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.

One small mystery, and another

Item One: Always pay close attention to your hash slabs

I've recently done some educational events, and I usually bring a few hash slabs. When I'm done, I usually give them a once-over to check for damage incurred from transportation, handling, and so forth (there's a bryozoan that has been breaking on one of the slabs, for one thing). I noticed something unusual on one of the slabs: a small triangular object that on close inspection proved to have interesting surficial markings of fine, gently curved lines running between the long axes of the object. The object is approximately 6.5 mm long by 3 mm wide. What there is of it appears to be either triangular in cross section, or two flaps of material (kind of like a lopsided tent) draped on the matrix. I say this because the tip overlaps a small circular fragment, and because there does not appear to be anything continuing down into the matrix from the two sides that are visible. The lining of the surface does not appear to be segmenting, but looks almost like it defines slightly imbricated sections of the structure. The wide end appears to be open, with the two visible edges slightly overhanging infilling matrix. The edges do not define a flat plane, but curve in slightly where they meet. The sides appear to be flat, making allowance for a slight overall curve and skew to the object. My interpretation is that the structure is a hollow or partially object of triangular cross-section, composed of material that grew in layers from the tip, with the wide end open. The host slab is one of those I collected last spring from the construction pile. It is certainly from the Decorah Shale, and probably came from the lower third or so of the formation, based on geography. Behold (scale bar in mm):

Earthquakes in Minnesota

I was co-leading a field trip for a Minnesota Master Naturalist group over the weekend, and at one point we strayed onto a tangent involving faulting and earthquakes in Minnesota. Given we've already covered one of the rarest of the geological rare birds of the state with "Minnesota's dinosaurs", let's add another notch to the rock hammer with the subject of earthquakes in the Land of 10,000 Lakes.

Gonioceras: when a nautiloid is also a shovel-flounder

In a previous post, we were briefly introduced to the local nautiloids, which for the most part inhabited straight, gently tapered shells. Plectoceras coils, but otherwise the local squidlings are reasonably predictable. There is one other notable exception, though: Gonioceras is one of the oddest-looking nautiloids you can run across. Picture the blade of a shovel: it's pointed, it's flat, it's got a ridge at the broad end that turns into a socket for the handle. Now, let's modify it a bit. First, trim the blade into a triangle. Now, chop off the fitting for the handle where it goes beyond the blade. Next, take the rest of the fitting section and extend it down to the tip, and make it hollow, so you've got a tube running the length of the blade. Taper it, and keep the underside of the blade flat. Finally, put an appropriately sized nautilus body in the tube. This is essentially what a Gonioceras looked like. One other thing: the septa (the divisions between chambers) are arranged in sine-wave-like curves, concave behind the aperture and convex on the flanges. The illustration below includes the central portion and one of the flanges of a shell, with the aperture toward the top, and should make things more clear:

Gonioceras occidentale from Illinois, plate LVII of Clarke (1897). This specimen shows part of the wedge-like flaring of the shell and the curved septa.

Of course, in the field, you're liable to just find a chunk with those distinctive wavy septa:

Gonioceras in the wild, skooshed flat and otherwise worse for wear, from the lower to middle Mifflin Member of the Platteville Formation.

Although Gonioceras certainly cannot be accused of simply doing what the other nautiloids were doing, its innovative structure does not appear to have caught on. In Minnesota, Gonioceras is primarily a Platteville Formation concern (Stauffer and Thiel 1941; Catalini 1987). It is generally thought of as a bottom-feeder, sometimes interpreted as a crawler but perhaps analogous to a flounder (Vickers Rich et al. 1996). Like other nautiloids, its propulsion would have moved it primarily backward, so the pointed end was the leading end of the animal when it had to get going.

References:

Catalani, J. A. 1987. Biostratigraphy of the Middle and Late Ordovician cephalopods of the Upper Mississippi Valley area. Pages 187–189 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Clarke, J. M. 1897. The Lower Silurian Cephalopoda of Minnesota. Pages 760–812 in Ulrich, E., W. Scofield, J. Clarke, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

Vickers Rich, P., T. H. Rich, M. A. Fenton, and C. L. Fenton. 1996. The fossil book: a record of prehistoric life (corrected republication of 2nd edition). Dover Publications, Inc., Mineola, New York.