Dryosaurus elderae and the revenge of Nanosaurus agilis

It's been a busy few days over at The Compact Thescelosaurus, with new alvarezsaurs, nodosaurs, and dryosaurs. For this post, I'm going to focus on Carpenter and Galton (2018), which not only describes new species Dryosaurus elderae, but also is quite important for previous subject Nanosaurus agilis, and in general ticks off several of my boxes anyway ("hypsilophodonts", Morrison Formation, National Park Service areas, etc.).

Figure 2 from Carpenter and Galton (2018), showing the distribution of bipedal Morrison ornithischians. A keen-eyed observer who's familiar with the Morrison fauna might notice the absence of Drinker nisti and Othnielosaurus consors, and an abundance of Nanosaurus agilis...

Your Friends The Titanosaurs, part 3: Andesaurus, Antarctosaurus, and Argentinosaurus

Plenty of name recognition as far as titanosaurs go in this post of "Your Friends The Titanosaurs", which features Argentinosaurus huinculensis as leading contender for the dinosaur heavyweight crown, Antarctosaurus wichmannianus as one of the most historically important and troublesome titanosaurs, and the somewhat less well-known but supremely steady Andesaurus delgadoi. Three questionable species of Antarctosaurus have been held over for next time.

Lower Decorah trilobites

Following last week with Strophomena, and having figured out what seems to be a good method of photographing small specimens, I thought I'd try my hand at photo-documenting and identifying some trilobites. The sample set is mostly limited to the lower third to lower half of the Decorah Shale of St. Paul, and the most relevant publications for these trilobites are DeMott (1987) and Rice and Hedbloom (1987); Midwest Paleo also has a fine photo-atlas and list. I'm reasonably satisfied that my identifications at the family or subfamily level are accurate. Genus, I'm not so keen on. Species, I didn't even dare; I would be just parroting.

Big Ordovician brachiopods: Strophomena and friends

Starting out in the Ordovician rocks of Minnesota and surrounding states, you run into a few kinds of fossils right away. Depending on the rocks you're looking at, these might include nut-like small brachiopods, fragments of branching bryozoans, ring-like or gear-like crinoid columnals, snail shells, conical horn corals and so on. Among the most noticeable of the common fossils are larger D-shaped shells, up to a few cm across. We've encountered these large shells a few times before in the Platteville and Decorah; they are the shells of brachiopods in the Order Strophomenida, one of the most abundant and diverse groups of brachiopods. To avoid any confusion with Strophomenida, Strophomenoidea, Strophomenidae, etc., and because we're all friends here, we'll just call these large shells "strophs". There are also smaller members of Strophomenida, but we'll leave them be for the present.

A good-sized brachial valve of a stroph in exterior view, embedded in a slab. On the left (A) is the valve viewed from the top, while on the right (B) is the profile, showing how steeply convex it is. Because of mud, this was the only fossil I could see on the slab when I picked it up, but the high profile of the shell clued me in.

Lingwulong shenqi

After our latest titanosaur entry and a week off, here's... another sauropod. This one's different though: it comes from early on in the history of sauropods, and shines a light on the early diversification of the group. I introduce Lingwulong shenqi, debuting as the world's oldest known dicraeosaurid at somewhere around 175 million years old, and the first substantiated dicraeosaurid from Asia. These "oldests" and "firsts" are more important than they might sound.

Your Friends The Titanosaurs, part 2: Aeolosaurus

Aeolosaurus is one of those dinosaurs that is referenced fairly often in the literature, but has not left a strong impression with non-specialists, kind of like a character actor who's appeared in dozens of movies and TV shows over the years. The name rings a bell, but it's hard to recall much detail. It's easily the least familiar dinosaur to have both a namesake clade (Aeolosauridae/Aeolosaurinae/Aeolosaurini, depending on your taste, with the last being the most commonly used) and three valid species. It's not Aeolosaurus's fault, after all, unless you want to blame it for its habit of bequeathing only bits of tails to posterity. If you saw one amble by, you would definitely remember it.

However, there are good reasons why Aeolosaurus keeps on making cameo appearances: it and its closest relatives have very distinctive caudal vertebrae, and these vertebrae are fairly common in rocks covering the last 15+ million years of Argentina and southern Brazil. Because of my caution regarding titanosaur divisions, I haven't added a formal Aeolosaurini to my own files, but there certainly appears to be a group of South American titanosaurs with distinctive Aeolosaurus-like caudal vertebrae. For the purposes of this post I'm happy to refer to them as aeolosaurinids. I'm not going to get into detailed diagnoses, but I *will* briefly describe these caudals. If nothing else, they're the thing to remember about Aeolosaurus and aeolosaurinids:

In titanosauriformes, the neural arches of the caudal vertebrae have a tendency to creep up on the anterior part of the centra, rather than being centered. Aeolosaurinids take this even farther, with the articulating processes, the prezygapophyses and postzygapophyses, swept forward by the general excitement and the neural spines sometimes directed anteriorly as well. In Aeolosaurus this can be exaggerated to the point that the postzygapophyses are as far forward as the anterior margin of the centrum (Martinelli et al. 2011), which means that the prezygapophyses have to be stretched over nearly the entire preceding centrum to reach their articulations with the postzygapophyses (the exact positions of the processes vary along the tail; Santucci and de Arruda-Campos 2011). You might think of the prezygapophyses as something like stereotypical Frankenstein arms or sleepwalker arms, sticking way out in front of the rest of a vertebra. The figure below probably does a much better job of demonstrating:

Caudals from several aeolosaurinids (A–E) and other titanosaurs, from Martinelli et al. (2011), showing how the neural arch and its processes are shifted to the anterior end of the centrum. It also shows the procoelous nature of titanosaurian caudals, a style of ball-and-socket articulation with the socket in front and ball in back. In Aeolosaurus this is not a perfect ball, but skewed dorsally. Anterior is left and posterior is right, in case you need the orientation. "MPMA\without number" is actually part of the holotype of Aeolosaurus maximus, so all three named species are supposedly included, but the specimen labeled here as "MPMA\without number" actually appears to be CPP 248 (supposedly C) per an earlier figure in the publication. Could C and D be mixed up?

Fun with nautiloids: an essay in futility?

Or: "What good is a seven-foot nautiloid?"

In most reconstructions of marine Paleozoic life, large orthoconic (straight-shelled nautiloids) get two jobs: they are either large menacing objects in the water, usually seizing some unfortunate trilobite; or they are parked on the seafloor, again often engaged with a trilobite. The general takeaway is that a giant nautiloid was a voracious predator. Trilobites presumably are selected as the prey because so few other animals that lived during the heyday of the giant nautiloids both moved under their own power and were bigger than a few inches, making them about the only group worthy of the honor of artistic predation (although we can guess that a not-insignificant part of a giant predatory nautiloid's diet would be smaller nautiloids). Another, less scientific reason for having a nautiloid capture a trilobite is that otherwise Paleozoic marine reconstructions would be a whole lot of small filter-feeding animals not going anywhere, which doesn't make for dramatic art.

Nautiloids and trilobites: natural enemies forever locked in combat. Also pictured: many snails.