No Sexual Dimorphism in the Dinosauria

A couple of weeks ago, Everything Dinosaur reported on the publishing of a new scientific paper that set out to re-define the Dinosauria.  A new model for plotting the various branches of the dinosaur family tree has been proposed, meat-eating dinosaurs such as Tyrannosaurus rex and Velociraptor find themselves aligned with the ornithischians (bird-hipped dinosaurs), whereas iconic dinosaurs such as Diplodocus and the less iconic Herrerasaurus might just be outside of the Dinosauria altogether.  A new study has just been published that postulates that there was no sexual dimorphism within the Dinosauria.

To read more about the reclassification of the dinosaurs: Root and Branch Reform of the Dinosaur Family Tree.

That’s science, fundamental principles get challenged and in the light of new evidence, accepted thinking can be overturned.

Challenging the Idea of Sexual Dimorphism in the Dinosauria

Also, last month, Dr Jordan Mallon, palaeontologist and post-doctoral fellow of the Canadian Museum of Nature, published in the journal “Paleobiology” a fascinating article which challenges another fundamental precept of dinosaurs.  Dr Mallon argues that there is no proof that male dinosaurs looked different from females.

Many Scientific Papers Had Proposed Evidence for Sexual Dimorphism in Dinosaurs (including Stegosaurs)

The difference between girl and boy stegosaurids.

Females may have had reduced plates that were more spiky.

Picture credit: Evan Siatta with additional annotation by Everything Dinosaur

The Emperor’s New Clothes Syndrome

Dr Mallon conducted a detailed statistical analysis on nine different dinosaur taxa.  Dinosaurs that have relatively well-documented fossil records, including skull material which is believed to represent animals of different ages (ontogenetic stages), dinosaurs about which sexual dimorphism has been reported, inferred or at least debated in the past.

The dinosaurs studied:

  1. Coelophysis bauri
  2. Coelophysis rhodesiensis
  3. Stegoceras validum
  4. Allosaurus fragilis
  5. Stegosaurus mjosi
  6. Kentrosaurus aethiopicus
  7. Tyrannosaurus rex
  8. Plateosaurus sp.
  9. Protoceratops andrewsi

With crocodilians and many birds displaying sexual dimorphism, take alligators and the often-cited peacocks for example, as these animals are closely related to the Dinosauria, then surely dinosaurs were sexually dimorphic too?  But it could it be a case of the “Emperor’s New Clothes”?  Scientists expected to find evidence in the fossil record demonstrating differences between the boys and the girls and a large body of evidence based on the shape and size of bones has been gathered to reinforce this hypothesis – but just how valid is this idea?

An Extensive Statistical Analysis Searching for Evidence of Sexual Dimorphism

Dr Mallon set about conducting a wide-ranging, in-depth statistical analysis of sexual dimorphism in the Dinosauria, the first such investigation, as far as we, at Everything Dinosaur, are aware.   Previously published papers were re-visited including the intriguingly titled “The King’s New Clothes: A Fresh Look at Tyrannosaurus rex”, written by our chum Pete Larson, twenty years ago.  In addition, amongst the numerous papers referenced in his thought-provoking piece: “Recognising Sexual Dimorphism in the Fossil Record: Lessons from Nonavian Dinosaurs”, Dr Mallon looked at the research undertaken in 2015 by Maiorino, Farke, Kotsakis and Piras* into Protoceratops andrewsi, one of the species subjected to statistical analysis in this new study.

In the Maiorino et al paper, a number of examples of sexually dimorphic traits were listed amongst the Ceratopsia (see table below).

Sexually Dimorphic Traits within Horned Dinosaurs

Sexual dimorphism in horned dinosaurs.

A table outlining proposed sexual dimorphism in horned dinosaurs.

Table credit: L. Maiorino et al (from a paper also referenced by Dr Mallon).

Skull Measurements Not Characteristic of Sexual Dimorphism

The team concluded that anatomical traits such as the height and width of the frill and skull size thus may not be sexually dimorphic.  When careful measurements were considered, it seems that the size of the frill and skull size could not determine the boys from the girls, at least not in this Asian Late Cretaceous dinosaur.

Trying to Spot the Girls Amongst the Boys (P. andrewsi)

Assessing sexual dimorphism in Protoceratops.

Using frill and skull measurements to identify sexual dimorphism in Protoceratops.

Picture credit: L. Maiorino et al

Dr Mallon concurs.  In the 2015 study, scientists measured a number of traits within a sample set of twenty-nine Protoceratops skulls.

They measured:

1 – Width of the frill.
2 – Postorbital width of the skull.
3 – Length of the frill.
4 – Width of the skull across jugals.
5 – Nasal height of the skull.
6 – Height of the frill.
7 – Length of external nares.
8 – Width of external nares.

The 2015 study could not determine two distinct clusters of measurements when the skulls were assessed.  If they had, then these two unambiguous groups would have supported the idea that Protoceratops at least, exhibited sexual dimorphism.

Protoceratops One of Nine Different Dinosaurs Incorporated within the Study

Protoceratops on display.

A Protoceratops exhibit on display at The Naturmuseum Senckenberg (Frankfurt, Germany). Picture credit: Everything Dinosaur.

Picture credit: Everything Dinosaur

Why is Sexual Dimorphism in the Dinosauria Such a Big Deal?

The fossil record can only tell us so much.  For instance, inferring behaviour is extremely difficult.  Most palaeontologists now agree that many dinosaurs lived in herds (should that be flocks)?  If sexual dimorphism can be identified that it will provide critical information about the role of sexual selection in the evolution of different types of dinosaurs, as well as supporting the idea that many dinosaurs indulged in visual displays and intraspecific combat.

Dr Mallon argues that quantifiable data to support this hypothesis is often lacking.  A review of existing literature was undertaken to identify quantitative evidence for differences between the boy dinosaurs and the girls and according to this new research, the good doctor could find no evidence for sexual dimorphism in the nine taxa studied.

This contradicts many existing perceptions, but as with all science it is a question of evaluating the evidence.

Looking for Medullary Bone

Dr Mallon states that he can’t be certain that dinosaurs were not sexually dimorphic, it’s just that the available evidence precludes its detection.  A helpful start would be to positively identify females by the presence of unlaid eggs in the body cavity or medullary bone within fossil material and indeed there have been some examples already found.  However, more data is needed before we can positively distinguish the males from the females.

Things would of course be much easier, if nonavian dinosaurs were still around today.  All we would need is some volunteers willing to lift the tail of a Triceratops or probe the private parts of a Pachycephalosaurus, would anyone want to give it a try?

* The Protoceratops paper: L. Maiorino , A. A. Farke , T. Kotsakis , and P. Piras . 2015. “Males resemble Females: Re-evaluating Sexual Dimorphism in Protoceratops andrewsi (Neoceratopsia, Protoceratopsidae)”. PLoS ONE 10:e0126464. doi: 10.1371/journal.pone.0126464.

An article published in December 2016 that looks at the potential evolutionary advantage of different sized members of the same Coelophysis flock: Sizing Up Early Dinosaurs – Variation an Evolutionary Advantage.

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