Tuesday, September 29, 2015

Eukaryote Evolutionary Dynamics Through The Proterozoic


No, attention on evolution need not only be on dinosaurs, mammals and other large creatures. The Proterozoic, which is dominated by microscopic life also has a very interesting evolutionary story to tell.

Take a look  at this absolutely lovely info-graphic showing number and type of non-metazoan  eukaryote fossils in sampled stratigraphic units through the Proterozoic.

This was compiled by  Phoebe A. Cohen and Francis A. Macdonald from a literature survey of described fossil assemblages of non-metazoan Eukaryotes from Proterozoic stratigraphic sections from all over the world. The results show a dynamic living world in which biodiversity of eukaryotes fluctuated, tracking global ecological triggers. The bottom panel showing lithology in which fossils are found is important. You can see that mudstone (shale) dominates. Certain types of organisms favor certain types of sediment. Also, certain parts of organisms are better preserved in certain sediment types. This means that lithology can introduce a bias to the fossil record. There is plenty of sandstone in the Proterozoic. But soft bodied organisms don't get preserved too well in sandstone.  Carbonates are not very well represented too as host lithology in the fossil assemblages recorded. The Proterozoic has lots of limestone and dolostone sequences, but just like sandstone may not preserve soft bodied organisms as well as mudstones do. Proterozoic limestones generally have been found to contain the tougher recalcitrant fragments of eukaryote cells, so there is scope for carbonate sequences to be examined in more detail for their fossil content. Moving on, take a look at the distribution of phosphatic rocks. They appear in small time windows in the Tonian in early Neoproterozoic around 1000 million years ago and then much later in the Ediacaran beginning around 630 million years ago. Phosphatic minerals preserve fine details of soft tissue, but such type of preservation is restricted to only thin time slices.

Paleogeography may also bias the fossil record. Some locations may have accumulated the right types of sediments at the right time intervals. Location with respect to nutrient inputs and basin configuration may result in peculiar or endemic biota in that particular region. Add to that is a sampling bias. Deposits in some countries as just better studied resulting in a better fossil record. A journalist asked me recently why is the Vindhyan Basin in Central India so rich in fossils. This was in the context of the Proterozoic fossil record. The answer is that Vindhyans are not the exception. There is an improving fossil record from many Indian Proterozoic basins. A recent review by Mukund Sharma and colleagues summarizes this record...  I suspect though that the Vindhyan fossil record has been examined in a more systematic stratigraphic context so as to discern macro evolutionary trends.  An excellent paper by Purnima Srivastava  summarizes such evolutionary trends in the Proterozoic fossil record of the Vindhyans. She documents simpler prokaryote communities and moderately diversified megascopic unicellular eukaryotes in the early parts of the Proterozoic. In the Neoproterozoic she documents  more diverse and complex megascopic eukaryotes including the emergence of multicellular plant (bryophytes and sporophytes) and animal clades (Ediacaran fauna).

Source: Srivastava 2012

Explanation-  Megafossils from the Vindhyan Supergroup. (a) Association of Chuaria and Tawuia comparable with Jacutianema from the Rewa Group. (b) Dichotomous branching from the Samaria Shale, Bhander Group. (c) Association of Chuaria and Tawuia comparable with Jacutianema from the Rewa Group. (d) Carbonaceous disc with a cluster of small spheroids or a scale of some metazoan, Dholpura Shale, Bhander Group. (e) Carbonaceous vesicle with dinoflagellate-like features and two notches, Samaria Shale, Bhander Group. (f ) Chuaria-like carbonaceous discsarranged within a Tawuia-like elongated vesicle, Sirbu Shale, Bhander Group. (g) Close view of the hold fast-like  structure of (f ). (h) Chuaria with prominent and well-preserved inner body/nucleus and outer ring, Sirbu Shale, Bhander Group. (i) Carbonaceous ring, a part of the hold fast, according to the model proposed by Kumar (2001) for a multicellular plant from the Sirbu Shale, Bhander Group. ( j) Chuaria with spines/notches/ budding, Samaria Shale, Bhander Group. (k) Carbonaceous disc with two spines/processes, Samria Shale, Bhander Group. (l) Very thin filaments exhibiting branching, Rohtas Formation, Semri Group, Lower Vindhyans. (m) Small carbonaceous disc, with an umbrella-like protrusion (Sirbu Shale), comparable with the problematic microfossil Kakabekia from the Gunflint Chert (Barghoorn & Tyler 1965). (n) Very small-sized carbonaceous globules scattered haphazardly in an organic gel-like matrix, Sirbu Shale, Bhander Group. (o) Carbonaceous vesicles attached on a branched filament, Dholpura Shale, Bhander Group. (p) Branched filaments of the Dholpura Shale, Bhander Group.

Continuing a little more on the journalist's question about fossil record in India, there are other locations in the Himalayas (Krol Formation, Lesser Himalayas) and Rajasthan (Marwar Super Group) that also contain latest Neoproterozic sediments and offer rich scope for exploring the biodiversity of a very interesting period of earth history.

Coming back to the paper, the authors after taking lithological and geographic biases into account find a pattern of increasing assemblage diversity from the early Proterozoic up to the Cryogenic Period, which sees a fall in diversity. Cryogenic Period or " Snowball Earth" was a phase in the Proterozoic which saw episodes of widespread glaciations. Fossil assemblage diversity increases again in the Ediacaran Period.

Here is an extract from the paper that highlights the important questions about Proterozoic evolution that the authors address:

Here, we assess the existing record of Proterozoic fossils and test the robustness of this record by investigating potential biases presented by taphonomy, fossil categorizations, regional sampling, and uncertainties in age models. We then layer on existing paleogeographic, geochemical, and climatological datasets and assess potential relationships between eukaryotic diversification and environmental change. Questions we seek to illuminate with improved datasets include: What was the relationship between eukaryotic diversification and a putative rise in oxygen (Lenton et al. 2014; Planavsky et al. 2014)? Did the breakup of the supercontinent Rodinia lead to changes in the diversity and distribution of microfossil assemblages (Valentine andMoores 1970;Dalziel 1997; Hoffman 1998)? Was the diversification of crown group eukaryotes and origin of biomineralization (Parfrey et al. 2011, Cohen et al. 2011) driven by tectonically modulated changes in ocean chemistry (e.g., Halverson et al. 2010; Squire et al. 2006)? Did increased sinking of newly evolved mineralized tests drive changes in the biogeochemical cycles and climate (Tziperman et al. 2011)? What were the effects of global glaciation (a.k.a. Snowball Earth; Hoffman et al. 1998) on microeukaryotes? Were microeukaryotic diversification and the appearance of metazoans driven by predation (Porter 2011), changing ocean chemistry, or other factors?,

The connection with the breakup of  Rodinia is intriguing. Rifting of continents beginning around 830 mya and 780 mya lead to emplacement of large igneous provinces. Weathering of these silicate rocks resulted in an increased supply of iron and phosphorous to the oceans, leading to increase in primary productivity with effects down the food chain. Likewise, an increase in oxygen content in sea water may have provided many an impetus for the evolution of eukaryote complexity and diversity. Predation, a metabolically demanding activity may have been favored, as may an increase in cell size. Certain elements like Zinc which play physiologically critical roles in eukaryotes also may have been more accessible in the enhanced presence of oxygen.

Geological processes and biological evolution are intertwined and the Proterozoic fossil record provides ample instances of it. The origins of multicellular animals lie in the latest Neoproterozoic. The much later Cambrian "Explosion" which represents the geologically rapid diversification of the triploblastic biosphere grabs all the attention, but it is relevant to point out that a lot of the molecular machinery that animal cells rely on had already evolved in unicellular eukaryotes in various protist and fungal groups. Their diverse fossil record in the Proterozoic provides us with a broader understanding of the evolution of complexity.

Cohen, P., & Macdonald, F. (2015). The Proterozoic Record of Eukaryotes Paleobiology, 1-23 DOI: 10.1017/pab.2015.25

No comments:

Post a Comment