Monday, November 30, 2015

Dinilysia's inner ear suggests it was a burrowing species, implications for the evolution of snakes

Modern snake skull, with inner ear shown in orange. 
Photo Credit: Hongyu Yi
Modern snakes probably originated as habitat specialists, but it is controversial unclear whether they were ancestrally terrestrial burrowers or marine swimmers. In a new paper Yi and Norell (2015)  use x-ray virtual models of the inner ear to predict the habit of Dinilysia patagonica, a stem snake closely related to the origin of modern snakes. Previous work has shown that modern snakes perceive substrate vibrations via their inner ear. The study's  data show that D. patagonica and modern burrowing squamates share a unique spherical vestibule in the inner ear, as compared with swimmers and habitat generalists. The authors built predictive models for snake habitats based on their vestibular shape, which estimated D. patagonica and the hypothetical ancestor of crown snakes as burrowers with high probabilities. This study provides an extensive comparative data set to test fossoriality quantitatively in stem snakes, and it shows that burrowing was predominant in the lineages leading to modern crown snakes.

Comparisons between CT scans of the fossil and modern reptiles indicate that snakes lost their legs when their ancestors evolved to live and hunt in burrows, which many snakes still do today.

The findings suggest snakes did not lose their limbs in order to live in the sea, as has been previously suggested.

Scientists used CT scans to examine the bony inner ear of Dinilysia patagonica, a 2-metre long reptile closely linked to modern snakes. These bony canals and cavities, like those in the ears of modern burrowing snakes, controlled its hearing and balance.

They built 3D virtual models to compare the inner ears of the fossils with those of modern lizards and snakes. Researchers found a distinctive structure within the inner ear of animals that actively burrow, which may help them detect prey and predators. This shape was not present in modern snakes that live in water or above ground.

The findings help scientists fill gaps in the story of snake evolution, and confirm Dinilysia patagonica as the largest burrowing snake ever known. They also offer clues about a hypothetical ancestral species from which all modern snakes descended, which was likely a burrower.

Dr Hongyu Yi, of the University of Edinburgh's School of GeoSciences, who led the research, said: "How snakes lost their legs has long been a mystery to scientists, but it seems that this happened when their ancestors became adept at burrowing. The inner ears of fossils can reveal a remarkable amount of information, and are very useful when the exterior of fossils are too damaged or fragile to examine."

Mark Norell, of the American Museum of Natural History, who took part in the study, said: "This discovery would not have been possible a decade ago -- CT scanning has revolutionised how we can study ancient animals. We hope similar studies can shed light on the evolution of more species, including lizards, crocodiles and turtles."

Citation

Yi, H. &  M.A. Norell. The burrowing origin of modern snakes. Science Advances, 2015; 1 (10): e1500743 DOI: 10.1126/sciadv.1500743

Sunday, November 29, 2015

Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs

A dinosaur nest. Photo credit: Kohei Tanaka
Extinct archosaurs' eggshell porosity may be used as a proxy for predicting covered or exposed nest types, according to a study published November 25, 2015 in the open-access journal PLOS ONE by Kohei Tanaka from the University of Calgary and colleagues.

Knowledge about dinosaur nests may provide insight into the evolution of nesting and reproductive behaviors among archosaurs, a group that includes living birds and crocodilians, as well as extinct dinosaurs. Unfortunately, little remains of prehistoric nests, and most information on extinct archosaurs is only gleaned indirectly through comparison with living relatives. Among extant archosaurs, two general types of nests are observed: open nests, where the eggs are uncovered and built by species that brood their eggs; and covered nests, built by species that incubate their eggs using external heat sources. Scientists try to infer the type of nest by looking at different characteristics of the eggs and the nest setting. The authors of this particular study proposed a statistically rigorous approach to infer nest type based on large datasets of eggshell porosity and egg mass compiled for over 120 extant archosaur species and 29 extinct archosaur taxa.

The researchers found a strong correlation between eggshell porosity and covered or exposed nest types among extant archosaurs, which indicates that eggshell porosity may be used as a proxy for nest type, which may help predict nest type in extinct taxa. Their results show that covered nests were likely used by more primitive dinosaurs, and the transition of theropods from covered to uncovered nests may have allowed the exploitation of alternate nesting locations. These changes in nesting styles may have lessened the odds of nesting failure due to predation, flooding, or torrential rainfall, and may have played a role in the evolutionary success of maniraptorans, including birds.

Citation
Kohei Tanaka, Darla K. Zelenitsky, François Therrien. Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs. PLOS ONE, 2015; 10 (11): e0142829 DOI: 10.1371/journal.pone.0142829

Tuesday, November 10, 2015

The evolution of venom, a new perspective


This is an Anderson's Pitviper (Trimeresurus andersoni), a venomous 
snake in the Andaman Islands of India. Photo Credit: Dr. Kartik Sunagar

In a new study published in  PLOS Genetics, scientists at the Hebrew University of Jerusalem have revealed new discoveries about the evolution of venom. The research points to a 'two-speed' evolution of animal venom, showing for the first time the significant roles played by different forces of natural selection.

Venom is a complex mixture of proteins and other toxic chemicals produced by animals such as snakes and spiders, either to incapacitate their prey or to defend against predators. The influence of positive selection (the process by which a protein changes rapidly over evolutionary time scales) in expanding and diversifying animal venoms is widely recognized.

This process was hypothesized to result from an evolutionary chemical arms race, in which the invention of potent venom in the predatory animals and the evolution of venom resistance in their prey animals, exert reciprocal selection pressures.

In contrast to positive selection, the role of purifying selection (also known as negative selection, which is the selective removal of deleterious genetic changes from a population) has rarely been considered in venom evolution.

Moreover, venom research has mostly neglected ancient animal groups in favor of focusing on venomous snakes and cone snails, which are both "young" animal groups that originated only recently in evolutionary timescales, approximately 50 million years ago. Consequently, it was concluded that venom evolution is mostly driven by positive selection.

In the new study, Dr. Yehu Moran at the Hebrew University's Department of Ecology, Evolution and Behavior and the guest scientist Dr. Kartik Sunagar examined numerous venom genes in different animals in order to unravel the unique evolutionary strategies of toxin gene families.

The researchers analyzed and compared the evolutionary patterns of over 3500 toxin sequences from 85 gene families. These toxins spanned the breadth of the animal kingdom, including ancient venomous groups such as centipedes, scorpions, spiders, coleoids (octopus, cuttlefish and squids) and cnidarians (jellyfish, sea anemones and hydras).

Unexpectedly, despite their long evolutionary histories, ancient animal groups were found to have only accumulated low variation in their toxins.

The analysis also revealed a striking contrast between the evolution of venom in ancient animal groups as compared to evolutionarily "young" animals. It also highlighted the significant role played by purifying selection in shaping the composition of venoms.

According to Dr. Yehu Moran, "Our research shows that while the venoms of ancient lineages evolve more slowly through purifying selection, the venoms in more recent lineages diversify rapidly under the influence of positive selection."

The findings enable the postulation of a new theory of venom evolution. According to this theory, toxin-producing genes in young venomous groups that enter a novel ecological niche, experience a strong influence of positive selection that diversifies their toxins, thus increasing their chances to efficiently paralyze relevant prey and predatory species in the new environment.

However, in the case of the ancient venomous groups, where the venom is already "optimized" and highly suitable for the ecological niche, the venom's rate of accumulating variations slows down under the influence of purifying selection, which preserves the potent toxins generated previously.

The proposed "two-speed" mode of venom evolution highlights the fascinating evolutionary dynamics of this complex biochemical cocktail, by showing for the first time the significant roles played by different forces of natural selection in shaping animal venoms.

According to Drs. Moran and Sunagar, "The 'two-speed' mode of evolution of animal venoms involves an initial period of expansion, resulting in the rapid diversification of the venom arsenal, followed by longer periods of purifying selection that preserve the now potent toxin pharmacopeia. However, species that have entered the stage of purification and fixation may re-enter the period of expansion if they experience a major shift in ecology and/or environment."


Citation

Kartik Sunagar & Yehu Moran. The Rise and Fall of an Evolutionary Innovation: Contrasting Strategies of Venom Evolution in Ancient and Young Animals. PLOS Genetics, 2015; 11 (10): e1005596 DOI: 10.1371/journal.pgen.1005596

Sunday, November 8, 2015

The origin of large body size and herbivory in giant Canary Island lacertids

Gallotia galloti. Photographer's Credit: Petermann
2005, via Wikimedia Commons
The Island Rule on body size claims big animals  evolve smaller body sizes due to the limited resource of food and limited habitat; while smaller animals that have no natural enemies in islands evolve larger body sizes.

 Andrej Čerňanský and colleagues discovered a fossil related to the genus Gallotia which is endemic to the  Canary Islands. They named it Janosikia ulmensis, after the Slovak national hero, outlaw Juraj Jánošík. Zoological Journal of the Linnean Society reports the results. The genus Gallotia has not been growing in size on the island – quite the contrary. The Gallotia ancestor came to the island with a large body size.

Until now, the genus Gallotia was considered a clear example of island gigantism; scientists supposed that a small lizard resembling the North African species Psammodromus colonised the Canary islands 20 to 18 million years ago. As it did not have any natural enemies, it supposedly gradually increased its body size several times. But the fossil record illustrating this  was absent.

“Currently, we have been able to do research, for the first time, on fossils and almost complete findings from about 22 million years old which are in the line leading to the lizards from the Canary Islands. Our fossil comes from Germany and precedes the period of the islands’ colonisation. It also shows us a completely different story, proving that the evolution of body size is much more complex than we had originally thought,” Čerňanský explained.

This stems from a whole set of anatomic features – like the huge size, as the skull alone was almost five centimetres long, the Slovak researcher said. This shows that this line had already grown to this size on the continent before colonising the islands; some other current species from the islands, on the other hand, represent markedly smaller animals. Thus, Janosikia turned the original idea upside down.

The fossil is crucial for understanding the island rule of evolution of body size, and also the herbivory of today’s large species of lizards. It also confirms the assumption of molecular biologists that the line leading to the genus Gallotia has its origin on the European continent; and, last but not least, it is one of the best preserved lizards from the Tertiary that reveals many important, so far unknown aspects of the evolution of dominant group of reptiles in Europe – the family Lacertidae, Čerňanský summed up.

Janosikia ulmensis is from the early Miocene of Ulm, Germany (∼22 Mya). The authors show that this species and the Oligocene Pseudeumeces cadurcensis (Filhol, 1877) are in fact crown lacertids, and the first known pre-Quaternary record of the total clade of Gallotia. Pseudeumeces confirms the early origin of crown Lacertidae in the Palaeogene of Europe. More importantly, these fossil taxa show that large body size was already achieved on the European mainland by the early Miocene. Furthermore, Pseudeumeces and Janosikia were faunivorous, thus demonstrating that insularity, not large body size, was crucial to the evolution of herbivory in this lineage. Body size change in Gallotia was more complex than previously thought, encompassing size increase [e.g. in the extinct Gallotia goliath (Mertens, 1942)], but more commonly involving miniaturization. The physical environment may play a crucial role in modulating the evolution of body size.

Čerňanský, A., Klembara, J. and Smith, K. T. (2015), Fossil lizard from central Europe resolves the origin of large body size and herbivory in giant Canary Island lacertids. Zoological Journal of the Linnean Society. doi: 10.1111/zoj.12340.