Evidence for Sexual Dimorphism in the Stegosaurs

Show most five year olds, a picture of a Stegosaurus and the chances are they will be able to name it.  The plated plant-eater with its small head, stout limbs and its tail spikes is one of the most recognisable of all the dinosaurs.  However, palaeontologists still know remarkably very little about this Late Jurassic herbivore.  A new paper, written by a researcher from Bristol University and published in the on line academic journal PLoS One (public library of science), suggests that the shape and size of those famous back plates varied between the males and females.  If the researcher’s conclusions are correct, male Stegosaurs had back plates that were more rounded and up to 45% bigger than the females.

One of the Best Loved but Not that Well Understood Dinosaurs

Still lots to learn about this Ornithischian dinosaur.

Still lots to learn about this Ornithischian dinosaur.

Picture Credit: Everything Dinosaur

In Everything Dinosaur’s annual survey of popular prehistoric animals, Stegosaurus is nearly always recorded in the top five.  It seems most popular with girls.  In this new research, University of Bristol MSc student Evan Saitta proposes that skeletal evidence from a bone bed in central Montana referred to as quarry JRDI 5ES, suggests sexual dimorphism in a Stegosaur species known as S. mjosi.  Evan spent a total of six summers working as part of a field team from Princeton University (New Jersey), excavating the Stegosaurus fossils as part of his undergraduate thesis.

To view the results of Everything Dinosaur’s most recent prehistoric animal survey: Top Ten Prehistoric Animals of 2014 (Part 1)

The top five in our most recent survey can be found here: Top Five Most Popular Prehistoric Animals of 2014

Professor Michael Benton, Director of the Masters in Palaeobiology at Bristol University commented:

“Its very impressive when an undergraduate makes such a major scientific discovery.”

This is certainly true, but let’s not get too carried away for the moment at least.  Take for example the species name Stegosaurus mjosi, there is some uncertainty whether the fossils represent a Stegosaurus.  About thirty years ago, palaeontologists discovered the fossilised remains of a large Stegosaur during field work on the oldest part of the Morrison Formation exposed in Montana.  This dinosaur was formally named in 2001 as Hesperosaurus mjosi.  Although there have been many fossils found, including an almost complete skull, it is still debated whether these fossils represent a distinct Stegosaur genus or a species of Stegosaurus.  The strata from which the fossils come from has been estimated to be around 155 million years old, it has been suggested that Hesperosaurus mjosi, or if you prefer the junior synonym Stegosaurus mjosi, is a basal member of the Stegosaur family.  Its exact phylogenetic place in the Stegosaur family tree remains controversial.

However, this issue does not detract from the research carried out into the shape and size of the plates.  The dig site known as JRDI 5ES, as it is one of the sites managed by the Judith River Dinosaur Institute, provides a fascinating and tantalising glimpse into the lives of these large, Late Jurassic plated dinosaurs.  The site contains the remains of at least five individuals preserved in mudstone.  Although two distinct types of plates can be observed, the rest of the bones indicate that this group of Stegosaurs represent a single species.  The bones, although disarticulated for the most part, are found within the same stratigraphic horizon and they were probably not transported very far prior to burial.  This is indicated by the presence of many smaller bones such as unguals and skull elements as well as a lack of wear on the bones from transportation.  The absence of shed crocodilian and Theropod teeth indicate that the corpses were not scavenged and so burial must have been quite rapid.  It could be inferred that this was a group of Stegosaurs that died together, could this species of Stegosaur have lived in social groups?  Does this suggest Stegosaur co-existence?

Suggested Silhouettes of a Male and Female

Females may have had reduced plates that were more spiky.

Females may have had reduced plates that were more spiky.

Picture Credit: Evan Saitta (additional annotation by Everything Dinosaur)

Detailed measurements and quantitative analysis indicated that there were two distinct types of plates present.  Of the fairly complete plates found within the bone bed, four could be described as large and wide, whilst five more could be classified as being taller and more spiky in appearance.  These are distinct characteristics and what is more, Mr Saitta’s analysis suggests that there are no plates present that show traits of both wide and spiky characters.

Evan commented:

“Simply looking at them by eye, you can identify two varieties.  But then you can also measure them and do a more quantitative analysis and demonstrate that, yes, there are two distinct varieties of plates, and that there don’t appear to be any clear-cut intermediates.”

Two Distinct Plate Morphologies have been Described

The widest plate morph compared to the tallest plate morph (scale = 10cm)

The widest plate morph compared to the tallest plate morph (scale = 10cm)

Picture Credit: PLoS One

The plate shapes suggest sexual dimorphism in Stegosaurs, males and females evolved different shaped plates.  But which is which?  The Bristol based scientist cannot say for sure whether the males or the females had the broader, larger plates however he speculates that the boys had the bigger more rounded plates.

“We know from modern animals that males typically invest more into their ornaments than do the females.  In this case, the broader variety reaches sizes 45% larger in surface area than do the tall plates.  And I argue that these wide plates would create a great ‘billboard’ for male Stegosaurs if they were using them to attract a mate.”

If the fossils represent a single species, then sexual dimorphism seems the “best fit” for an explanation.  Analysis of the fossil material indicates that the animals were fully grown and so the shape of the plates cannot be put down to dinosaurs at different stages of growth.  Plates from all positions on the body can be classified as being of the rounded or taller shape so the variation cannot be due to the location of the plate on the animal.  Other isolated Stegosaur fossils found in Wyoming and ascribed to the same genus/species support the idea of sexual dimorphism in plate shape as these specimens too show only one plate shape.

Evan Saitta Cutting a Thin Section of Stegosaur Plate for Ontogenetic Analysis

Evan Saitta prepares a section of plate for cross-sectional study.

Evan Saitta prepares a section of plate for cross-sectional study.

Picture Credit: Judith River Dinosaur Institute

A spokesperson from Everything Dinosaur commented:

“This is a remarkable study.  We made one of our “palaeontological predictions” that 2015 would bring Stegosaurus into the limelight once more and this new research provides tantalising evidence of sexual dimorphism in Ornithischian dinosaurs.  We can speculate that the plates evolved as defensive armour or possible temperature regulators and over countless generations of females selecting mates with the most impressive plates, two distinct sets of plates formed in the species.  At first glance the spiky, taller plates associated with the females may have offered better defence against predator attack.  The more rounded plates ascribed to the males may not have been as effective.  The male Stegosaurs sacrificing some defensive capability in order to have a more impressive visual display to woo a female.  Although sexual dimorphism is not common in extant reptiles and birds, there are examples seen today.  For instance, the male peacock has sacrificed the ability to camouflage itself from predators by evolving a very obvious, huge tail feather display, all for the purpose of attracting a mate.”

Female peacocks may have influenced the tail length and plumage of the males by selecting males in the peacock population that had slightly longer tails and more flamboyant plumage. These traits are then passed onto offspring and this form of sexual selection by females leads to more ornate tail feathers in males as they have an advantage in terms of mating success.  It could be inferred that sexual selection by female Stegosaurs led to the evolution of larger, broader plates on the backs of the males.  The bigger more rounded plates providing the males with an advantage when it comes to mating, despite being less helpful when it came to fending off an attack from a Theropod.

The conclusions from this study are likely to be debated for a very long time.  This new paper provides a new “angle” on the purposes of those enigmatic plates and further research into “Sophie” the Stegosaurus stenops specimen at the Natural History Museum in London may help to “round off” the debate.

To read more about “Sophie” the Stegosaurus specimen exhibited at the Natural History Museum: All about “Sophie”

To see the list of Everything Dinosaur’s “palaeontology predictions” for 2015: Palaeontology and Fossil Predictions for 2015

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