Tuesday, January 29, 2013

[Paleontology • 2013] Sexual selection in prehistoric animals: detection and implications

Sexual dimorphism in pterosaurs.life reconstruction of the sexually dimorphic adult Darwinopterus showing the crested male and uncrested female (right).
reconstruction: Mark Witton. http://dx.doi.org/10.1016/j.tree.2012.07.015

Many fossil animals bear traits such as crests or horns that probably functioned as sexually selected signals or weapons. Interpretations of these structures as functioning in mate choice or intrasexual contests are often controversial, with interpretations based on biomechanics or physiology being favoured by many. Although testing hypotheses based on sexual selection can be difficult, especially given that there is no single, reliable means of recognising sexual selection, we argue that it is not impossible; indeed, there are now several cases where sexual selection is strongly supported. In other cases, a careful study of features such as sexual dimorphism, ontogeny, and allometry, coupled with testing of alternative hypotheses, will be necessary to distinguish between possible explanations for exaggerated features.

Keywords: fossil; prehistoric; sexual selection; ornament; weapon; crest; horn; exaggerated trait

Sex-related features of Darwinopterus. The male (right) has a large head crest, but this is absent in the female (left). The coloring of the animals is very uncertain, but the dramatic differences in crest structure are shown by the new fossil evidence reported in a study by Lu et al. [Art: Mark Witton]

Revealing ‘Fossil Sexual Selection’
| Sexual selection can be inferred from the fossil record

The term “sexual selection” refers to the evolutionary pressures that relate to a species’ ability to repel rivals, meet mates and pass on genes. One can observe these processes happening in living animals but how do paleontologists know that sexual selection operated in fossil ones?

Paleontologists have thought it challenging, even impossible, to recognize sexual selection in extinct animals. Many fossil animals have elaborate crests, horns, frills and other structures that look like they were used in sexual display but it can be difficult to distinguish these structures from those that might play a role in feeding behavior, escaping predators, controlling body temperature and so on.

However in the new study, the paleontologists argue that clues in the fossil record can indeed be used to infer sexual selection. “We see much evidence from the fossil record suggesting that sexual selection played a major role in the evolution of many extinct groups,” said study co-author Dr Darren Naish of the University of Southampton.

“Using observations of modern animal behavior we can draw analogies with extinct animals and infer how certain features improve success during courtship and breeding.”

Modern examples of sexual selection, where species have evolved certain behaviors or ornamentation that repel rivals and attract members of the opposite sex, include the male peacock’s display of feathers, and the male moose’s antlers for use in clashes during mating season.

“The fossil record holds many clues that point to the existence of sexual selection in extinct species, for example weaponry for fighting, bone fractures from duels, and ornamentation for display, such as fan-shaped crests on dinosaurs. Distinct differences between males and females of a species, called ‘sexual dimorphism’, can also suggest the presence of sexual selection, and features observed in sexually mature adults, where absent from the young, indicate that their purpose might be linked to reproduction.”

“Some scientists argue that many of the elaborate features on dinosaurs were not sexually selected at all,” Dr Naish said.

“But as observations show that sexual selection is the most common process shaping evolutionary traits in modern animals, there is every reason to assume that things were exactly the same in the distant geological past.”
Specimen ZMNH M8802, a female individual of Darwinopterus, from the Tiaojishan Formation (upper Middle-lower Upper Jurassic) of Liaoning Province, China.
(A) Partially prepared, exposing the skull, neck and left forelimb, but with the egg still almost completely encased in the rock. (B) Fully prepared with the entire skeleton and egg now completely exposed. [photo: Junchang Lü̈]

Fossil Sheds Light on Pterosaur Sex and Reproduction

Researchers in China have discovered an adult pterosaur fossil preserved together with one of its eggs. The pterosaurs, also known as pterodactyls, were winged reptiles that lived during the Jurassic period, and the presence of an egg with one of their fossils indicates that this pterosaur was definitely female. Now, researchers report details of the egg and compare the males and females of this pterosaur species, Darwinopterus.

Junchang Lü from the Chinese Academy of Geological Sciences in Beijing, along with colleagues in China and the United Kingdom, excavated the Darwinopterus fossil from sedimentary rocks in China’s Liaoning Province, and they estimate its age to be approximately 160 million years old. The nearly complete skeleton is described in the 21 January issue of the journal Science.

The discovery helped researchers learn more about male Darwinopterus by comparing previously discovered fossils with the new female fossil. The new find confirms that males had relatively small pelvises and large cranial crests, or extensions of bone on the top of their skulls. The females, on the other hand, had much larger pelvises but no cranial crests, according to the researchers.

“Darwinopterus females had no crest at all,” said Lü. “In the males, it was long and low-extending—from just above the nasal opening to the apex of the skull, just behind the orbit... Pterosaur males had a huge array of different kinds of head crests, varying from small keels on the jaws to huge sail-like structures that might reach as much as five times the height of the skull.”

Details of the fossilized egg imply that the pterosaur’s reproductive strategies were not like those of birds, as most researchers had previously suspected, but more like those of crocodiles or other reptiles. Lü and the team of researchers say that the egg is relatively small compared to the pterosaur’s body and that it was likely soft and covered in a parchment-like shell.

They note that, today, birds the size of a pterosaur would likely produce eggs nearly three times the size of this fossilized egg, since rigid bird shells must contain all the resources needed to sustain the developing embryo inside. Instead, the soft, fossilized shell suggests that Darwinopterus females buried their eggs like reptiles, paying little attention to them as they soaked up nutrients from the ground.

“The eggs could absorb water from their environment, and they likely increased in size and mass during this period—possibly doubling the original mass by the time they hatched,” said Lü. “This also means that the adult did not have to invest so much water in the egg, which could have been quite advantageous: Less material investment in the egg and less mass to carry around while the egg was being produced in the female’s body.”

And how did this one particular pterosaur meet its fate in order to become such a beautifully preserved fossil? The researchers have a few ideas about that too.

“It seems that the left forearm is broken, so we suspect that Mrs. T—that’s our nickname for this specimen—had an accident, perhaps in a storm or possibly as a result of a volcanic eruption, which were very common in this part of China at the time,” said Lü. “She ended up in the water—in a lake—and then sank to the bottom, drowning on the way. The egg was expelled from the body as Mrs. T began to decay.”

Revealing ‘Fossil Sexual Selection’ 

Fossil Sheds Light on Pterosaur Sex and Reproduction

Robert J. Knell et al. 2013. Sexual selection in prehistoric animals: detection and implications. Trends in Ecology & Evolution, 28(1): 38-47. doi: http://dx.doi.org/10.1016/j.tree.2012.07.015