Thursday, May 15, 2014

Sherlock Holmes And Long Term Evolutionary Patterns In Dinosaur Body Size

This week's (in Pune, India) Sherlock Holmes Elementary featured a murder mystery involving a smuggled dinosaur fossil. The fossil is believed by some palaeontologists to be entombed in rocks of the earliest Paleocene, making that dinosaur a survivor of the end Cretaceous mass extinction. Other palaeontologists strongly disagree with this survivor fauna scenario, which the drama turns into a motive for murder. There was a fair amount of geology and palaeontology in the episode and a reference to a term "dead clade walking".  This term is real life was coined by David Jablonski a palaeontologist from University of Chicago.  The term means that a clade or a lineage has survived a mass extinction but its fate has been sealed. Over a period of time say a few million years after the mass extinction that group does eventually go extinct.

Some dinosaur species may have survived the mass extinction but by early Palaeocene they were certainly all gone... except one lineage.. the Availea or birds. They prospered in the Cenozoic, radiating into a hundreds of species. Their success may have had deep roots and one important factor may have been the small size of their ancestors.

A paper in PLOS Biology explores the long term evolutionary patterns of dinosaurs and finds a positive relationship between high rates of evolution and small size-

Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage:

Author Summary

Animals display huge morphological and ecological diversity. One possible explanation of how this diversity evolved is the "niche filling" model of adaptive radiation—under which evolutionary rates are highest early in the evolution of a group, as lineages diversify to fill disparate ecological niches. We studied patterns of body size evolution in dinosaurs and birds to test this model, and to explore the links between modern day diversity and major extinct radiations. We found rapid evolutionary rates in early dinosaur evolution, beginning more than 200 million years ago, as dinosaur body sizes diversified rapidly to fill new ecological niches, including herbivory. High rates were maintained only on the evolutionary line leading to birds, which continued to produce new ecological diversity not seen in other dinosaurs. Small body size might have been key to maintaining evolutionary potential (evolvability) in birds, which broke the lower body size limit of about 1 kg seen in other dinosaurs. Our results suggest that the maintenance of evolvability in only some lineages explains the unbalanced distribution of morphological and ecological diversity seen among groups of animals, both extinct and extant. Important living groups such as birds might therefore result from sustained, rapid evolutionary rates over timescales of hundreds of millions of years.

In general the rapid-evolvers would be the smallest-bodied species -- the ones that reach reproductive age quickly and while they are still small. Rates of evolution depend on generational time. Which predicts that large-bodied/long-generation-time species would have more difficulty adapting to rapidly-changing extinction conditions.

The authors also mention that body size evolution in many non-avian dinosaur lineages seem to follow Cope's Rule, an increase in body size of descendant species over time. The earliest species in that lineage would be small and over time there would be a trend towards evolution of larger bodied species.

What could cause such a trend? Is larger body size advantageous and hence being favored by natural selection? Again the relationship to mass extinctions is intriguing. The authors mention that during the late Triassic mass extinction many branches of dinosaurs became extinct. If larger bodied dinosaur species were disproportionately killed off the survivor species of dinosaurs in early Jurassic would have been small bodied. This could explain Cope's Rule in a rather novel way (as explained by my adviser Anthony Arnold - who has worked on size evolution in foraminfera - in an email to me) - "since it means that rather than evolution favoring size increase, mass extinction selectively removes larger-bodied species, leaving behind the smaller guys as survivors. Since they don't have much room to get smaller the survivors could even speciate at random and the only direction in which the variance has room to expand is toward larger size.

Maybe!.. arm waving.."

So ecological crises may lead to species sorting based on size with smaller size being favored and then evolution of a trend towards larger size that reflects simply an inability to become any smaller!..

No comments:

Post a Comment