About Protoceratops sexual dimorphism

A study published earlier this year by Maiorino, Farke, Kotsakis and Piras provided us an eloquent example of how assessing the sexual dimorphism in extinct animals is a very difficult field to clear and virtually impossible to establish with certainty. Although several past authors have hypothesized that the two different forms of skull sampled on many known specimens of Protoceratops andrewsi (don't get misunderstood with P. hellenikorhinus), these theories stay pure speculation, since empirically dimorphism reported might depend by ontogenetic and/or intraspecific factors (otherwise possibly conditioned by paleopathological or taphonomic factors too) and therefore not necessarily sexual ones.

Does it mean that it's impossible to be sure about dinosaurs sexual dimorphism?

One of the most commonly methods used in scientific statistics is said ANOVA (an acronym which is for ANalysis Of Variation) that is very convenient to test various groups compared in order to create a statistic significance.

Applying this simple concept to Protoceratops specimes is easier than you think, at least theorically.

The alleged traits that have been pointed as indexes of sexual dimorphism by Dodson (1976) in the skulls of several specimens (about thirty well-preserved skulls of Protoceratops andrewsi are known) are basically four:

(1) postorbital width of the skull

(2) nasal height of the skull

(3) width of the frill

(4) height of the frill

Simply put, if most or all of the skulls would have presented the total masculinity or femininity in all four characters, the Dodson theory had been confirmed. But the study showed just relevant ontogenic variations, while following those that are suspected stretches of sexual dimorphism have resulted in a variation that's more than 90% (really too much to confirm the theory), the authors mention anyway a lower percentage in P. hellenikorhinus (actually a reference to a paper by Lambert et al.) although the currently known sample of P. hellenikorhinus is not sufficient for confident identification of sexual dimorphism so if no major sexual dimorphism occurs in P. andrewsi, it is unlikely that major sexual dimorphism occurred in the skulls of P. hellenikorhinus as well.

Nasal horn shape was referred by authors as the only possible element which shows a pretty sharply dual variability of supposed sexual dimorphism in the cranium.

Fairly, these are the last words of the paper: “Ultimate identification of sexual dimorphism in non-avian dinosaurs is quite difficult”.

And we all agree.