Your Friends The Titanosaurs" post. Originally I thought I could do it in one post, but after fourteen pages of notes and forty-plus references, this seemed perhaps overly ambitious. Therefore, this post will focus on the history of study (and a healthy bibliography, for those of you collecting papers), and a couple of future posts will cover more detailed descriptions, location on the body, distribution in time, space, and the titanosaur family tree, and functions. If you want spoilers...

Sunday, March 3, 2019

Tiny frogs of the Chinle

I am embarrassed to realize that after five full years of doing this, with a header mentioning "National Park Service paleontology" and "the Mesozoic", I had not done anything with the Chinle Formation or Petrified Forest National Park. This week offers a fine opportunity to correct this oversight, with the publication of the first frog fossils from the Chinle Formation.

The Chinle Formation is a terrestrial unit, with its colorful rocks deposited in various floodplain, river, lake, and other settings during the Late Triassic. The mosaic of settings led to a variety of rock types, from conglomerates to mudstones, with a healthy supply of volcanic ash from eruption centers to the west. Because we're talking about the Late Triassic, before the end-Triassic extinctions gave dinosaurs the opportunity to fill most of the empty terrestrial niches, the Chinle is full of fossils from all sorts of unusual and obscure animals. (And also petrified wood and freshwater mussels. Lots and lots of petrified wood and mussels!) It and its correlatives are usually good for at least one surprise every few years. Lurking among all of the phytosaurs and aetosaurs and so forth were representatives of lineages that would blossom later. Some of them we know from bones (dinosaurs, early croc relatives, pterosaurs, etc.), others can be suspected based on time and place but haven't yet been found. With this week's announcement, frogs move from the potential to the confirmed (Stocker et al. 2019).

The Chinle Formation is divisible into a number of members, depending on where you are. In ascending order, the units in the Petrified Forest NP area are the Mesa Redondo, Blue Mesa, Sonsela, Petrified Forest, and Owl Rock members. For the frogs, we're dealing with the Blue Mesa and Sonsela members, representing approximately 223 to 213 million years ago. (Following the stratigraphy of the Chinle Formation requires a certain amount of effort and dedication. Not only are different members found in different places, but usage has varied over time.) The three frog-producing localities are spread from the park to the famous Placerias Quarry near St. Johns, Arizona (Stocker et al. 2019).

An outcrop of the Sonsela Member, from the park website (NPS/Andrew V. Kearns).

Frogs, being small and delicate, are not exactly heavily represented in the fossil record. The frog lineage, Salientia, is known back to the Early Triassic, but if you were doing a Compact Thescelosaurus-like project on Mesozoic salientians you'd be finished pretty quickly. Salientia includes early stem-frogs plus the crown group Anura, which is the group consisting of all living frogs, their most recent common ancestor, and everything else that falls within that group. There are to date two Early Triassic stem-frogs, the well-known Triadobatrachus of Madagascar and the somewhat less famous Czatkobatrachus of Poland, with the next named salientian being a respectable frog's jump all the way into the Early Jurassic of Arizona, Prosalirus of the Kayenta Formation. The Kayenta has historically proven stubborn about this whole "absolute dating" thing, but an age in the first half of the Early Jurassic is the consensus, so that's a gap of something approaching 60 million years (approximately 250 to 190 million years). The unnamed Chinle salientian practically splits the difference (Stocker et al. 2019).

The Chinle salientian is known from five tiny fossils. Fortunately, four of them are ilia, which are very distinctive bones in frogs: there is a cup-like socket for the femur and a long thin blade directed anteriorly, lengthened by cartilage, part of the mechanism that gives frogs their spring. In the case of the Chinle form, they are also well into the microvert range: approximately 6.2 mm long, or about a quarter of an inch, the kind of stuff you find when you run sediment through screens as opposed to spotting while hiking around. The whole animal would have been less than about 25 mm long, or about an inch, in the realm of modern miniature frogs (Stocker et al. 2019). So, if you were thinking that ur-frogs might have been prehistoric giants, well... not so much in the Chinle. There *were* enormous amphibians in the Chinle, specifically the metoposaur Koskinonodon and its equally enormous taxonomic history, but it isn't closely related to anything living and it didn't look much like a frog.

The form of the hip joint and shaft suggest that the Chinle form was well on the way to modern frog jumping mechanics (Stocker et al. 2019). It may have been a true anuran, but only time and more fossils will tell. The presence of a stem-frog or true anuran in the Chinle also has some implications for frog distribution and paleoecology. The two Early Triassic stem-frogs lived outside of the tropics, whereas the Chinle form was more or less at the equator in western Pangea, showing that the frog lineage had spread across the supercontinent within the Triassic. Also of interest is the persistence of salientians from the Chinle into the Kayenta; over the length of the Chinle, the local climate became more and more arid (Stocker et al. 2019).

A Triassic frog clings to the snout of a phytosaur, used with permission by Andrey Atuchin (supplied by Adam Marsh/PEFO).

References

Stocker, M. R., S. J. Nesbitt, B. T. Kligman, D. J. Paluh, A. D. Marsh, D. C. Blackburn, and W. G. Parker. 2019. The earliest equatorial record of frogs from the Late Triassic of Arizona. Biology Letters 15:20180922. doi:10.1098/rsbl.2018.0922.

Sunday, February 24, 2019

The many moods of Rauffella

Earlier in February, I attended a Geological Society of Minnesota fossil lab hosted by Jeff Thole and Macalester College. At some point someone brought out a fossil that they weren't familiar with; it looked a lot like the final photo in this post, a light-colored object that resembled loops of cord. What this person had was one of the most characteristic but least scientifically appreciated fossils of the Decorah Shale: the trace fossil Rauffella (specifically R. palmipes, as we'll get to later).

Sunday, February 17, 2019

Your Friends The Titanosaurs, part 9: "Campylodon", Clasmodosaurus, Choconsaurus, and Daxiatitan

I've come up against some judgement calls at this point. There are two varieties for this exercise. The first includes a group of names based on poor material that have had little study but which are sometimes/often stuck into Titanosauria, usually from historical inertia or the "provenance argument". The provenance argument goes something like this: "the only described sauropods from this time and place are titanosaurs, ergo this is a titanosaur". The provenance argument is a seductive bit of taxonomic stereotyping, but I'm wary of it. For one thing, the implicit flip side is "therefore there cannot be anything else here than titanosaurs". When the provenance argument is correct, it's not because you put in any effort or had any kind of insight, and when it isn't, you look stupid for being lazy. When confronted with this dilemma and, say, a handful of teeth which have no particular characteristics that would put them in one group or another, I don't see what's wrong with "indeterminate sauropod". The other type of judgement call includes the frustrating taxa that sometimes end up in a clade and sometimes end up outside of it, generally because they are in that fuzzy transitional zone.

For this particular entry, I had cause to consider examples of both types: in the historical orphan bin are "Campylodon" ameghinoi (=Campylodoniscus) and Clasmodosaurus spatula, and in the fuzzy clade bin is Chubutisaurus insignis. I decided that the position of Chubutisaurus is too unstable to be confident it's a titanosaur at this point, and so left it out. We can always cover it later on, with some of its similarly ambivalent cronies. As of this writing, others in a similar equivocal position which I currently have outside of Titanosauria include Angolatitan adamastor, the two Huanghetitan species, Ligabuesaurus leanzai, Mongolosaurus haplodon, Phuwiangosaurus sirindhornae, Tastavinsaurus sanzi, Triunfosaurus leonardii, and Wintonotitan wattsi. Two things they almost all have in common: late Early Cretaceous age, and when they do show up in Titanosauria they're just barely over the line. The other two were more problematic because there's not much material and they've never attracted much interest. I decided to cover them here together as examples.

Sunday, February 10, 2019

Bajadasaurus pronuspinax

Like their cousins the rebbachisaurids, the dicraeosaurids have been on a roll over the past year. I featured the new oldest dicraeosaurid, Lingwulong shenqi, last year. This week saw the publication of another new dicraeosaurid, from a much more typical time and place (the Early Cretaceous of South America), but with a unique style all its own: Bajadasaurus pronuspinax.