New publication: Retention of fish-like odontode overgrowth in Permian tetrapod dentition supports outside-in theory of tooth origins (Haridy et al., Biology Letters)
Title: Retention of fish-like odontode overgrowth in Permian tetrapod dentition supports outside-in theory of tooth origins
Authors: Y. Haridy, B.M. Gee, F. Witzmann, J.J. Bevitt, R.R. Reisz
Journal: Biology Letters 15(10): 20190514
DOI to paper: 10.1098/rsbl.2019.0514
General summary: most studies of teeth in both extinct and living animals have focused on what we call the 'marginal dentition'; this comprises the teeth at the edges of the mouth. You may of course wonder what other teeth there could be - humans only have marginal dentition. However, many other extinct tetrapods and even some living ones have up to hundreds of small teeth in other places in their mouth, namely on their palate (roof of the mouth). There can also be non-marginal dentition of the jaw, in the hyobranchial region (around the throat and sometimes associated with gills), and arguably even on the skin when you look at fish like sharks (this is why scratching a shark from the back to the front feels very prickly compared to vice versa).
In this study, we looked at palatal dentition in a variety of early tetrapods, more specifically in a dissorophoid temnospondyl amphibian (Cacops) and an early reptile (Captorhinus) to look at how tooth replacement occurred. In the marginal dentition, the typical pattern is for a tooth to fall out and to be replaced by a new one in the same approximate position. In humans for example, it's not like one of your baby canine falls out and is then replaced by three new teeth positioned 1 cm back of where the old one was. This creates some semblance of organisation, although there are plenty of exceptions; some animals replace with progressively larger teeth, which leads to overall fewer tooth positions over the lifetime of the animal.
Part D of the above figure captures essentially the entire diversity of stages of tooth replacement. Some of the teeth are just barely being embedded; in part I, the tooth is starting to be covered by enveloping bone (abbreviation 'eb'; not a formal term), and in part E, the tooth has just reached the point of being fully buried. Some of the other teeth, like the one in part H, have been buried for quite a while, and they're starting to be resorbed (see the resorption bay ['rb'] below it). You should also notice that there is no vertical stacking; the next tooth does not come in directly above a previous position.
Insights into the origin of teeth
Teeth are naturally a structure of great interest to a wide range of scientists because of their ubiquity across tetrapods and the obvious import they have for a given animal's ecology. However, the origin of teeth happened so far back in vertebrate evolution (possibly before jaws evolved) that it has remained controversial about how and where exactly teeth proper show up in the evolution of vertebrates (quite far back and in some fish is the existing consensus). There are two main hypotheses: "inside-out" and "outside-in." The "inside-out" hypothesis says that teeth originate in the mouth first and then move outwards onto the body in fish, which is why sharks have sharp toothy scales (try scratching one from tail-to-head). The "outside-in" hypothesis says that teeth originate on the body (but obviously don't have the primary function of eating / prey capture that they have now) and then move into the mouth. We didn't set out to use data from Permian tetrapods to test this - it's not an intuitive way to examine tooth origins considering that the first teeth show up in some early fish. But our data show that the palatal tooth replacement of tetrapods is the same as that in both palatal teeth and dermal teeth of much earlier fish. The pattern first appears in the dermal teeth, including in fish that don't even have jaws (see the phylogeny below). As such, this adds some support to the "outside-in" hypothesis.
Below is a schematic showing the step-wise progression of tooth development under the "outside-in" hypothesis that we find support for in our study. In (A), there are only odontodes on the dermal skull (essentially the outside) - 'odontode' is a term used to refer to any dentine-containing tooth-like protuberance (which includes actual teeth). This step could have well preceded the origin of jaws, since tooth-like structures on your head would not be used for feeding, or at least not in the same way that they are in the mouth. Also, many dermal odontodes (abbreviation 'do') aren't as pointy as teeth in the mouth. In (B), the tissues that can produce dermal odontodes would migrate into the oral cavity (mouth) in both the upper and lower regions. Why exactly this would occur remains unclear. In (C), the odontodes in the mouth would be subject to different selection pressures - for example, those on the lower jaw (teal) would be different than those on the roof of the mouth (orange). This would then lead to the differentiation of different teeth of different sizes, spatial distributions, and replacement patterns, as seen in (D).
The history of this study
There's often a perception by non-scientists that science always happens by careful, intentional design with crisp execution of the study (and also that we always wear lab coats). That is NOT how this study went down. Some people more familiar with my work may remember a paper that we published back in 2017, also on palatal dentition. That study was looking specifically at little toothy plates that sit in membranes covering these large openings in the roof of the mouth in temnospondyls. Just out of curiosity, we decided that we should probably slice up some of the bones of the palate that have these teeth as well for comparison, so we lovingly destroyed this palatal bone of a Cacops back in the early part of 2017. When I sat down to image them, I was immediately drawn to this perplexing observation - why were there teeth buried in the bone? Like full teeth, with enamel and everything? The phrase "uhhh Yara, what is this," is what kicked off this project. You can see one of the very first, uncleaned images that I took (complete with my traditional hot pink scale bar) below. A better (cleaner) version of this is included in the first figure in our paper.
About the blog
A blog on all things temnospondyl written by someone who spends too much time thinking about them. Covers all aspects of temnospondyl paleobiology and ongoing research (not just mine).