Maniraptoran Dinosaurs Show No Decline In Disparity Before Mass Extinction

Its hard to unravel and unpack complex phenomenon like patterns of faunal turnover during mass  extinctions. The methods chosen, the materials (fossils) available for study and the granularity of the study influences the results.

My last post was about a modeling study that concluded that for 40 million years before the mass extinction,  extinction rates exceeded the evolution of new species in many dinosaur lineages. Dinosaur diversity was declining towards the end of  the Cretaceous. Only a few herbivorous dinosaurs showed an increase in diversity.

Now a different study by Derek W. Larson, Caleb M. Brown , David C. Evans published in Current Biology focuses on just one sub group  of dinosaurs and comes to a different conclusion. This is an analysis of over 3000 teeth  of bird-like maniraptoran dinosaurs from 18 stratigraphic units in Western N. America which shows that there was no decline in disparity in different maniraptoran lineages for the last 18 million years before the mass extinction. The maniraptors were largely omnivorous but may have included specialized carnivorous and as well as herbivorous species.

Disparity is a measure of the range of variation of morphology, in this case teeth shape. This in turn is something of a proxy for the range of ecologic niches being exploited.

The authors remark-

Overall, these inferred ecomorphological patterns indicate that toothed maniraptorans, including toothed birds, were ecologically diverse and stable leading up to the end-Cretaceous mass extinction, at least in North America.

An interesting angle explored  in the paper is why toothed bird-like maniraptors and some lineages of toothed birds died out, while their physiologically and morphologically close relatives, the ancestors of modern birds, the Neornithes,  survived the mass extinction.

In the fallout of the bolide impact that marks the end of the Cretaceous, terrestrial food webs that relied on photosynthesis would have collapsed. However, seed banks derived from plants, including relatively abundant angiosperms, could have been a common, nutrient-rich resource that would have persisted among the detritus, which itself has been suggested as a key food source related to species survival across the boundary. With clearing of vegetation, either during a global firestorm or widespread burning of dead ground cover, exposed seeds in these areas worldwide could have been exploited by granivores. This pattern is observed in modern fire succession communities, where granivorous birds are the first avians to re-occupy disturbed habitats due to food resource accessibility. A persistent seed bank, which can remain viable for more than 50 years in modern temperate forests, most likely outlasted the catastrophic ecosystem collapse caused by a bolide impact and associated infrared thermal pulse, acid rain, darkness, and impact winter. Toothed maniraptorans, with their feeding systems adapted to faunivorous or potentially folivorous diets, would have been restricted to food chains dependent on living plant matter and unable to access these seed banks. Therefore, dietary specialization toward granivory in some lineages of crown group birds may have been one of the key factors in their survival through the end-Cretaceous mass extinction.

Although very similar to maniraptors, the Neornithes had evolved a unique dietary specialization. The presence of a beak, which served as a crushing apparatus, may have helped these ancestors of modern birds exploit seeds as a food source.

Fossils And The Origin And Diversification Of Birds

Stephen Brusatte and colleagues have a fine review article in Current Biology that brings together findings from the fossil record and molecular phylogeny work that throw light of the long history of bird evolution from Jurassic-Cretaceous to modern species.

Did you know .. that the remarkable Jehol Biota from northeastern China from 130  to 120 million years ago preserves thousands of bird fossils and accounts for nearly half of the global Mesozoic bird diversity? The Jehol Biota represents fossilization in wetland and lake sediments. The fine grained sediment size would have aided the superb preservation of these creatures. This fauna included small arborealists, semi-aquatic birds and large generalists but certain modern ecotypes like large aerial forages and aquatic specialists are not present. The End Cretaceous mass extinction produced empty ecologic niches for a greater diversity of bird forms to evolve.

..or that birds retain a single functional ovary and oviduct and a single oocycte is ovulated, shelled and laid per 24 hour cycle. Microstructural egg shell characteristics and small clutch size evolved incrementally in bird-like dinosaur ancestors who did retain two ovaries though. Earliest birds like Jeholornis and enantiornithines ( a basal group of birds) apparently had one ovary indicating that birds may have lost one ovary perhaps due to body lightening in response to the evolution of flight.

There is plenty of information in this essay on the long evolutionary history of bird like characters in dinosaur ancestors and the subsequent diversification of early (and now extinct) and post Cretaceous modern birds. It is not true that the end Cretaceous mass extinction affected only non-avian dinosaurs. Early birds had diversified into distinct groups by late Cretaceous and the mass extinction wiped out many of these lineages as well. Some lineages of the early birds (neornithines)did make it through the mass extinction. Molecular phylogeny indicates that all modern lineages formed within the first 15 million years after the extinction and then diversified quite rapidly and are today represented by 10,000 odd species.

But why write so much? The old adage " a picture is worth a thousand words" can be so true!

Just take a look at this lovely inforgraphic.

 Source: The Origin and Diversification of Birds

It summarizes the evolutionary relationship (phylogeny) of dinosaurs and birds and superimposes the evolution of traits that we recognize as typical of birds on a timeline from Triassic to the earliest birds (where Avialae/Aves branch out) . What we see is that these features evolved piecemeal over a 100 million year period in dinosaurs and some in the earliest birds, but not as a crazy spurt of morphological innovation that would have marked the geologically sudden appearance of a radically different creature.  Traits like bipedal posture, hollowed bones, wishbone, three fingered hands, wings and feathers, all appeared at successive stages in dinosaurs. Other traits like keeled breastbone to support flight muscles, endothermic metabolism and rapid growth, highly reduced tail, true muscle powered flight, and the loss of that one ovary, appeared in early bird lineages. The long story is that a lineage of therapod dinosaurs very gradually evolved bird-like characters so much so that experts find it difficult to separate bird-like dinosaurs from dinosaur-like birds.

Creationists smirk that experts can't even decide what is a bird and what is a dinosaur. Or that bird-like dinosaur fossils are younger than the earliest birds and so dinosaurs can't be the ancestors of birds. They are missing the point that the later appearance in the fossil record of bird-like dinosaurs is simply an artifact of preservation potential. This means that older bird-like dinosaurs haven't been found yet and that after birds branched out from dinosaurs the ancestral dinosaur lineage survived alongside birds. So, the sampled bird-like dinosaurs are not the direct ancestor species of birds but co-existing cousins. More importantly, the difficulty in taxonomic identification means that morphological transformations are being captured in the fossil record. This is strong evidence for evolution.

Meanwhile, just a thought from another paper I read recently on styles of diversification in the fossil record by Douglas Erwin. In the article he points out that the evolution of morphological novelty and spurts of diversification are often de-coupled  i.e.  novelty may arise in a species but that may not immediately result in an adaptive response in terms of exploitation of new resources and ecological space. Many novel traits that we recognize as typical of birds evolved much earlier in dinosaurs and it is not clear whether their evolution lead to an adaptive radiation in that dinosaur lineage. What did happen though is that at some point a threshold was crossed during maniraptoran (the closest relatives of birds) dinosaur evolution. We can think of  this as the dinosaur-bird transition. A collection of traits which had evolved piecemeal and under different evolutionary circumstances worked really well together and were co-opted and modified to serve different functions. The bird body plan then very successfully diversified into many different and new ecological roles.

Brusatte, S., O’Connor, J., & Jarvis, E. (2015). The Origin and Diversification of Birds Current Biology, 25 (19) DOI: 10.1016/j.cub.2015.08.003

Erwin, D. (2015). Novelty and Innovation in the History of Life Current Biology, 25 (19) DOI: 10.1016/j.cub.2015.08.019