Thursday, November 4, 2010

Emerging New Research 1 – Recent Aquatic Amniote Literature Reviewed

Now that I have gotten the news of the recent symposium out of the way, I’d like to present a new type of post for me – an annotated review of recent literature on aquatic amniotes and matters relevant to the study of their paleobiology. My intention with this is to highlight the primary utility I see in recently published work, including some curious details that seem often get ignored, and occasionally review an “oldie but goodie” from among the pieces of less-cited literature that I worry hasn’t gotten the recognition it deserves. I will do my best to be open and honest, but don’t want this to be a way of building a long list of enemies either, so if I do not nitpick every paper’s problems and instead focus on what strengths it has, I hope you’ll forgive me. If you are the author of one of these papers and feel I haven’t gotten the point of it, or misrepresented it in some way, I would really appreciate your comments on the blog, or a direct message to me. This is not meant as a critique of research, only a way of pointing out some recent work that might have been missed by the greater aquatic amniote paleontological community. The literature is vast and growing everyday, and I’d like to consider this an aspect of service to our community of scientists – I’m going to read these anyway, so I may as well save you all the trouble of finding these papers and get the news out.

Each of these will have a link to a journal site (or preferably a site at which the paper is available open access), so I hope that this will serve as an aid for all of you that haven’t the time to review all the literature. Though many of these papers I get directly from authors or through journal subscriptions, many of the papers I review I received thanks to the gracious efforts of David Janiger (LACM). David lists recent papers and offers pdfs to people on the MARMAM listserv (by the way, has a Marmam listserv section). If you already get papers thanks to his help, or are reading this blog, thank him already! He has been doing this for a while, and it is incredibly valuable.

Fourteen nuclear genes provide phylogenetic resolution for difficult nodes in the turtle tree of life.
Anthony J. Barley, Phillip Q. Spinks, Robert C. Thomson, H. Bradley Shaffer
Molecular Phylogenetics and Evolution 55:1189-1194

Bradley Shaffer Lab page
author's page for pdf

This short and sweet paper about a nuclear gene-based phylogeny of modern turtles nicely resolves some of the persistent problems of the relationships among turtle families, particularly due to the taxon Platysternon (big-headed turtle). The taxon sampling here isn’t extremely extensive (19), but it gets at least two taxa from each family of interest, particularly with respect to the Kinosternidae, Chelydridae, Emydidae, and Chelonioidea. As a start at getting a molecular phylogeny that can resolve these contentious relationships, this study does well. I found it particularly satisfying that kinosternids and chelydrids are sister taxa here, together as sister taxa to the Chelonioidea. Parham et al (2006) found similar, but not the same results, having these three clades not as a monophyletic group, but paraphyletic nested sister taxa to the Emydidae, Platysternon, Geomydidae, and Testudinidae (for more papers by Jim Parham related to this, visit his site). The taxon sampling of the present study is not extensive, so ultimately after many more taxa are sampled for these same genes and other molecular and morphological data, we may find this remains stable, or not. Either way, this has some interesting relevance to the origins of sea turtles, and the relative age of these clades.

Understanding the diet composition of marine mammals: grey seals (Halichoerus grypus) in the Baltic Sea).
Karl Lundström, Olle Hjerne, Sven-Gunnar Lunneryd, Olle Karlsson
ICES Journal of Marine Science 67:1230-1239

Author's page

The literature on the diets of marine mammals is huge, even though a concrete idea of exactly what animals eat (taxonomic diversity, abundance) and how this diet varies with age and distribution is a difficult matter to resolve. The best global look at this I know of is a paper more than 10 years old (Pauly et al 1998), even though copious reports on stomach contents exist. Large-scale studies are difficult to coordinate in this way because so much of the existing literature on diet is uneven in its quality and coverage, often sampling only strandings or not accounting for abundance or taxonomic diversity. Here, this group has done so for the grey seal, Halichoerus grypus, in the Baltic Sea. The divided their sample of hunted seal specimens (which they got in cooperation with hunters and fisheries sources) by regions of the Baltic and age categories. What I really love is their care for the data on abundance AND diversity of stomach AND intestinal contents, and that they report all their data, and even correct it for Erosion-class-specific size and numerical correction factors. As one might predict from the nature of biology, when they looked closely at these details, they found differences in the diets of young and old, and between regions. These differences seem largely to be concerned with commercially caught fishes, which is of practical importance, but one could easily envision using this same data to compare paleodietary measures of the same specimens (isotopes, dental microwear, etc.). Gosh, I need to contact some people in Sweden!

Mid-Cenomanian vertebrate faunas of the Western Interior Seaway of North America and their evolutionary, paleobiogeographical, and paleoecological implications
Stephen L. Cumbaa, Kenshu Shimada, Todd D. Cook
Palaeogeography, Palaeoclimatology, Palaeoecology 295:199-214

Stephen Cumbaa's page
Kenshu Shimada's page
Todd Cook's page

This is a much-needed review of the copious marine vertebrate fauna of the Western Interior Seaway by some of the best people on this topic I know. Though this is obviously of interest here because of the numbers of marine reptiles many of us think of living in the Western Interior Seaway, this sea was obviously more densely populated with bony and cartilaginous fishes. These authors know these fishes well, and being a little familiar with these collections and taxa, I can see that this was a terribly massive amount of work to compile all of these faunal distributions, primarily because of the care it would take to go through all the hundreds and thousands of tiny fish teeth from these sites. Even though they only report on diversity of each site, not abundance, it is because of the clear limitation that these sites are biased by some degree of time-averaging. Without knowing how long each deposit/accumulation took to get there, abundance is a difficult thing to assess. In the end, this thorough study of the faunas of the WIS is incredibly useful and should play a part in estimates of biodiversity of marine reptiles as well, which presumably preyed on these taxa.

Single source sound production and dynamic beam formation in echolocating harbour porpoises (Phocoena phocoena)
P. T. Madsen, D. Wisniewska, K. Beedholm
The Journal of Experimental Biology 213:3105-3110

Author's page with pdf

I believe that this might be one of the less-conspicuous papers to those that study fossil cetaceans, as it could easily be misconstrued as simply another paper studying the acoustics of echolocation. But these researchers did something very clever – they attached suction-cupped microphones to both pairs of phonic lips of a porpoise to basically see whether both pairs were functioning to produce whistles. They weren’t, as one could glean from the title, and this has major implications for the functional role of the second pair of phonic lips, as well as questions about the evolutionary loss of them in sperm whales. Sperm whales usually fall out as primitive whales in most phylogenetic studies, and because of that one might consider it because they lack the second phonic lips that this is a primitive trait. But some (Cranford et al, 1996) have suggested it was secondarily lost, and this may be the case. If you are working on fossil cetaceans, this study itself may not be directly useful to your work with fossils, but it is important to your understanding of the adaptive and non-adaptive roles of anatomical structures, and how that might influence your perception of how structures associated with echolocation may be retained. I hope those looking into the evolution of echolocation will seriously consider this work.

Hydrodynamics of a ship/whale collision
Gregory K. Silber, Jonathan Slutsky, Shannon Bettridge
Journal of Experimental Marine Biology and Ecology 391:10-19

Author's page with pdf

This study utilized models in a flow tank to simulate the effects of the bow wave of a vessel and how that might interact with a whale. The model of the whale was outfitted with an accelerometer, and though not dynamic itself (no autonomous movement, and obviously no behavior), the accelerometer helped convey the data about how the whale model responded to wave propagation around the vessel as it passed. I’m personally more keen on seeing further data on whale strikes in real situations, but this study goes as far as one could without using living animals to get at some fundamental questions about flow around boats and whether whales are in any special danger of impact due to hydrodynamics. Though the authors stress how this might affect policies about vessel speed in whale-inhabited waterways (which is important), I would argue that one also must use this to consider how to modify vessel design to minimize the hydrodynamics that enhance the danger of collisions with whales. The former is obviously cheaper and easier to implement, but the latter is certainly worth considering.

Change in the foraging strategy of female South American sea lions (Carnivora: Pinnipedia) after partutition
Massimilliano Drago, Luis Cardona, Enrique A. Crespo, Nestor Garcia, Santiago Ameghino, Alex Aguilar
Scientia Marina 74(3):589-598
(free pdf at journal site)

The study of stable isotopes as a proxy for diet is widely recognized as a good technique for sorting out not only the diets of modern marine mammals, but those of fossil taxa as well. Curiously enough, much of the data on the isotopic composition of primary producers and potential prey species that are the sources of these isotopes in marine mammals is lacking for many environments, as is data on how the diets of these animals can change in small ways for short periods of time that may or may not be reflected in tooth enamel. If the changes are temporary, serial sampling may pickup some oddity, but without the sort of data that would answer the question. Plus, tooth enamel records only the isotopic record of the individual while the tooth is being formed, and the data presented here about the isotopic signature of coastal-benthic predation by post-partum female Otaria is likely never to be recorded in their teeth because their teeth have already developed. If one were to study the dental microwear of these same individuals and compare it with the data from their tooth-enamel based isotopes, one would find conflicting signals. It isn’t that either method is wrong, but that they sample the diet of the animal in different temporal and life history scales, which should be seen as an opportunity, not a limitation. In this case, these authors focus their work on the immediately relevant results of how these sea lions’ diets change after giving birth, but I would argue that for the paleobiologist, this is a clear reminder of the need for consideration of the minor changes in behavior that come with life history events that can have a big impact on the dental microwear or isotope record of individual specimens being studied. Hence, the most obvious solution to that is to consider these limitations when dealing with small samples (which some fossil taxa force us to work with), and when possible, use large samples.

Age and growth of franciscana dolphins, Pontoporia blainvillei (Cetacea: Pontoporiidae) incidentally caught off southern Brazile and northern Argentina
Silvina Botta, Eduardo R. Secchi, Monica M. C. Muelbert, Daniel Danilewicz, Maria Fernanda Negri, Humberto Luis Cappozzo, Aleta A. Hohn
Journal of the Marine Biological Association of the United Kingdom

With a large sample size (N=291) incidentally caught in fishing nets over an 11 year period, the authors were able to report the rate of growth for this species in a detailed fashion, with care taken for how gender may have a role. Though they found no significant differences between genders, their data seem to show that females start out consistently a little larger than males, even though their rates of growth do not seem to differ, except perhaps in the first two years of life (after which female growth rates are slightly greater than males). Although this is of minimal interest to a paleobiologist, it is important data to keep straight when further discussions about sexual dimorphism are brought up for fossil cetaceans.

Does dispersal across an aquatic geographic barrier obscure phylogeographic structure in the diamond-backed watersnake (Nerodia rhombifer)?
Matthew C. Brandley, Tim J. Guiher, R. Alexander Pyron, Christopher T. Winne, Frank T. Burbrink
Molecular Phylogenetics and Evolution 57(552-560)

Frank Burbrink's page

Ok, this may be mistaken for something marginally related to aquatic amniotes, but this is getting at something worth understanding with regard to the origins of aquatic amniotes – how can freshwater systems be a barrier or a habitat, and when does it matter? Joshua Samuels (John Day) and I have been talking about this together for years, and hopefully we’ll come to writing something about it someday, but in the meantime, let’s focus on the paper. The authors here simply compared the relatedness of populations of watersnakes around the Mississippi River, finding that the best explanation for the divide in the genotypes of populations was the Mississippi River as a barrier. Not only that, but that the divergence date of this barrier is likely to have been during the Pleistocene, during an interglacial when the river may have been much wider. This is particularly compelling, considering that this is a watersnake, not a dry-desert-loving rattlesnake. This taxon can cross the Mississippi River, yet reproduction must be a behaviorally complex enough event that the river has succeeded in being a barrier to these populations sharing with each other reproductively. What does this mean to the paleobiologist? Perhaps the notion of a river as a barrier, at least in some fashion (perhaps not distribution, but widespread dispersal) is possible not only for fully terrestrial taxa, but semiaquatic ones as well.

There are plenty more reviews coming, and I hope to have some new material on Eocene Sirenia and trichechid tooth development to share soon as well.