Tuesday, June 28, 2016

Considering the Wild Diversity of Frills Among Horned Dinosaurs

The frills sported by horned dinosaurs are fascinating, as are the creative and wide ranging hypotheses proffered to explain this “exaggerated” feature.  Two of the most difficult aspects of drafting this post may have been (1) keeping from getting mired in the vigorous debate over explanations for those frills (I am deliberately cursory in my treatment below), and (2) refraining from a title that grossly punned on the word “frill” (e.g., A Frilling Debate).

Bill Shipp’s experience is every amateur paleontologist’s dream and, at the same time, the source of gut wrenching envy.  Shipp, a physicist, bought a ranch in Montana and, knowing this was fossil country, secured the assistance of a local amateur paleontologist to show him how to go about hunting for fossils.  First day out, Shipp found a large bone, part of a larger cache of bones that would eventually be identified as the remains of a new species of horned dinosaur.  He nicknamed the roughly 76-million-year-old specimen “Judith” for the Judith River Formation in which it was found.  (Judith may or may not be female or male; distinguishing dinosaurs by gender is hugely problematic.)

(The story of Judith’s discovery, excavation, and identification has been widely covered, including an article in the Washington Post titled This Dinosaur Had a Heartbreaking Life.  Now She’s Famous – And An Inspiration, by Sarah Kaplan, May 18, 2016; an article on the Canadian Museum of Nature’s website titled Judith the New Horned Dinosaur; and the scientific article naming the species that appeared in PLOS One titled Spiclypeus shipporum gen. et sp. nov., a Boldly Audacious New Chasmosaurine Ceratopsid (Dinosauria: Ornithischia) from the Judith River Formation (Upper Cretaceous: Campanian) of Montana, USA, by Jordan C. Mallon et al., May 18, 2016.)

The animal’s scientific name is Spiclypeus shipporum; the generic portion means “spiked shield,” while the species portion honors Bill Shipp.  That generic name helps explain my interest in this horned dinosaur (beyond my jealousy over Shipp’s incredible luck).  Of its frill, Washington Post writer Sarah Kaplan notes that it was “massive” and “spiked” and “would have put the most well-dressed Elizabethan courtiers to shame.”

Paleoartist Mike Skrepnick’s painting of Judith (see the Canadian Museum of Nature’s website) depicts the animal as it may have appeared in life and this image is probably the most immediate inspiration for this post.  The frill, in Skrepnick’s vision, was stunning, spiked and covered with a colorful hide sporting two concentric patterns looking like huge, angry eyes.  Though the “eyes” reflect an educated guess by Skrepnick, the array of spikes that marked the side margins and dorsal portion of the frill, some angling forward and others backward, are no guess.  Rather, they follow the fossils that Shipp and others uncovered.  Truly amazing.

And, to top it off, that same site includes a digitized 3D rendering of Judith’s skull that can be manipulated.  Makes the frill even more dazzling.

Horned dinosaurs, the ceratopsians, hold a particular attraction for me given their often wild cranial ornamentation, described in the literature as “exaggerated” or, less often, “bizarre.”  Horns and frills varied dramatically and are largely what paleontologists rely on to distinguish among these plant-eating species.  Indeed, as paleontologist Scott D. Sampson learned early in his career, “Key differences [among horned dinosaurs] were pretty much limited to variations in ornaments on the skull roof. . . . In other words, even if you were to find a complete skeleton, without the top of the skull it would be difficult to identify exactly which dinosaur species the bones belonged to.”  (Dinosaur Odyssey:  Fossil Threads in the Web of Life, 2009, p. 159.)

Though ceratopsian horns came in marvelously different configurations, for me the frill is the captivating feature of the skull.  I’m most familiar with the frill of Triceratops horridus, a younger ceratopsian cousin of Judith’s, particularly that of the Triceratops specimen nicknamed “Hatcher,” (after John Bell Hatcher who found the original bones in 1891), one of the stars of the Smithsonian’s Last American Dinosaurs exhibit, the tide-me-over exhibit until the new National Museum of Natural History’s new dinosaur hall opens in 2019.  Hatcher (a cast) appears below.

I’ve always considered the Triceratops frill to be substantial and pretty impressive.  Here’s paleoartist Mauricio Antón’s reconstruction of a living Triceratops, replete with a somewhat colorful frill extending from the back of the skull (this artwork appears in the Last American Dinosaurs exhibit).

But, it turns out that the Triceratops frill, though robust in my eyes, is actually rather on the lesser side and, certainly, not fancy compared to others appearing in the ceratopsian ranks.  Yale’s Peabody Museum of Natural History displays a wonderfully large number of horned dinosaur skulls.  Among them are those of Torosaurus, whose frill featured broad holes in the bone, and Centrosaurus (I think that’s the correct genus name) with a pair of long pointed spikes sweeping forward from the frill’s top margin.

(I’m avoiding the debate over the relationship between Torosaurus and Triceratops; for some, the latter is a juvenile version of the former.)

Here is an array of drawings of various ceratopsian species as they may have appeared in life.  It suggests the great variety in frills that marked these horned dinosaurs.  Though I cannot speak to the scientific accuracy of these particular drawings, they are certainly in keeping with where paleoartists have, of late, taken ceratopsians.  (This file was created by MathKnight based on drawings by Nobu Tamura and is licensed, according to Wikimedia under Creative Commons Attribution-Share Alike 4.0 International license.)

There they are, frills differing in size and orientation, often spiked, covered in possibly wildly colored hide adorned with startling patterns.

This diversity naturally gives rise to the question:  To what end?  What was the purpose (or purposes) of the horned dinosaurs’ frills?

In pursuit of an answer I came upon an example of science at work, a dance of point-counterpoint, a tug of war with hypothesis challenging hypothesis.  Yes, lots of head butting among scientists staking out positions on this question.  For the following discussion, I have largely followed Scott Sampson’s treatment of the debate specifically over the purpose of the frill as he lays it out in Dinosaur Odyssey.  I recognize that he’s no disinterested commenter on the competing arguments, coming down in favor of one of these hypothesis over the others.  Still, for purposes of this post, his treatment of the debate in Dinosaur Odyssey serves as a good basis for an introduction.  And, really, all I want to do is suggest how rich the array of explanations offered over the years for this cranial feature has been.  The debate is over primacy of any one hypothesis, not its exclusivity.  An exaggerated feature may have served one purpose primarily, and others secondarily.  And, clearly, the debate is not over yet, and probably never will be.

Here then is a very brief run through several of the proffered explanations with Sampson’s take on each.  On occasion, I add an additional voice or two.

At the outset, it’s important to stress that, central to Sampson’s evaluation of most of these various hypotheses is whether the hypothesis in question can explain why there is such a wide diversity in this exaggerated cranial feature.  Significantly, he posits that, through the working of natural selection, one would expect just a few efficient designs for frills to emerge, not the riot we see.

The frill, some have claimed, served as a platform for muscle attachment to enable large jaws to chew powerfully through vegetation.  Sampson categorically dismisses this hypothesis, noting that “muscle strength derives more from cross-sectional thickness than length, and various bony indicators suggest that the jaw muscles did not extend significantly onto the frill.”  (p. 159, 161)  Frankly, this mechanically based explanation seems to be almost uniformly rejected by current writers.

Defense is probably the most popular explanation for the frill.  Certainly that was brought to mind for visitors to the old dinosaur hall in the Smithsonian’s Natural History Museum where a T. rex skeleton seemed posed to launch an assault on a Triceratops skeleton.  But, wait, cautions Sampson.  Though ceratopsian frills may appear massive and stout, they were not of a uniform thickness, and could narrow down in spots to just a millimeter in thickness.  Not much defense there.  Furthermore, the frills were served by a rich blood supply.  The risk, it would seem, is fairly high of bleeding out quickly from the nip of predator.  Perhaps the strongest argument against the frill protecting the animal from predators is the incredible variety in frill morphology.  A defensive purpose doesn’t explain that variety.  One would expect a very limited number of frill configurations, those effective for defense.

Thermoregulation for these large beasts, so another argument goes, was accomplished by running blood across the bone of the frill, dissipating body heat in the process.  Think of the ears of the elephant which help to give off excess heat.  Sampson cautions that the ceratopsian cranial structures aren’t well-designed to do that, and, critically, this explanation certainly doesn’t account for the variety in frills.

Species recognition is a more recently stated hypothesis to account for such exaggerated features as the ceratopsian frill.  Species occupying the same ecological area, it is argued, would need to identify fellow members of their own species and distinguish them from members of other ceratopsian species.  This might be important for protective and other social behaviors, as well as for ensuring the identification of appropriate mates.  Distinctive frills would allow that, and one virtue of this hypothesis is that it explains the frill variety.  Yet Sampson counters, “One of the problems with this idea is that there is minimal evidence of multiple closely related animals sharing the same ecosystem.”  (p. 162)

Rather, the frills, Sampson concludes, were mating signals, in other words, they were “objects for courtship and combat with members of the same species.”  (p. 162-163)  The focus here is on reproduction and what is at play, according to Sampson, is sexual selection.  “The most obvious examples of mating signals [in extant and, presumably, extinct animals] are visual ones, involving unique coloration and/or physical structures such as horns, crests, and elaborate tails.”  (p. 163)  The main display sign at the Last American Dinosaur exhibit clearly embraces this hypothesis, featuring, as it does, this drawing of two horned dinosaurs getting down and dirty with each other.

Sampson supports this hypothesis further by noting that, where there are sufficient numbers of specimens of a particular species that allow one to distinguish morphological changes occurring at different stages of life (from young to juvenile to young adult to fully adult), it is evident that the frills take their prominent place only when the animal reaches adulthood, that is, sexual maturity.  Although there’s little or no evidence of sexual dimorphism among dinosaurs (that is, one sex having ornamental features the other does not), Sampson writes, “[N]ot all [extant] animals with bizarre structures are sexually dimorphic.”  (p. 165)

In recent years, the debate over explanations for the frills and other exaggerated cranial features among dinosaurs has, it appears, centered largely on whether the species recognition hypothesis or the mating signals hypothesis is most supported by what evidence there is.  Paleontologists Kevin Padian and Jack Horner have fought strongly for species recognition, arguing, in part, that the so-called mating signals are merely a subset of species recognition functions.  Further, they are adamant in contending that, by definition, without sexual dimorphism there cannot have been sexual selection.  The mating signals camp features, in addition to Sampson, Robert J. Knell and David W.E. Hone among others.  Some of their positions in favor of mating signals are described above.  The flavor of the debate between these two camps is captured well in three pieces that appeared in the Journal of Zoology in 2011 – a paper by Padian and Horner (The Evolution of ‘Bizarre Structures’ in Dinosaurs:  Biomechanics, Sexual Selection, Social Selection of Species Recognition), a responding letter by Knell and Sampson (Bizarre Structures in Dinosaurs:  Species Recognition or Sexual Selection?  A Response to Padian and Horner), and a response by Padian and Horner to that response (The Definition of Sexual Selection and its Implications for Dinosaurian Biology).

Frankly, from my reading of these and other articles on the subject, there seems to be common ground that both parties agree upon (or, at least, don’t disagree upon).  I am particularly taken with a recent piece by David Hone in which he argues that these exaggerated structures in ceratopsians were used for “socio-sexual signaling,” a phrase, he believes, better reflects the possibility that these structures might have served, to differing degrees among species, both social and sexual signaling.  (Hone et al., Positive Allometry for Exaggerated Structures in the Ceratopsian Dinosaur Protoceratops andrewsi Support Socio-Sexual Signaling, Palaeontologia Electronica, January, 2016.)  But, then again, this may not satisfy the various contestants in this arena.

After all is said and done (or argued and rebutted), it may come down to Sampson’s characterization of this debate:
Considering dinosaurs as a whole, we can be certain that we’re missing a large part of the mating signal picture.  Barring the invention of time travel technology, this situation is unlikely to alter substantially, and the study of dinosaurian social lives is destined to remain a highly speculative endeavor.  (p. 168)
In closing, I have to share the brilliantly insightful (and funny) comment made by paleontologist Peter Dodson (Sex in the Cretaceous – How to Tell the Girls from the Boys, American Paleontologist, Fall 2011):
This brings us to the question of sexual dimorphism among dinosaurs.  We must acknowledge at the outset that this is a problem for humans, not for dinosaurs.  (p. 22)
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