I’m hoping to feature some more University of Alberta-related research over the next few weeks, and first up is an interview with Stephanie Blais, a UALVP grad student studying ischnacanthid acanthodians. Stephanie recently published a paper in the Journal of Vertebrate Paleontology with new information on the origin of vertebrate teeth. So without further ado, here are five questions for Stephanie Blais:
1. What inspired you to conduct this study?
This particular study is really just part of a general interest in the origins of teeth. I don’t know if anything really “inspired” it per se, but I’ve always been interested in teeth as indicators of ecological role, and never really thought about how they originated, and then I learned that nobody actually really knows how they evolved. Which is weird, because they’re so widely studied and they’re probably the most numerous vertebrate fossils out there! So I decided to look at fossils of some of the oldest animals with ‘true’ teeth, and noticed some of the specimens from the MOTH locality (NWT) have weird tooth-like scales, which is where this study came in.
The next step is to figure out what function these may have had, and also to look at the other kinds of early gnathostome (jawed vertebrate) teeth and see how they are related.
2. What are the inside-out and outside-in hypotheses all about?
These are the two main hypotheses about the origin of the vertebrate dentition. They’re hard to sum up without getting into a lot of evo-devo, and they touch on quite a few points, but I’ll try. It’s also difficult to explain both without bias, but I’ll give it a go. Bear with me.
Basically, the ‘inside-out’ hypothesis suggests that the developmental machinery that produces teeth originally evolved in the pharynx of jawless vertebrates and eventually became transferred to the oral cavity. This would mean that teeth evolved before jaws. Proponents of this hypothesis have also suggested that conodonts (which lacked odontodes or any form of external denticles or armor) had the first vertebrate teeth, and this would mean that pharyngeal denticles and external denticles have completely different evolutionary histories.
The opposing ‘outside-in’ hypothesis is that teeth are essentially modified head scales that became specialized along the margins of the mouth in early jawed vertebrates (so teeth evolved after jaws). According to this hypothesis, internal and external denticles share a common evolutionary origin. A modified version of this hypothesis suggests the pharyngeal denticles in some thelodonts developed due to migration of cells or tissue with odontode-producing potential from the outside of the body to the pharynx through the branchial openings.
3. What are ischnacanthid acathodians, and what do they tell us about the evolution of teeth?
Ischnacanthids are members of a larger group of small, spiny fishes called acanthodians, which are related to both sharks and bony fishes. They are interesting because they have many kinds of dentition, although they’re unique in having special dermal tooth-bearing bones in their jaws. They’re also interesting because there are articulated Ischnacanthids from the Silurian, with well-developed teeth. Although other groups had teeth, we mostly find disarticulated specimens or, more often, isolated teeth and scales. The development of their dentition is more difficult to puzzle out, and that’s what I’m interested in. Study of ischnacanthids, other primitive acanthodians, and shark-like fishes from the Silurian and Early Devonian can hopefully help us to understand how the first teeth developed, and how different kinds of early teeth are related to each other.
4. What is so special about the MOTH locality?
The MOTH locality is one of the best sources of Early Devonian fish fossils in the world. Hundreds of specimens, from over 70 different species, have been collected from this one site in the Mackenzie Mountains. What really makes MOTH outstanding, though, is that a large proportion (I don’t know if it’s been figured out for the whole assemblage, but it’s probably around 40-50% for acanthodians) of the specimens are of articulated, complete or nearly complete fishes. That is probably partly due to sampling bias, but that’s still hundreds of articulated specimens. And a lot of those are perfectly preserved down to the nodes on the ridges on their (microscopic) scales!
As far as ischnacanthids are concerned, articulated specimens of this quality are pretty much unheard of anywhere else. Usually you only find their isolated jaw bones, maybe with a bit of jaw cartilage attached if you’re lucky. There have been articulated ischnacanthids described from other localities, but none are as well-preserved as those from MOTH.
5. And finally, because it is a law that all palaeontological news stories must eventually come back to T. rex…what does this study tell us about the teeth of T. rex?
You know, surprisingly, the journalists I have spoken to (all two of them) didn’t ask about T. rex. They did ask how these tooth-like scales relate to our teeth though – maybe there are two options to the law – T. rex and/or humans?
Regarding T. rex, I guess my answer will depend on what, if anything, I find out about tooth homology among early gnathostomes. If ischnacanthid teeth are homologous to bony fish teeth, then they could be regarded as the “tooth ancestors” of T. rex teeth, without which T. rex wouldn’t have been able to Rule The Dinosaurs! … And which of course would then make them valid for paleontological study. I rather think they might be.
Blais SA, MacKenzie LA, Wilson MVH. 2011. Tooth-like scales in Early Devonian eugnathostomes and the ‘outside-in’ hypothesis for the origins of teeth in vertebrates. Journal of Vertebrate Paleontology 31:1189-1199.