The Bedeviling Fossil Teeth of Squatina

When Tiberius Cornelis Winkler first described the teeth from what is now known as Squatina prima (Winkler, 1874), he called them the “most curious” of the teeth in the “remarkable” collection of fish fossils he was then identifying.  The Dutch surgeon turned paleontologist focused on their “extraordinary shape,” a slender, conical crown set on a “very unusual” root that had a triangular base.  These specimens were among the fish fossils from the Heersian (no longer an accepted stratigraphic name) in the middle Paleocene Epoch (approximately 61 to 59 million years ago) that Winkler treated in his Mémoire Sur Quelques Restes De Poissons Du Système Heersien (appearing in the Archives de Musée Teyler, volume IV, 1878).  (In the Notes section below, I consider the apparent conflict between the 1874 date in the animal’s full scientific name and the 1878 date of publication of the Archives de Musée Teyler, and provide a source for my comment on the Système Heersien or Heersian.)

Winkler lamented how little he’d been able to learn about these teeth.  He’d scoured the literature but found no description and no clue as to their taxonomic niche; so he really knew nothing or next to nothing about them.  Nonetheless, he exercised naming rights.

It seems to me we might call the fish that possessed these remarkable teeth
                              Trigonodus primus Winkler
in light of the triangular shape of the root.  (p. 14)

The genus name Trigonodus means something like “triangular knots or nodules.”

Though Winkler realized these teeth were from a shark, he was unsure about the family to which the shark might belong, and clearly had no clue about its generic name Squatina, which had been first used by zoologist A. M. Constant Duméril in 1806 (Zoologie Analytique, ou Méthode Naturelle de Classification des Animaux, p. 102.)  Duméril described the genus as follows:

The squatine (squatina), commonly called the angel, is a unique species, whose principal characteristics derive from the shape of the fins which are notched [?] at their origin, and the position of the mouth at the end of its rounded head.

(“Notched” is probably not an accurate translation from the French, more on that later.)

Ah, the angel shark.  A harmless and retiring fish that remains with us today.  Winkler experienced conflicting emotions over his fossil teeth from angel sharks – a ready appreciation of their singular appearance coupled with frustration over their challenging taxonomy.  Even today, it would seem, identifying extinct species within the Squatina genus can seriously irritate paleontologists.

Pictured below is a beautiful Squatina tooth that appeared amid several pounds of washed and screened material from a Late Paleocene formation (probably about 58 million years ago) on the Potomac River.  With microfossils as my intended quarry, I separated out, and set aside, the larger elements from the matrix, then spent many hours searching through the smaller bits of shell and quartz.  In a fit of frustration, all of it nearly went into the trash when nary a microfossil appeared.  That would have been a real loss.  One would have thought that, by now, I’d have learned the important life lesson –

Never write it off (whatever “it” might be), always check it out.

In the Notes below, I give appropriate credit to music critic Neil Strauss for this “check it out” life lesson.

The initial image above (on the left) shows the Squatina tooth from the side – this is how the tooth would sit in a bottom jaw.  The crown emerges from the root at a right angle!   Winkler was indeed correct on this – a “most curious” tooth.  The second image above (on the right) is a view looking at the tooth from the tongue's perspective.  (A slight break in the crown is evident on the right side of the tip.)

The first picture below is of the labial side of the tooth and shows a little apron of the crown that dips over the edge of the root.  The second image is from the backside of the root (the crown apron is at the top).

Finally, the last image below offers a difficult perspective - the apex of the crown is pointed straight at the viewer.  But, I think, this captures the triangular shape of the root that presumably inspired Winkler’s original name.

The angel shark itself is “bizarrely shaped.”  (Leonard Compagno, et al., Sharks of the World, 2005, p. 137.)  This modestly sized fish is (and probably was), on average, about 5 feet long (1.6 meters) and, as shown below in a picture of an extant shark, resembles a ray or skate (indeed, squatina is Latin for “a skate”).  (This picture of a Squatina dumeril was taken by Donald Flescher, NOAA/NEFSC, and was downloaded from the NOAA Fisheries Service website.)

The angel shark is remarkably flat, with its eyes located on the top of its head.  The Squatina is a stealth hunter, lying in sand and mud on the bottom waiting for a passing victim.  One of the features that distinguishes this shark from a ray is that the shark’s large pectoral fins are not attached to its head (hard to see that in the picture above).  I think that’s what Duméril was getting at in his description of the angel shark – “the fins which are notched [?] at their origin . . . .”  “Notched” is probably not really what Duméril intended with “échancrées” but it seems the best of the limited options offered by Google Translate.  Perhaps “separated” would work better.

Why is the fossil record of the Squatina such a taxonomic challenge as to its species?

In a nutshell, it’s because the teeth have not changed much over time and the genus has survived for such a surprisingly long period of time.

The long winded explanation is that what we have mostly from extinct angel sharks (and from any other shark) are their teeth and, unfortunately (for identification purposes), the shape of the teeth of this genus is paleontologically “conservative.”  As paleontologist Bretton W. Kent (Fossil Sharks of the Chesapeake Bay Region, 1994, p. 25) observed,

Not only are the teeth similar in different regions of each jaw, but tooth form changes very little during the evolution of this group.  Consequently, it is extremely difficult to identify fossil species of Squatina based solely on teeth.

“Extremely difficult to identify fossil species” are words that chill a collector.  But, according to Kent, although teeth alone don’t usually do it for identification, the prospect is better if you know the “stratigraphic position” from which the tooth came.  In other words (as I understand this), the differences in teeth among different species can be so subtle that identification may depend upon the rock layer of origin.  This, I think, brings geological age to bear on the process as was the case with my little tooth.  Conveniently, there are only three Squatina species represented in the Chesapeake Bay area’s fossil record and only one comes from Paleocene formations, the Squatina prima.  Hence, what I found is Squatina prima.

I was comfortable with that identification, based on Kent’s analysis, until I turned to Jim Bourdon.

In confronting Squatina teeth, Jim Bourdon, amateur paleontologist par excellence, artist, originator of the indispensible website The Life and Times of Long Dead Sharks (commonly referred to as “elasmo” after its URL:  www.elasmo.com), threw up his hands in frustration over the taxonomic confusion.

In the introduction to his treatment of the Squatina genus on “elasmo” titled Squatina Duméril 1906, Angel Shark – Upper Jurassic – Recent, Bourdon derided the species names – hassei, prima, and subserrata – as merely “chrono-buckets,” into which all of the teeth from the same stratigraphic position (he didn’t use the phrase "stratigraphic position," but I think that’s what he meant) are dumped.  In essence, “there appears to be no rhyme or reason for assignments to a particular species.”  (Well, there actually is a “reason” but probably not a good one in this instance – the geologic formations of origin.)  His annoyance at this state of affairs was palpable when he wrote,

I have not the time, material or inclination to straighten out this mess, so I'll merely accept these buckets and use them as such.

As Kent did, Bourdon drew attention to the “conservatism” of the Squatina.  The long life of the genus is a striking attribute, one for which Bourdon offered a note of, perhaps, grudging admiration (maybe, in some small way, this made up for the aggravation the genus’ teeth caused him):

Not too many vertebrate genera have remained around for 150 million years – Squatina is represented by complete skeletons in Germany's Upper Jurassic . . . .

One last question, why “angel” shark?  In fact, in the 1500s, the fish was called the “monkfish” because it looked like a monk.  (Compagno, p. 137.)  From the top, I suppose the head might resemble that of a tonsured friar.  Angel?  I haven’t come across an explanation yet, but the large pectoral wings moving independently of the head may have seemed like angel wings.  Regardless, in the end, we still have to deal with the angel shark's bedeviling little teeth.

Notes

1) The early publications considered in this post are in French.  I’ve relied on the Google Translation function and some common sense to render them into somewhat passable English.

2) The 1878 publication date of the volume in which Winkler’s description of Trigonodus primus appears seems to conflict with the 1874 date commonly associated with Winkler in the full scientific name of Squatina prima.  I haven’t been able to determine if Winkler’s memoir was actually first published separately in the earlier year and then subsequently bound in this later volume.  Perhaps it is significant that he begins his piece by noting that he acquired this collection of fossil fish teeth on the first day of 1874.

3) Regarding the lack of current acceptance of  Système Heersien or Heersian as a stratigraphic name (it is the named source of the fossil teeth that Winkler described in his memoir), see Geert De Geyeter, et al., Disused Paleogene Regional Stages from Belgium:  Montian, Heersian, Landenian, Paniselian, Bruxellian, Laekenian, Ledian, Wemmelian and Tongrian, Geologica Belgica, volume 9, No. 1-2, 2006.  (This link will ask to save the PDF to your computer.)

4) I must give credit where credit is due.  I’ve appropriated and massaged writer Neil Strauss’ credo – “I’ll check it out.”  In response to enthusiastic recommendations about music, movies, or people, Strauss, New York Times music critic for a decade, would say, “I’ll check it out.”  And sometimes the result would be life changing.  See, Neil Strauss, Preface to Mingering Mike by Dori Hadar and Mingering Mike, 2007.  Worthy of a dedicated post itself, Mingering Mike’s story is recounted in a recent Washington Post article by Katherine Boyle, D.C. Outsider Artist Mingering Mike’s Works to be Exhibited at Smithsonian, March 1, 2013; and in a couple of NPR stories by Xeni Jardin, May, 2007.