Project EARTH-17-ES1: How do thee vary? Let me count the ways. A quantitative investigation of changes in species through the Phanerozoic

Supervisors: Erin E. Saupe (Oxford) and Jonathan Todd (NHM)

Species are often considered to be the fundamental units of taxonomic, ecological, and evolutionary biology. Although over 1.7 million extant species have been described, extinct species do not trail far behind considering the incompleteness of the fossil record, with over 250,000 described. In order for these fossil species to be of relevance to studies of diversity, ecology and evolution, we need to quantify to what extent fossil species correspond to extant ones (Jablonski et al. 2003). Very few studies, however, have explicitly tested this relationship (Hendricks et al. 2014).

Palaeontologists have generally assumed the conceptual equivalence of morphological species through time. This equivalence, however, may be time-dependent for two, related reasons. First, recent work shows that many living ‘traditional’ benthic marine invertebrate species are comprised of genetic clades that are narrowly-distinct with regard to morphology (‘cryptic’ species). Since the probability of preservation of such fine-scale morphological features generally decreases with age, cryptic species diversity, if present, would likely not be obtainable from much of the Palaeozoic record. Second, it can be postulated that morphological variability among closely-related species has decreased through time. This hypothesis stems from the idea that increasing benthic alpha diversity (Bush and Bambach 2004) may have led to increasingly finer niche partitioning in order for more and more species to coexist. Increased specialization associated with the occupation of narrower niches would lead to morphologically less-disparate and more-tightly structured species.

lamarck

By Lamarck, 1819 [CC0], via Wikimedia Commons

 

 

 

 

 

 

 

The ideal focal taxon with which to test this hypothesis and to quantify such patterns is the bivalve mollusc order Pectinida (scallops). This group contains abundant and diverse clades of well-preserved, taphonomically-resistant shells from the Palaeozoic to Recent. The student will quantify morphological variability among Recent populations and species, and compare their taxonomic and geographic structure to members of comparable, well-studied extinct clades of this order at three time slices within the Phanerozoic (Eocene, Jurassic, Carboniferous). Extant species of two abundant and easily-obtained genera, Argopecten and Chlamys (family Pectinidae) from the temperate and tropical Atlantic will be studied. Using these two clades, the student will (1) assess the morphological variability among and between Recent species and test species-level delimitations using DNA; (2) use these results to determine within and between species-level disparity of focal taxa in the Palaeozoic, Mesozoic and Cenozoic; (3) statistically analyse changes in morphological disparity throughout the Phanerozoic to Recent; and (4) test whether species exhibit decreasing geographic/stratigraphic ranges, and/or narrower niches through time.

The project will represent a significant contribution to our understanding of how species evolve both morphologically and ecologically over geological time scales, and will allow us to better integrate fossil data with modern diversity dynamics. This interdisciplinary project will provide the successful PhD candidate with highly-valued and sought-after tools for investigating macroevolutionary and macroecological processes, including geometric morphometrics, statistical and ecological modelling, database construction, and molecular genetics. The student will have the opportunity to travel abroad to collect data, and to present results at major, international conferences (e.g. AGU, GSA, PalAss).

Further reading:

Bush, A. M., & Bambach, R. K. (2004). Did alpha diversity increase during the Phanerozoic? Lifting the veils of taphonomic, latitudinal, and environmental biases. The Journal of geology, 112(6), 625-642.

Hendricks, J. R., Saupe, E. E., Myers, C. E., Hermsen, E. J., & Allmon, W. D. (2014). The generification of the fossil record. Paleobiology, 40(04), 511-528.

Jablonski, D., Roy, K., Valentine, J. W., Price, R. M., & Anderson, P. S. (2003). The impact of the pull of the recent on the history of marine diversity. Science, 300(5622), 1133-1135.