Death Valley National Park geology, Part III: Mesozoic through Paleogene

We made it out of the Precambrian. We made it out of the Paleozoic. We aren't nearly out of Death Valley, though. There's still about 250 million years to go, during which the valley was converted from marine property to land, became gently toasted by massive subterranean plutons, and was eventually pulled apart into the modern topography over about 15-20 million years of nonstop faulting, with volcanic eruptions and inland lakes thrown in for variety.

And the state fossil of Minnesota is...

...nothing. Despite the map on "List of U.S. state fossils" [note, 2018/2/22: this has since been fixed], Minnesota does not have an official state fossil. There was a push to draft the giant beaver into this position, but the rodent fell short. The immediate political reason for its failure can be grasped from its scientific name: Castoroides ohioensis. Should the state fossil of Minnesota refer to another state? Certainly not!

Death Valley National Park geology, Part II: Cambrian through Permian

When we left Death Valley National Park (DEVA) last week, we'd gotten partway through the Cambrian, into the early stages of a passive continental margin with shallow marine deposition. We return to find that predictable shallow marine deposition held sway until fairly late in the Paleozoic, when things got complicated in a hurry.

Death Valley National Park geology, Part I: Proterozoic to Cambrian

As I mentioned a few months ago, one of the main components of my day job over the past few years has been preparing paleontological resource summaries for National Park Service Inventory and Monitoring networks. Between new and updated summaries, I've completed ten (Greater Yellowstone, Mediterranean Coast, Northern Colorado Plateau, Northern Great Plains, Northeast Coastal and Barrier, Northeast Temperate, Sonoran Desert, Southeast Coast, Southern Colorado Plateau, and Southern Plains). Each network has its own foibles and presents different challenges. Some are much more easily tackled than others; smaller parks generally have less to worry about than larger parks, with small urban cultural or historical units usually being the simplest. After finishing the Southern Plains Network last spring, I knew that the Mojave Desert Network (MOJN) would be one of the obvious candidates for the next project. It was completed under a superseded format, so it was already in the hopper, and we were already working with Death Valley National Park and Lake Mead National Recreation Area, so the iron was hot. The attraction for me was the challenge. The MOJN parks have superb geological records exposed by the combination of Basin and Range faulting and restricted plant cover, and they cover vast areas.

Mostly bryos and burrows

It's still a little early to be out and about in the metro rocks and expect good luck (for one thing, the Decorah is still in winter-spring-transition mud mode) or consistent weather, but we're getting close, and sooner than usual. Here's one seasonal photo and then a few interesting pieces I hadn't featured before. These are all photos of Decorah Shale fossils from the St. Paul side of the river.

Lower Decorah a couple of weeks ago, after having been relatively warm and dry for several days. These two fragments were covered with half-burrows (imagine a tubular burrow split along its long axis), and several other pieces from the same area and probably a similar stratigraphic level had the same kind of abundant half-burrows.

Meroktenos thabanensis: not Early Jurassic, just ahead of its time

It's been about a month since Meroktenos thabanensis made its (re)appearance, but I'm just now getting around to it. A bunch of other topics muscled ahead of it, which is fitting, I suppose. Prosauropods (by which the reader should understand "sauropodomorphs what ain't sauropods"; if you should want me to write "basal sauropodomorph" over and over again instead, I accept large cash bribes) are among those dinosaurs which never seem to get much respect. They have been fertile subjects for arm-waving, though, one result of which being that a survey of popular-audience dinosaur books will turn up wildly differing depictions over the years. Bipedal or quadrupedal? Herbivorous, omnivorous, predatory, or scavenging? A stem leading into sauropods, a bunch of rungs, or a distinct empire of prosauropods? Gleefully extreme lumping or splitting? What about herrerasaurids? What about Teratosaurus? (if that last one means anything to you, congratulations, you're in your mid-30s or older, or you study rauisuchids.) Prosauropods have also not been helped by a certain vague uniformity of body shape, nor have they been done any favors by geography. North America has an unimpressive record for these dinosaurs, so far including Anchisaurus (let's not kid ourselves about Ammosaurus being distinct), Sarahsaurus, Seitaad, the unnamed Nova Scotia form, and odds and ends. For a group that did not have horns, frills, spikes, bony armor, crests, ridge-backs, spectacular size, or big pointy teeth, and had the poor sense to go extinct before the appearance of tyrannosaurs or dromies to prey upon them, not being well-represented in North America has not helped their exposure. But I digress.

Teenage Wasteland: the question of adulthood in dinosaurs

It took quite a while before people started noticing that (non-avian) dinosaur growth didn't quite work the way it had been supposed. In fact, it seems to have taken quite a while before people permitted the notion of growth to enter their studies in the first place, which is how we got stuck with Brachyceratops and cheneosaurs (we would have all been better off without Procheneosaurus, but that is another story). By the 1970s, researchers had warmed to the idea that dinosaurs might change radically during growth. By the 1990s, suggestions that certain horned dinosaurs such as Brachyceratops and Monoclonius were really younger examples of other horned dinosaurs were at large. There was a hint of things to come in the punchline to the horned dinosaur story, which was that the full suite of horns and other excrescences did not appear until late in growth, but the implications were not yet clear.

It was about ten years ago or so that the rumblings started making it to the conferences. At the 2007 Society of Vertebrate Paleontology annual meeting, Jack Horner et al. presented an abstract concerning their findings on pachycephalosaurids, namely Dracorex was Stygimoloch was Pachycephalosaurus, because for reasons that remain mysterious Pachycephalosaurus liked to style in spikes as a kid but dropped them as it matured. The work was formally published in 2009, and can be read for free. More followed. Anatotitan represented old large examples of Edmontosaurus annectens. Most controversial, of course, was the "Toroceratops" hypothesis, that Torosaurus was just what Triceratops turned into as it grew up.

The issue of dinosaur growth comes up again because of a review paper by David Hone et al., which summarizes the various anatomical markers people have used to argue for a given specimen being at a certain growth stage (various fusions in the skeleton, bone texture, size, appearance of features thought to indicate reproductive maturity, etc.) and tackles how various growth stages have been defined. You should definitely read the paper if you are interested in this topic (again, it's free), and two of the authors have put out additional statements.

At the heart of the controversy is how we define things like "juvenile", "subadult", and "adult". When we use these terms, there are certain assumptions attached to them, but not everybody shares the same assumptions, and not everybody clearly states what they mean when they use them. This is a particular problem when dealing with a group of animals with growth histories that do not necessarily match up with the categories we're trying to put them in. Essentially, the more we look, the fewer dinosaur specimens turn out to fit our criteria for adulthood, indicating that reproductive maturity occurred before skeletal maturity. Once you've got "subadults" doing most of the breeding, you've got some important implications for selection pressures, ecology, classification, and so forth, and you're left with the possibility that dinosaurs may have more or less abandoned "adulthood" as we typically understand it. This would be pretty darn significant. Take the Morrison Formation, for example. What if, instead of the "functional unit" of Apatosaurus ajax or Diplodocus carnegii or Camarasaurus supremus being the giant sauropods we know from museum mounts, the basic reproductive units of these species were much younger and smaller individuals? For one thing, you've greatly reduced the amount of food needed to support viable populations.

Inevitably, my thoughts return to "Toroceratops". One of the major issues, at least in my consideration, is a question of selective pressures. Let us assume that "Toroceratops" is accurate, and let us further assume that reproduction began during the Triceratops stage. I make this assumption in consideration of the great numerical disparity between Triceratops and Torosaurus specimens. It would seem to be a poor way to run a large vertebrate taxon to have so many individuals keel over just before reaching reproductive maturity. Under these two assumptions, my original question was what selective pressures would be responsible for turning Triceratops into Torosaurus if it was already breeding quite happily (we presume) as Triceratops and so few individuals seemed to be living long enough to make that change. In other words, with a perfectly functional Triceratops, what is the advantage of going Torosaurus? But what if this was asking the question from the wrong direction? What if instead of there being selective pressure to attain the Torosaurus form, there was pressure to prolong the Triceratops form, making the Torosaurus form a relic of an earlier stage of the Triceratops line? In other words, might Torosaurus not represent the "adult" of Triceratops, but rather something like a geriatric form that still appeared late in growth because there was limited selective pressure for or against it? One implication of this idea is that there should be geologically older taxa that shifted from Triceratops-like morphologies to Torosaurus-like morphologies earlier in growth.

Further reading:

Campione, N. E., and D. C. Evans. 2011. Cranial growth and variation in edmontosaurs (Dinosauria: Hadrosauridae): implications for latest Cretaceous megaherbivore diversity in North America. PLoS ONE 6(9):e25186.

Hone, D. W. E., A. A. Farke, and M. J. Wedel. 2016. Ontogeny and the fossil record: what, if anything, is an adult dinosaur? Biology Letters 12(2).

Horner, J. R. and M. B. Goodwin. 2009. Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS ONE 4(10):e7626.