The Pacific Mastodon

Mastodons have been in the news this week, thanks to a new paper by Alton Dooley et al. that makes a case for distinguishing a new Pleistocene species, Mammut pacificus, from the familiar Mammut americanum. The paper is freely available, so give it a look if you're interested in the technical side of fossil proboscideans (mammoths, mastodons, elephants, and friends). If you're interested but not quite up to speed on the details, there's also an in-depth interview with the lead author.

The skull of the type specimen of M. pacificus, Figure 1 in Dooley et al. 2019. From the caption: "Cranium in: (A) dorsal, (B) ventral, (C) left lateral, (D) right lateral, (E) posterior, (F) distal end of left tusk (I1), lateral, and (G) right tusk (I1), lateral view. Teeth include left and right M2–M3. (A–E) are images of a resin cast of the holotype cranium on exhibit at the Western Science Center. All images are orthographic views of photogrammetric models. Scale = 10 cm."

Although mastodons and mammoths are among the most familiar extinct animals, our understanding of their species is still fairly hazy. For those of you who are unfamiliar with the issue, the short answer is that paleontologists historically loved to name proboscidean species. The long answer is too long for a post, but there's a little 1,800-page two-volume monograph by Henry Fairfield Osborn that may provide some illumination. You can read and download the volumes from the Biodiversity Heritage Library (both) and Internet Archive (1, 2). Two caveats for the reader: both volumes are large files (88 and 103 mb, respectively), and Osborn had some* ideas about taxonomy and evolution that have not quite stood the test of time. The 1920s and 1930s were a splitting time, and Osborn could split with the best of them. Over the following decades researchers have gradually settled on a shortlist of a few species and genera, but there are still a lot of questions, and given the results from the original binge of species-naming there hasn't been much of an appetite for creating more.

*By "some" I mean "all of them".

Western North America appears to have been mammoth country; mastodons are not particularly abundant, with many finds only coming in the past couple of decades. Dooley et al.'s project started with a mastodon found at Diamond Valley Lake West Dam near Hemet, California. This mastodon, now on display at the Western Science Center and known familiarly as "Max", has small third molars for its size. (As the interview relates, this fact only became apparent when Dooley was working on exhibit text, and then the project grew from there.) The authors began making comparisons to other mastodon specimens. What they concluded, after several years of work, was that mastodons from California and southern Idaho shared a small suite of characteristics unlike the more familiar mastodons of eastern North America. Aside from the small third molars, these include six fused sacral vertebrae (usually five in M. americanum), femora with relatively thicker shafts, no mandibular tusks (M. americanum sometimes has tusks in the lower jaw), and males with relatively thinner tusks at the base.

Figure 33 from Dooley et al. 2019. The caption there reads "Red circles mark all known M. pacificus localities, while blue circles mark the M. americanum localities that produced teeth used in this study and represented in Table S2. Note that while there are many additional M. americanum localities that were not included in this study and that are not indicated on the map, there are no known M. americanum localities in California. The M. americanum locality in Oregon is a non-diagnostic specimen that was included as M. americanum in this study, but that could represent M. pacificus."

Dooley et al. found not only that all of the mastodons that could be studied from California shared these characteristics, but that the differences extended well into the Pleistocene, into the Irvingtonian land mammal stage. The mechanism for species separation would most likely be ecological: mastodon fossils are particularly rare in the mountain and desert country that intervenes between California and the rest of North America, and there may simply have not been enough suitable suitable mastodon environments in that region to keep the California population connected to the eastern population. In recognition of this distinct population, Dooley et al. coined the name M. pacificus, the Pacific mastodon. Moral of the story? Keep looking at your fossils, even if it's something seemingly well-known; you never know when something unusual might turn up.

References

Dooley, A. C., E. Scott, J. Green, K. B. Springer, B. S. Dooley, and G. J. Smith. 2019. Mammut pacificus sp. nov., a newly recognized species of mastodon from the Pleistocene of western North America. PeerJ 7:e6614. doi:10.7717/peerj.6614.

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.

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.

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...

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.