Titanosaur osteoderms: distribution

My plan had been to wrap up everything in this post. (Well, originally it was to do everything in one post. Ha.) However, again the contents were beginning to sprawl, and I felt guilty about the lengths of a couple of recent posts. After all, this isn't a peer-reviewed journal article, is it? There are different expectations for a blog post. Therefore, we'll just have distribution here, and one more post for the functions and conclusion. For the previous installments, go here (introduction) and here (characteristics).

Your Friends The Titanosaurs, part 11: Drusilasaura, Elaltitan, and Epachthosaurus

It's an all-Argentina installment this time around, including two large titanosaurs that seem to have gotten lost in the flood of titanosaur taxa (Drusilasaura deseadensis and Elaltitan lilloi) and a more modestly sized stalwart of the clade (Epachthosaurus sciuttoi).

The other side of the slab

Here we have the opposite side of a big ol' rock.

Titanosaur osteoderms: characteristics

Now that we've had a look at the background and history of study for titanosaur osteoderms, we can get into the nitty-gritty of their anatomy and where they were positioned on the body. It turns out there's still quite a lot we don't know about these structures.

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.