June 30 , 2016

by matthewbonnanadmin

The success of dinosaurs, and all other vertebrates for that matter, was and is predicated on sexual reproduction. In most cases, male and female individuals in a population of a particular species must come together and mate. There are some exceptions – for example, some populations of lizards are all females but can reproduce through a process known as parthenogenesis (Gilbert, 2010). But can you tell males from females in the fossil record? That answer depends on what vertebrates we’re talking about. For example, Chondrichthyans, the cartilaginous fishes which include sharks, stingrays, and their relatives, reproduce via internal fertilization (Liem et al., 2001). Male chondrichthyans possess claspers, cartilaginous extensions of their pelvic girdle that are inserted into the cloaca (urogenital opening) of the female to inject packets of sperm (Jones and Jones, 1982). Females lack these reproductive structures. Therefore, it is possible to sex fossil chondrichthyans based on the presence or absence of claspers (Lund, 1985; Benton, 2005). It is also possible to sex fossil humans on skeletal differences, particularly on the basis of pelvic structure: the human female pelvis is wider and has a larger sciatic notch angle than those of males (Jurmain et al., 2009).

But for dinosaurs, and most reptiles and birds for that matter, the structure of the skeleton itself does not offer up many clues to help us sort male from female. Dinosaurs did not give live birth but instead laid shelled eggs as do their living relatives the birds and crocodylians (Fastovsky and Weishampel, 2012). Thus, there is no known morphological difference in the pelvis between male and female archosaurs. Even when there is size sexual dimorphism (meaning the male or female sex is significantly larger than the other), it can be difficult to determine this from the skeleton alone. In fact, in my own research with Jim Farlow and Simon Masters, we investigated femur shape across one-hundred and one alligators (Bonnan et al., 2008). Alligators have sex sexual dimorphism, where males are significantly larger and more robust than females. In our study, all the femora (the plural of femur) came to us from deceased alligators of known age, sex, and (if female) reproductive status (had they laid eggs, had they ever been gravid [that is “pregnant” or carrying eggs]). Although we found there to be a difference in femur shape between male and female alligators, this difference only accounted for ~3% of the total shape change in the sample (Bonnan et al., 2008). And this was only really possible to figure out because we knew for certain the age, sex, and reproductive status of the alligators in question. If this pattern holds for dinosaurs, it is doubtful that even if a subtle difference in bone shape were present that we would ever know it was there or whether it truly reflected size sexual dimorphism.

However, many reptiles (including birds) do something pretty amazing: they tap into their own bones to shell their eggs (Wink and Elsey, 1986; Schweitzer et al., 2007). You’ve probably never stopped to think about where the calcium for building the skeleton comes from in developing reptiles and birds, but it has to come from somewhere. That somewhere is the egg shell itself (Blom and Lilja, 2004). And where does that calcium come from in the first place? You guessed it: the mother’s own skeleton! Now, in turtles and alligators, the calcium is stripped directly from the bones themselves (Wink and Elsey, 1986; Schweitzer et al., 2007), but in birds something truly remarkable happens – a specialized bone, called medullary bone, is formed which can act as a source of calcium for shelling eggs (Schweitzer et al., 2007). The bone is called medullary because it forms in the so-called “marrow” cavity of the long bones (like the femur) and the technical term for this hollow space is called the medullary cavity. In other words, birds do not strip calcium directly from their bones but instead lay down a specialized bone tissue inside their long bones and other parts of their skeletons (Chinsamy et al., 2016) whose primary function is to be tapped for calcium when eggs are shelled.

What does this all mean? What it means is that only female bird skeletons possess medullary bone since it is necessary for shelling their eggs. In contrast, male bird skeletons would not possess medullary bone because, well, they’re not in the business of laying eggs. Now, birds are dinosaurs (Fastovsky and Weishampel, 2012), and so it might be hypothesized that female non-avian dinosaurs were also in the business of producing medullary bone to shell their eggs as well. As it turns out, Mary Schweitzer and her colleagues found out that, yes, indeed, medullary bone did form in at least one non-avian dinosaur: none other than Tyrannosaurus rex itself (herself?) (Schweitzer et al., 2005)! This was discovered through the careful study of fossil bone histology, the creation of thin slices of bone that can be examined under the microscope and compared to those of modern animals. The implication is you can, indeed, sex a T. rex. The discovery of medullary bone and soft tissues in Tyrannosaurus rex is a remarkable achievement whose history goes well beyond the scope of this blog post, so I strongly encourage those interested to read Mary Schweitzer’s own account of this fascinating story (Schweitzer, 2010).

Since Schweizter and her colleagues’ discovery, medullary has also been reported in another predatory dinosaur, Allosaurus, and two ornithischian dinosaurs (from the ornithopods which include the so-called ‘duck-billed’ dinosaurs) (Lee and Werning, 2008; Hübner, 2012). Thus, it may very well be possible to identify particular individual non-avian dinosaurs as female without relying on deciphering more subtle cues from skeletal morphology. Moreover, since medullary bone can only occur in sexually mature individuals, it is possible in some cases to determine the minimum age at which sexual maturity was reached in certain non-avian dinosaurs (Lee and Werning, 2008). These data have further enhanced our understanding of the growth patterns in non-avian dinosaurs, and suggest that many of these animals grew at rates equivalent to those of birds and mammals (Lee and Werning, 2008).

When these types of patterns begin to emerge, it is easy and tempting to conclude that all female non-avian dinosaurs, like birds, laid down medullary bone to shell their eggs. It would also seem like a “slam dunk” that finding medullary bone in non-avian dinosaur skeletons tells you that you have a female individual. However, a new study by Anusuya Chinsamy and colleagues on medullary (?) bone in a sauropod dinosaur provides a cautionary counterpoint about drawing such conclusions too quickly (Chinsamy et al., 2016). Chinsamy is herself an expert on dinosaur bone histology, and, like Schweitzer, has thoroughly examined many slices of fossil bone from reptiles to birds to non-avian dinosaurs to mammals (Chinsamy-Turan, 2005, 2011; Chinsamy and Hurum, 2006).

As a side note, I have had the privilege and pleasure to work with Anusuya when she helped our team decipher the age and growth rate of a then new transitional sauropodomorph dinosaur, Aardonyx, from South Africa (Yates et al., 2009).

What Chinsamy and her colleagues have found is that medullary bone and other forms of endosteal bone (bone that forms inside long bones, vertebrae, and so forth) are sometimes indistinguishable from one another (Chinsamy et al., 2016). Moreover, endosteal bone can form from injuries or stress and thus be vascularized (that is, invaded by blood vessels), another key characteristic used to distinguish bird medullary bone from other forms of bony deposits (Chinsamy et al., 2016). The upshot of all of this is that Chinsamy et al. (2016) found what would be considered medullary bone in an armor scute, a toe bone, and a tail vertebra of a sauropod dinosaur. Thus, one conclusion would be that, here, too, was evidence for the identification of a female individual non-avian dinosaur. However, it is also possible that the presumed medullary bone is in fact not related to shelling eggs and is instead another type endosteal bone formed for pathological reasons or due to stress (Chinsamy et al., 2016).

Disagreement in science is often viewed as a weakness. There is the tendency when scientific disagreements occur for outsiders to throw up their hands and say, “There they go again – we’ll never figure this out!” or “Those paleontologists never agree on anything … its all speculation!” However, the opposite is, in fact, true – that is that disagreements over data in any branch of science, including vertebrate paleontology, is a sign of a discipline’s health. The more we understand about what we do know and what remains unclear, the stronger the science becomes. This is because such disagreements often inspire additional research, either by the original investigators or by other scientists (including undergraduate and graduate students), that then push the envelope of our understanding further and which often uncover previously unknown information. It is this constant tension between finding consilience and disagreement over the interpretation of data that keeps a field like vertebrate paleontology active, vibrant, and strong.

So, can we truly sex T. rex? Will we unravel the clues to understanding how to identify sex in non-avian dinosaurs and other fossil reptiles? Like all things in vertebrate paleontology, time and fossils will tell, as will the patience, persistence, and tenacity of the scientists who pursue these questions.

Submitted by: Matthew F. Bonnan, Stockton University

References Cited

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Blom, J., and C. Lilja. 2004. A comparative study of growth, skeletal development and eggshell composition in some species of birds. Journal of Zoology 262:361–369.

Bonnan, M. F., J. O. Farlow, and S. L. Masters. 2008. Using linear and geometric morphometrics to detect intraspecific variability and sexual dimorphism in femoral shape in Alligator mississippiensis and its implications for sexing fossil archosaurs. Journal of Vertebrate Paleontology 28:422–431.

Chinsamy, A., and J. Hurum. 2006. Bone microstructure and growth patterns of early mammals. Acta Palaeontologica Polonica 51:325–338.

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Yates, A. M., M. F. Bonnan, J. Neveling, A. Chinsamy, and M. G. Blackbeard. 2009. A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism. Proceedings of the Royal Society B: Biological Sciences 277:787–794.