Obvious sexual dimorphism in pterosaurs?

. From Tetrapod Zoology with this caption, "Cover of TREE featuring Knell et al. (2012). Image (featuring male and female Pteranodon with obvious sexual dimorphism) by Mark Witton." While this might be true, rigorous phylogenetic testing says otherwise. These are distinct species.

Figure 1. From Tetrapod Zoology with this caption, “Cover of TREE featuring Knell et al. (2012). Image (featuring male and female Pteranodon with obvious sexual dimorphism) by Mark Witton.” Naish’s statement has been shown to be false. Attention to detail and rigorous phylogenetic testing provides other results (Fig. 2) in which these two inaccurately portrayed Pteranodon specimens actually represent distinct species. In general, smaller skulls with smaller crests are phylogenetically closer to the ancestral germanodactylids with smaller skulls and smaller crests. Not only does that make sense, it can be demonstrated phylogenetically (Fig. 2). See the juvenile? Cute, but inaccurate. (see below).

An April post at Darren Naish’s Tetrapod Zoology was headlined, “Dinosaurs and their ‘exaggerated structures’: species recognition aids, or sexual display devices?” (refs below).

Naish wrote, “Our latest paper is devoted to a discussion of the species recognition hypothesis and, specifically, why we think it’s problematic and should be discarded. We’ve noted that dinosaur workers have increasingly taken to mentioning species recognition whenever they discuss exaggerated structures (see list of citations in Hone & Naish 2013), so now is a good time to try and set the record straight.”

Pteranodon skulls

Figure 2. Click to enlarge. A family tree of Pteranodon and Nyctosaurus derived from Germanodactylus. Note the size increase is gradual. So is the crest size increase. Of all these many specimens, can you tell which are male and which are female? I can’t. There are no two skulls, sans crests, that are identical in morphology. You can’t divide this set of illustrations into two genders, and yet, according to Naish you should be able to. Attention to detail indicates there’s also much more to skull morphology than crest shape.

Is it possible to ascertain that small crested specimens are females?
Or are small crested specimens just in the lineage of large crested specimens? Contra Naish, species recognition helps us identify various dinosaurs with all their crests, horns and feathers. The same is true of pterosaurs and you can test this with phylogenetic analysis (Fig. 2). If this was gender identification in Pteranodon, there should be just two forms relatively close to one another for each of the above skulls. But no two here are even that much alike. Instead they form a series of gradually evolving shapes, which defines them as distinct individuals and species. Interestingly, there is a derived size decrease in specimens R, S, T and Z4.

Juvenile Pteranodon
The image in figure 1 also shows a juvenile Pteranodon with an appropriately short crest and short beak. Well, we have a juvenile Pteranodon (Fig. 2) and it doesn’t have any indication of a short beak, but retains the proportions of an adult, and a specific adult at that, YPM 2594, which is likewise known from a posterior skull without a complete rostrum.

Pteranodon is the Cretaceous equivalent of Darwin’s finches.
Rapid diversification in Pteranodon originated as individual variation that was enhanced over the generations by natural selection. Pteranodon was not just one or two species, but dozens, as is plainly evident when you look at dozens all once (Fig. 2). That’s the value of accurate reconstruction, something most pterosaur workers avoid like the flu.

Getting back to sexual dimorphism
Naish wrote, “And we absolutely reject Padian & Horner’s (2013) argument that sexual dimorphism is essential for the recognition of sexual selection: there’s unambiguous evidence from the living world that sexual selection is at play even when dimorphism is absent (Hone et al. 2012, Knell et al. 2012, 2013).”

To Naish’s point, there is always sexual dimorphism, whether presented in skeletal differences, beards, breasts, genitals, feathers or pheromones. Sometimes it’s more obvious. Sometimes less. In pterosaurs no one has ever been able to document male vs female differences in the skeleton, even when there’s an egg between the legs of an obvious female, a subject we looked at earlier. Males, as best as we can ascertain were outwardly identical. Crests appeared on various darwinopterids during speciation, as we looked at earlier. Some have even been named new genera by various workers.

Unfortunately, at least in pterosaurs,
there’s no evidence for gender identification that can stand up under the scrutiny of phylogenetic analysis. We don’t see only two kinds of Pterodactylus. We see several (with not one tested specimen identical to another). We don’t see only two kinds of Rhamphorhynchus. We see dozens. Lack of reconstructions and lack phylogenetic analysis leads paleontologists like Darren Naish into supposition and assumption. What’s “obvious” to him is only obvious because he hasn’t tested it to see if it is indeed true. Testing, not supposition, is what makes good Science.

And by the way,
Naish and those who are in his camp are among those who think (without testing) that tiny pterosaurs are juveniles of larger taxa. Bennett (2006) made this supposition, but his study did not stand up to phylogenetic analysis.  Tiny Solnhofen pterosaurs of all sizes and shapes nest at the bases of several major clades and are the transitional taxa from one clade to another as each phylogenetic series gradually decreases then increases in size.

Darren Naish and Modular Evolution
Naish wrote,  “So it’s almost as if the head and neck were evolving at different rates from the rest of the body: in other words, Darwinopterus looks like a classic case of ‘mosaic evolution’ or modularity (hence the species name). This much-discussed evolutionary phenomenon has been considered controversial, in part due to a lack of good examples: Darwinopterus looks like one of the best yet discovered, and this isn’t lost on Lü et al. (2009).”

A lack of good examples indeed! This hypothesis applies only to Darwinopterus in order to cover up a lack of good phylogenetic analysis, which you can see here.

Of course, there’s no such thing as “modular evolution,” made famous during the Darwinopterus bungle, but Naish embraced this fairy tale without criticism. If true we’d see it on other taxa and in other clades, but we don’t It was invented for Darwinopterus. If modular evolution were true we’d have less confidence in our reconstructions of partial skeletons because heads would be evolving leaving the legs far behind, as imagined for Darwinopterus. In reality the palate evolves along with the toes and everything else. In reality Darwinopterus is a dead end taxa leaving no known descendants, but it did convergently evolve a large skull and smaller naris. Phylogenetic analysis settles that issue.

That being said…
There’s much more in Naish’s blog that is good Science and worth considering. If interested, follow this link. He’s just got an Achilles heel when it comes to pterosaurs.

So, there’s more to Pteranodon that just size and crests.
We know of a variety of post-crania as well, some robust and some gracile with various humerus shapes (Fig. 3). Only a few Pteranodon specimens are known from associated skulls and post-crania. So, which post-crania go with which crania? We can only guesstimate now and earlier some effort was made toward this goal. Small ones might have been young or might have been primitive (more likely the latter, since juveniles, like Ptweety or so rare and the small ones are not virtually identical to any large ones (isometric growth).

Post-crania Pteranodon

Figure 3. Click to enlarge. Various Pteranodon specimens known from post-crania. Note the yellow box includes one of the largest specimens, but it has an unfused extensor tendon process, which may mean it is a very large Nyctosaurus with fingers.

If Naish is right
and there is “obvious” sexual dimorphisim in Pteranodon, let him (or someone!) present three or four pairs of males and females, then subject them to phylogenetic analysis. The gender pairs should match with their mates if true.

And finally, we need to talk about the “female” pelvis in Pteranodon.
It’s morphologically closer to Nyctosaurus as reported here. And, yes, it’s a big pelvis, but we have further evidence of Pteranodon-sized Nyctosaurus, because the former fused the extensor tendon process and the latter did not, as reported here , and we know of a very larger unfused extensor tendon process on what others have identified as a Pteranodon based on its size.

No matter what crap they throw*,
I’m not going to let Naish and Witton disfigure and fantasize pterosaurs when good solid evidence is available to counter their traditional and mistaken hypotheses. Credit will also be given when appropriate.

* By “crap,” I mean blackwashing statements that contain no pertinent specifics or evidence. You’ll know them when you see them.

References
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.SMNS
Bennett SC 1992. Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Knell R, Naish D, Tompkins JL and Hone DW E 2012. Sexual selection in prehistoric animals: detection and implications. Trends in Ecology and Evolution28, 38-47.
Naish D and Cuthill IC 2012. Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? Lethaia 45, 139-156.
Naish D, Tomkins JL and Hone DWE 2013. Is sexual selection defined by dimorphism alone? A reply to Padian and Horner. Trends in Ecology and Evolution. http://dx.doi.org/10.1016/j.tree.2013.02.007

3 thoughts on “Obvious sexual dimorphism in pterosaurs?

  1. “The same is true of pterosaurs and you can test this with phylogenetic analysis (Fig. 2). If this was gender identification in Pteranodon, there should be just two forms relatively close to one another for each of the above skulls. But no two here are even that much alike. Instead they form a series of gradually evolving shapes, which defines them as distinct individuals and species. Interestingly, there is a derived size decrease in specimens R, S, T and Z4”
    This assumes your phylogeny is accurate. I don’t see how the tree posted disproves sexual dimorphism. You have “basal” forms diverging into two distinct branches – long crests and wide crests. This is exactly what you would expect in a single species exhibiting sexual dimorphism. The only “odd man out” is Z3, but I’m not sure what characters you used to unite it with the broad crested specimens. Of course, the dimorphism hypothesis requires other assumptions that are hard to test.

    Juvenile Pteranodon – privately held specimen so it really is not worth discussing. It is not available for anyone else to study, examine, measure, etc. We can’t base a hypothesis off something that is not reproducible.

    “Pteranodon is the Cretaceous equivalent of Darwin’s finches.”
    No. Not in any way. The Galapagos finches adapted to radically different niches rapidly in order to reduce competition. This is evidenced by specific adaptations to their niche (particularly beak shape, which Darwin wrote about). Crest shape, whether gender or species identification, is not an adaptation of that same type or order. Pteranodon lived in a relatively homogeneous environment (Interior Seaway, southeastern shore). Unless there are some unknown features of these different specimens I see no indication that they were adapted to substantially different ways of life.

    “To Naish’s point, there is always sexual dimorphism, whether presented in skeletal differences, beards, breasts, genitals, feathers or pheromones.”
    Not really. The only for-sure difference will be gonads. Having worked with Condors for a number of years I can tell you that there is not always an external physical difference between genders. They have the same plumage, are the same size, and have the same skeletal dimensions. If you found the bones of all the California Condors in the world today you wouldn’t be able to determine gender. The same is true for other extant theropods.

    “Tiny Solnhofen pterosaurs of all sizes and shapes nest at the bases of several major clades and are the transitional taxa from one clade to another as each phylogenetic series gradually decreases then increases in size”
    Wouldn’t you expect juvenile taxa to nest at the base of a clade? Especially is ontogeny recapitulates phylogeny? So I don’t see how this disproves Naish.

    “So, which post-crania go with which crania? We can only guesstimate now and earlier some effort was made toward this goal.”
    Unless you have VERY tight stratigraphic controls this is a useless exercise.

    “Small ones might have been young or might have been primitive”
    How can you reject the juvenile hypothesis by just saying “isometric growth”? That is far from a sure thing AND, again, juvenile taxa should nest as basal to/in a clade. What specific characteristics do you use to differentiate between juvenile specimens and adult but basal specimens? Especially since you explicitly refute fusion as a guide (and that may be justified). How can you tell? Is there a repeatable test that you have to distinguish between them?

    “If Naish is right and there is “obvious” sexual dimorphisim in Pteranodon, let him (or someone!) present three or four pairs of males and females, then subject them to phylogenetic analysis. The gender pairs should match with their mates if true.”
    If you are saying what I think you are saying then you are mistaken. Each male should not be a sister group to just one female. You would expect two “clades”, one male and one female, with juvenile specimens at the base. Basically like your Figure 2, actually.

    All of this ignores the biggest problem with any sexual dimorphism hypothesis with fossil organisms. You need to have organisms representing one discrete population for your results to be very solid. The further the stratigraphic distance (and geographic, in the case on non-flying organsims), the harder it is to support such a hypothesis since the changes you are seeing could be the result of 200,000 years or so of evolution, not dimorphism. I don’t know how tightly controlled the stratigraphy is for the Pteranodon specimens listed here (and in TREE) but that would be where I would start critically analyzing the claim – not misusing phylogeny.

  2. You cannot reasonably “test this with phylogenetic analysis.” It’s been attempted, in mammals and in birds. Juveniles tend to group basal in a distribution sample, adults elsewhere. This is evidence of ontogenetic transformation, and is true even when considering paedomorphism (retention of juvenile features and proportions through ontogeny) and peramorphism (radical alteration and disjunction of juvenile features and proportions through ontogeny). I’ve given you the references before, and so have others, about the flaws of assumptions you make on phylogeny, that merely listing specimens as OTUs comes with the problem that you refuse to contextualize your OTUs. Fail, massive Science fail. You refuse to correct that when it’s brought up to you, that you artificially break taxa into sets and pretend they are equivalent to full, modern taxa. Just imageine what would result if you tried this the way others had with phylo analysis and included known adults and juveniles into trees with congenerics: Lions, tigers, leopards, puma, etc.

    But this comment is not enough. You refuse to listen.

  3. Wow, “There’s something very wrong with ‘our’ (= your!) pterosaurs”, indeed. I don’t have time to respond to the many, many misunderstandings and misinterpretations here, but… why have you screwed with the ordering of the authors in the references? Weird.

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