Many of the fossils I painstakingly collected over the years have entered into a curious limbo, resting now in gallon-sized plastic baggies that are distinguished only by general category of fossil (teeth, shells, etc.) and where found. These fossils await a final disposition though I do know they won’t be kept. (My previous post described what a poor job I did of curating the bulk of the collection.) So far, a few fossils have resisted the deconstruction process as I’ve filled these large bags. The pufferfish fossils highlighted in the previous post are one example. A second group of fossils has managed to distract me and avoid the fate of those fossils in the big baggies. This is a small clutch of tiny dolphin teeth collected along the shoreline of the Calvert Cliffs where the Calvert Formation is exposed (the portion of that formation yielding these teeth dates from possibly a bit less than 17 to about 14 million years ago). I initially set them aside for two reasons: I find them beautiful in their simplicity and, until now, I had never attempted to try and assign them to Miocene dolphin genus and species. (Clearly, decommissioning my fossil collection will take forever if this is going to be my usual pattern of behavior.)
Some initial groundwork with terminology and relationships is necessary. The cetaceans (commonly slapped with the general label whales) are divided into two main groups. The odontocetes or toothed whales are a taxon which also includes dolphins and porpoises. The other large category of cetaceans are the mysticetes or baleen whales. Another point of clarification. The small cetacean teeth I collected from the Calvert Cliffs shoreline are most likely from dolphins and should not be ascribed to porpoises: the latter, extant from perhaps the mid Miocene onward, were denizens of the Pacific, and, reportedly, their fossils are particularly scarce prior to the Pleistocene. (For more on this issue concerning porpoises, see text by Joy Pierce Herrington, in Fossil Marine Mammals (Digital Version), Volume IV of IV - Part 2, North Carolina Fossil Club, edited by Richard Chandler, 2017, pages 6, 118.)
Early on in this taxonomic endeavor, I was encouraged by learning of the recent publication of a superb description of the various toothed whales of the Calvert Cliffs written by paleontologists Stephen J. Godfrey and Olivier Lambert. (Miocene Toothed Whales (Odontoceti) from Calvert Cliffs, Atlantic Coastal Plain, USA, Chapter 2 in The Geology and Vertebrate Paleontology of Calvert Cliffs, Maryland, USA, Volume 2, Turtles and Toothed Whales, edited by Stephen J. Godfrey, Smithsonian Contributions to Paleobiology, Number 107, 2023.) It is an expansive treatise on these odontocetes, including discussions and descriptions of 16 dolphin species whose fossils have been found in material shed from the portion of the Calvert Formation along which I collected my dolphin teeth. (This count is based on Figure 2.73 which lists the named odontocetes by stratigraphy. I have assumed, possibly in error, that Shattuck-Zones 8 to 13 are responsible for my teeth.)
Yet, this volume, for all of its many merits, turns out to be a thin reed (at least for me) upon which to attempt any extensive taxonomic attribution of the dolphin teeth I have on hand. I went so far as to create a spreadsheet listing each of the dolphin species described by Godfrey and Lambert, excerpting any text about, or pictures featuring, teeth. A few of the authors’ descriptions and images prompted one or two rash and wild guesses as to identity, but I ultimately concluded not to suggest any. Discretion is by far the better part of valor in this instance. Beyond the Godfrey and Lambert volume, I searched extensively elsewhere for some sort of single guide to fossil dolphin teeth, something similar to the many volumes devoted to fossil shark teeth, and came up empty. Clearly, I hoped someone had done the heavy lifting in creating such a guide.
There are several key reasons why my taxonomic exercise failed. I have already mentioned the evident wear and tear on my specimens. But, in reality, at the heart of my difficulties is the profound impact of evolution on the earliest harbingers of the cetacea that made their way from the land back to the water. Dolphins, like all of the other toothed whales, evolved from mammals that had typically mammalian teeth with highly varied morphology. Consider the differences among the teeth in your own mouth. Over time, as the stem odontocetes evolved to become fully aquatic, their teeth changed markedly. I was quite taken by a paragraph on these evolutionary changes to their teeth written by paleontologist Steve Brusatte in his terrific book The Rise and Reign of the Mammals: A New History From the Shadow of the Dinosaurs to Us (2022).
Gone are all the complex cusps and ridges; gone is the lineup of incisors, canines, premolars, and molars; gone is the replacement of baby teeth with adult teeth; gone is the ability to chew. Instead, all the teeth are conical pegs, which simply cut meat off fish or other whales, which the odontocete then swallows. Some odontocetes barely use their teeth in feeding and lazily swallow their prey whole. (p. 280)
I drew three key conclusions from Brusatte’s overview of the changes that occurred.
First, the end result of the process is, in his words, “conical pegs.” Yes, the conical or peg-like aspect to all of these teeth renders them relatively indistinguishable without close study. This is unlike fossil shark teeth which come in a plethora of different shapes and sizes which are key to sorting them by genus and species, a task relatively easy for some of them. (By the way, the teeth of porpoises are not conical but, rather, spatulate or cupped at their end.)
That said, there is in fact some diversity in the dolphin teeth that could be found along the Calvert Cliffs. At the most extreme, there are the teeth from the three species of the Squalodontidae family that are present here. The squalodons or, as they are sometimes called, “shark toothed dolphins,” have very distinctive teeth. Consider those depicted below which appeared in the Godfrey and Lambert volume (figure 2.2, scale bar is 10mm).
Certainly, a couple of these are nothing like the Calvert Formation dolphin teeth in my collection.
Godfrey and Lambert do identify a few potentially distinguishing features of the teeth of several other dolphin species, ranging from tiny cusplets to keels running down the teeth. Nevertheless, given the state of my dolphin teeth and the inherent "sameness" of nearly all such teeth, I don’t feel I have the necessary guidance I need to go further. Frankly, it's true that, by and large, Miocene odontocete teeth (other than those from squalodons) are reasonably and justifiably covered by Brusatte’s descriptive phrase “conical pegs.” Further, isolated odontocete teeth (excepting those of squalodons) are seldom appropriately diagnostic as to genus or species. This specific point is made quite convincingly by a paleontologist on the discussion site The Fossil Forum in an exchange of messages regarding a search for ID guides to odontocete teeth. He posted on June 11, 2013: "I have one piece of advice for you: Prepare for failure and disappointment. The general attitude amongst marine mammal researchers is that, with the exception of really distinctive species, isolated odontocete teeth are generally not identifiable even to the genus level, and often not even to the family level. . . . Odontocete teeth do not preserve the same breadth of diagnostic information that shark teeth have."
A second point to be made from Brusatte’s description is that it suggests that teeth in the jaws of odontocetes are homodont, that is, they do not really vary in morphology by location on the maxilla or mandible – top, bottom, front, or back of the jaws, the teeth are all similar, which they generally are. Godfrey and Lambert label the teeth of odontocetes (other than squalodons) as “roughly homodont.” This works to reinforce the sameness of the fossil dolphin teeth we might find along the shoreline.
(As an aside, at this point it strikes me that, were dolphins cartilaginous like sharks, that is, not having a bony skeleton, the work of paleontologists like Godfrey and Lambert would be incredibly more difficult. Having the remains of bony skeletons to aid in determining genus and species makes a critical difference.)
Finally, Brusatte noted that, in the course of their lives, odontocetes have a single set of teeth. The impact of this on the relative abundance of odontocete teeth fossils is significant. I’ll acknowledge that some Miocene dolphins and extant dolphins are very much polydonts, that is, they have many teeth, some species a couple of hundred or more. Nevertheless, when one considers that an individual shark might in its life span shed tens of thousands of teeth, it’s no wonder shark teeth litter the shoreline along the Calvert Cliffs, while dolphin teeth don’t.
All of this adds up to what I found to be insurmountable hurdles for assigning the teeth I have to genus, much less species, and helps explain the "missing" guide to fossil odontocete teeth.
In conclusion, although I won't be assigning my dolphin teeth to genus and species and, as a result, they will remain unknown, I will assign them to the “keep” category. Beauty, simplicity, and relative scarcity win out. So, a bunch of teeth that I can only assign to a collecting location and the broad category of “dolphin teeth,” nevertheless avoids the bardo to which I’ve condemned many fossils from my collection, some of which could be identified. Quite ironic!
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