Sea Water Chemistry And Shell Mineralogy: Tales Of Mesozoic Bivalves

Years ago when I was in the second year of college, I along with friends, went for a fossil collection tour to the town of Ariyalur in South India. Rocks of Cretaceous age outcrop all around, and these strata have now become one of the most famous fossil localities in India.

We collected ammonites, echinoids, plant leaf impressions on clay and bivalves... lots and lots of bivalves.. In the picture below are the remains of my collection of molluscs. On the top left is an oyster with a clam clinging on to one of its valves. Bottom left is another oyster with its jagged valve margin. In the middle is a largish clam and to the right is an oyster whose layered shell structure is clearly visible.

I have some photomicrographs too taken from thin sections given to me by a friend.

In the above image the foliated shell microstructure of a piece of a bivalve can be clearly seen in cross polarized light.

And in the image below, a coarser prismatic crystal structure of a shell fragment is visible in the center of the image.

Most molluscs groups (including bivalves) in today's tropical seas built their skeletons using the CaCO3 polymorph aragonite. I say tropical seas, because molluscs with calcite skeletons are more common in temperate waters, such as for example in the marine communities living on the continental shelf of the southern coasts of Australia. In the Cretaceous seas though, even at tropical latitudes, calcite bivalves were common. In this apparent puzzle lies a very interesting story of climate change, sea floor spreading, changing sea water chemistry, the evolutionary decline and success of different bivalve groups during the Mesozoic, the emergence of bivalve reefs and the localization of hydrocarbon reservoirs.

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