Tooth development in Trichechidae Part I

Alright, so this is not a complete post, but it's a start.
This Monday I will be visiting the Mammalogy Collection at the AMNH for one last data collection trip for the study of tooth development in the sirenian family Trichechidae. The Trichechidae is the family that includes modern manatees, as well as a number of fossil forms.
Aside from a comparatively abundant Pleistocene fossil record of manatees primarily found in Florida (subspecies Trichechus manatus bakerorum Domning 2005), most fossils of manatees are fairly scarce and poorly preserved, including the two Miocene taxa, Potamosiren and Ribodon, both from the Amazon River Basin of South America. Both of these taxa are known from little more than isolated teeth and some fragmentary maxillary or mandibular chunks.
But, thanks to two features of the skulls of fossil forms that pull their relationship closest to the modern manatees, there is a couple of oddball fossil forms from Belgium and Germany that ally closely with the Trichechinae (Potamosiren, Ribodon and Trichechus), the Miosirenae (Miosiren and Anomotherium)
Over the next several posts I will try to demonstrate some of the key features of all of these taxa, and how understanding some of the unique specializations in modern Trichechus (especially how those features in modern Trichechus vary inter- and intraspecifically), we can better grasp how these features evolved, possibly as a response to a rapid increase in abrasives in their mouths thanks to the uplift of the Andes.

the problem with microwear #1 (of many to come)....or... "if seacows eat seagrass, why can't we consider them grazers?"

Although I haven't published as much as I would like on the subject yet, many of you that know me know that I have spent an inordinate amount of my research life focused on the study of microscopic damage to tooth surfaces, known as dental microwear. Most of this has been focused on marine mammals, particularly members of the Sirenia and Desmostylia, although most members of these groups are extinct. I presented one of the more thorough studies of dental microwear in modern and some fossil Sirenia at the Society of Vertebrate Paleontology meeting in Bristol, UK this year.

But, to give some insight into the reality of such work, the sort that makes me pull my graying hair out, I thought I would share here. WARNING, I may rant about the flawed science of many microwear studies, but only to highlight the complexity of the problem, I do NOT intend this as a criticism of my valued colleagues endeavoring to get to the answers of a VERY complicated, messy bit of science. They deserve credit for having the guts to put it out there, knowing that in the end they will inevitably fall short of the full story - that's the way science goes, sorry everybody. I learned this perhaps a bit too late, but just get used to it and publish!

See, I digressed already!

Ok, so the problems with dental microwear are many, many, many.... but among other things, many folks attempt to apply systems of ecology to many groups of animals across large fields of diversity (and hence, morphology, physiology, and evolutionary background). The classic is the idea of the hippo-ecomorph. There are many fossil mammals with large bodies and short limbs, such as Teleoceras, Coryphodon, etc., that commonly get lumped into being hippo-like in their morphology (which is superficially true) AND lifestyle (which is rarely, if EVER supported by data). The anecdotal comparisons with hippos that most paleontologists make usually only serve to demonstrate their ignorance of modern hippo ecology (being noctural grazers with little to no social system, only found in groups because of their reliance on a scarce resource - water). But nonetheless, you will still find references of Teleoceras as a hippo ecomorph in textbooks and such, even though several thorough studies have shown that the only evidence potentially telling about this indicates that they were very much NOT like hippos.

So how does this relate to dental microwear? In the strange world of classic microwear (excluding some more elaborate confocal microscope-using methods), there is:

1. an SEM method that visualizes very small portions of the tooth at a high magnification
2. a light microscopy method that visualizes a larger area of the tooth at a lower magnification

The differences in these methods are great, but the basic idea is that teeth incur damage from what they eat (or more likely, the dirt that is on whatever they are eating), and the small bits of damage can be characterized in different ways that roughly correlate to different diets of grass, browse, or a mix of both.

BUT, as anyone that has ever raised an herbivorous mammal, had a garden, or even made a salad understands, not even the simplest diets can be broken down that simply AND not a single animal on this planet (except for maybe the koala) can ever be described as being a strict consumer of a single plant type. Plants come in all shapes and sizes, as well as all sorts of material properties and abrasiveness. The general notion that the silica nodules, known as phytoliths, that are found to surround vascular bundles in grasses are the cause of the scratchy wear in grazers itself is an example of this issue. Not only have phytoliths been demonstrated to not all have the hardness needed to wear enamel (Sanson, 2007), but many plants that do not have phytoliths wear enamel in very similar patterns. The best example I know are seagrasses and the wear found on the teeth of manatees, Trichechus manatus. Manatees eat a lot of seagrass, yet not a single seagrass has phytoliths inside, so what causes the wear? My research on this of late has pointed in the direction of substrate, specifically siliclastic substrate that some seagrasses like to grow in. In the end, the simple answer to the question of what causes wear is that for seacows, it isn't phytoliths. For all animals in general, it might be better put as - consider all the options before you rule any single thing out, and consider the system at hand. In the end, the data for one ecosystem may ultimately NOT be comparable to another for just this reason.

I will try to continue these rants to cover other aspects of microwear, including issues with methodologies, assumptions, dietary interpretations, and the ever-persistent attempts to apply microwear to fossil organisms, including dinosaurs, despite clear differences in mastication, ecology, etc that should act as BIG warning labels to most people that microwear should not, could not, and cannot be applied in the same way for every animal that ever wore a tooth. The simple notion of using data from one study and comparing it with that of another is a complex matter that needs addressing as well, so I will try to bring it up here as well.

So much to do, so little time! Thanks for your patience and time.
Brian