Friday, May 1, 2015

Teeth and Denticles ~ A Loss of Clarity

In vertebrate paleontology, increasing knowledge leads to triumphant loss of clarity.
~ Paleontologist Alfred Sherwood Romer as quoted by evolutionary biologist Keith Thomson

The more I learn, the less I know for sure, and, yes, this can be a good thing.

I have been working through a sample of matrix from the Archer City Formation in Texas (early Permian Period, roughly 295 to 290 million years old).  This particular sample abounds in fossils, though without much apparent diversity.  This squares with my reading of Lower Permian Freshwater Sharks and Fishes of Texas and Oklahoma (2014) by Kieran Davis, a commercial collector (I think).  The predominant shark genus in the Archer City Formation, according to Davis, is the freshwater Orthacanthus of the Xenacanth family, whose teeth are fairly easy to identify, at least to the genus level, because the teeth have a strange and striking double-bladed arrangement.  These pictures show the teeth from the so-called lingual side (facing the middle of the mouth).

Orthacanthus teeth are often larger than those that I have turned up.  For a fuller treatment, see Gary D. Johnson’s Dentitions of Later Palaeozoic Orthacanthus Species and New Species of ?Xenacanthus (Chondrichthyes:  Xenacanthiformes) From North America, Acta Geological Polonica, Vol. 49, No. 3, 1999.

Along with the Orthacanthus teeth in this material are a great many other tooth-like fossils which, based on Davis’ guide, I initially identified as denticles from the Orthacanthus.  Two clusters of specimens representative of these finds are pictured below.

But then there are these specimens.

They don't resemble the denticles included in Davis' guide.  Is that because these specimens are broken, consisting of only crowns with no root element, or are they actually teeth but from some other fish taxa?  I should note that Davis observes that he found few fossil fish specimens (other than the Orthacanthus fossils) while collecting in the Archer Formation.

At that point, the question became:  What do I know about shark denticles and what they look(ed) like?  Not much,  So I delved into the literature.

Denticles, also called placoid scales, are teeth-like structures embedded in the skin of extinct and extant sharks, rays, and skates (the elasmobranchs) which differ by species.  Further, denticles on a single individual may also differ in shape and size depending upon their location on the skin.  Researchers believe that, depending upon species and dermal location, denticles may play roles in locomotion (reducing drag or turbulence), protection from predation, protection from abrasion, reproduction, or luminescence.  See, for example, M.J. Johns, et al., Taxonomy and Biostratigraphy of Middle and Late Triassic Elasmobranch Ichthyoliths from Northeastern British Columbia, Geological Survey of Canada, Bulletin 502, 1997, p. 17; and Reef Quest Centre for Shark Research, Skin of the Teeth.

The actual composition of shark teeth and denticles is basically the same.  According to Johns and her colleagues,
A typical elasmobranch tooth or scale consists of a crown with an outermost shiny layer of enameloid followed by dentine.  Dentine layers enclose the pulp cavity which is filled with blood and nerve tissues.  The crown sits on a base (tooth) or pedicle (scale) . . . .”  (Taxonomy and Biostratigraphy of Middle and Late Triassic Elasmobranch Ichthyoliths, p. 15.)
Ancient jawed vertebrates exhibited this “dermal skeleton,” but most groups lost it, but not the sharks, rays, and skates.  (Qingming Qu, et al., Scales and Tooth Whorls of Ancient Fishes Challenge Distinction Between External and Oral “Teeth,” PLOS ONE, August 2013, p. 1.)

My doubts about the identity of my Archer City “denticles” were not resolved as I considered the basic shapes of shark denticles depicted in the literature.  These mostly do not resemble those Davis identifies as Orthacanthus denticles.  Consider these drawings of denticles from common extant sharks that appeared in Lewis Radcliffe’s The Sharks and Rays of Beaufort, North Carolina (Bulletin of the United States Bureau of Fisheries, Vol. XXXIV, document issued April 6, 1916).  These drawings show (from top to bottom) denticles from Cacharhinus isodon (Finetooth Shark), Galeocerdo cuvier (Tiger Shark), and Cetorhinus maximus (Basking Shark).  Only the last from the Basking Shark favor at all those I found in the Archer City material.  (I've given the scientific names currently applied to these sharks, not those Radcliffe used for them.  Many more denticles from different shark species are pictured in Radcliffe's book.)

Additional descriptions and illustrations of denticles appear in many other sources, such as the study by paleontologist Marjorie Johns and her colleagues cited earlier (Taxonomy and Biostratigraphy of Middle and Late Triassic Elasmobranch Ichthyoliths from Northeastern British Columbia); some of the articles by paleontologist Wolf-Ernst Reif (such as Types of Morphogenesis of the Dermal Skeleton in Fossil Sharks (Paläontologische Zeitschrift, Volume 52, No. 1-2, June 1978); and the gallery of pictures of shark and ray denticles on the website of the Australian Museum.  With regard to this last source, the Bashing Shark denticles shown there do not bear much resemblance to my "denticles," despite Radcliffe's drawing suggesting otherwise.

So, where am I on my “denticles”?  Well, learning more hasn't conquered the uncertainty, but a more informed exploration continues.  Davis may be right that most or some of those simple hooks I’m finding are Orthacanthus denticles.  Right now, I really don’t know.

There’s another aspect of the relationship between teeth and denticles that interests me.  The basic similarity between teeth and denticles has sparked a debate over, and uncertainty about, the evolutionary origins of teeth.  Two hypotheses are in contention:  (1) The outside-in hypothesis posits that dermal denticles (which can be found not only on the external skin and sometimes near the mouth, but also, indeed, inside the mouth) evolved into teeth as we know them.  (2) The inside-out hypothesis, as I understand it, argues for the independent development of these two kinds of structures (perhaps. it would be better labeled the inside and outside hypothesis).  For background on these hypotheses, see, for example, Peter S. Ungar, Teeth:  A Very Short Introduction, 2014, p. 48-50; Philip C.J. Donoghue and Martin Rücklin, The Ins and Outs of the Evolutionary Origin of Teeth, Evolution & Development, published online September 15, 2014; and Gareth J. Fraser, et al., The Odontode Explosion:  The Origin of Tooth-Like Structures in Vertebrates, Bioessays, Volume 32, No. 9, September, 2010.

For much of two decades, according to paleobiologists Donoghue and Rücklin, the outside-in hypothesis held sway until various pieces of evidence helped the inside-out thesis gain traction.  As a consequence, they state at the outset of their recent article,
The diversity of jawed vertebrates is predicated on the two formative evolutionary innovations of teeth and jaws, the origins of which appear to be becoming increasingly unclear.
The origins of which appear to be becoming increasingly unclear.  Initially, I understood this sentence to mean that the more the issue’s been studied, the less certain things have become.  That was the tale I assumed the authors were about to tell.  But, not so.  Their article, in fact, is a point by point refutation of the evidence mounted for the inside-out hypothesis.  They make their case and then conclude, with a garnish of scientific language:
This suggests that the traditional ‘outside-in’ hypothesis is the best explanation for the evolutionary origin of teeth through expansion of odontogenic competence from the external dermis to the internal epithelia.  [Translation:  Outside-in prevails because denticles in the skin evolved into teeth inside the mouth.]
They are not experiencing any loss of clarity, while other researchers haven’t ceded the field on this issue.  But, for me, uncertainty prevails.

Returning to the epigraph that opened this post.  I came upon it in an essay by evolutionary biologist Keith Thomson (subject of recent post) which is part of the collection titled The Common But Less Frequent Loon and Other Essays (1993).  After quoting his old teacher, paleontologist Alfred Sherwood Romer, Thomson undercuts the witty and thought-provoking statement by calling it Romer’s “little joke.”  What Romer meant, writes Thomson, is that, in vertebrate paleontology, the discovery of a truly transitional fossil lying between two taxa, say fishes and tetrapods, leads to a loss of clarity - is this fossil "a late fish or a very early tetrapod?"

Damn that constraining context.  I searched unsuccessfully for the document Thomson cites for Romer's sentence because I wanted so much to free it.  Then, very recently, I learned that Thomson uses it in another work, Morphogenesis and Evolution (1988), where he does the deed himself by largely jettisoning this limiting context and meaning.  All fields of science, he asserts, go through a repeating cycle, where, in the face of increasing knowledge, a synthesis that seems to tie the field together unravels (loss of clarity), later to be replaced by the emergence of another synthesis, and so on.  In light of that application of the Romer quotation, I think I'm allowed to liberate it still further, turning it into a maxim, thereby giving it the universality (and overstatement) it deserves:
Increasing knowledge leads to triumphant loss of clarity.

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