Monday, March 26, 2012

Questing After Species - Tales of Squirrels and Sharks

“What does the Questing Beast look like?” 
“Ah, we call it the Beast Glatisant, you know,” replied the monarch, assuming a learned air and beginning to speak quite volubly.  “Now the Beast Glatisant, or, as we say in English, the Questing Beast – you may call it either,” he added graciously –“this Beast has the head of a serpent, ah, and the body of libbard, the haunches of a lion, and he is footed like a hart.  Wherever this beast goes he make a noise in his belly as it had been the noise of thirty couple of hounds questing.”
“It must a dreadful kind of monster,” said the Wart, looking about him anxiously. 
“A dreadful monster,” repeated the other complacently.  “It is the Beast Glatisant, you know.” 
“And how do you follow it?” 
This seemed to be the wrong kind of question for King Pellinore immediately began to look much more depressed than ever, and glanced over his shoulder so hurriedly that his visor shut down altogether.
                                             ~ T.H. White, The Sword in the Stone (1939)

The King Pellinore of the Arthurian legend, as he is envisioned by T.H. White, is a befuddled, fumbling, gentle old man, in endless pursuit of the Questing Beast, an animal, despite the resemblance of its various parts to other animals, that is so clearly an entity apart, a different thing.  It’s that type of a seemingly eternal hunt that I share with Pellinore (though possibly I share other of his attributes as well).  In my case, I am in search of an understanding of what defines a species when working with fossils; as far as I can tell, this may be a quest without end.  This posting is an inchoate report from the quest.

To the amateur, the non-scientist, the literature on what constitutes a species is often a wild wood sporting thickets of thorny concepts and impenetrable language.  There are some capable guides who will take someone like me along different paths through the wood, though these paths may not lead to the same places.  There are, to say the least, disagreements as to what is a species and how to delimit such a thing.

There is no gainsaying the importance of this effort.  Paleontologist Donald Prothero asserts,
The species is the fundamental taxonomic unit in nature, the only such category that has biological reality.  How we determine whether a particular group is a phylum, class, order, or genus is largely arbitrary – but species have an existence outside the minds of scientists. (Bringing Fossils to Life:  An Introduction to Paleobiology, 1998, p. 35.)
I’ve been reading what the preeminent evolutionary biologist Ernst Mayr (1904 – 2005) believed constitutes a species.  Of particular interest is what he wrote about the change in naturalists’ concept of a species early in the 20th century.  Prior to this period, according to Mayr (What Evolution Is, 2001, p. 161 et seq.), the traditional typological species concept held sway – for the most part, physical attributes were what largely distinguished one species from another.  Darwin, for example, noted in On the Origin of Species (1st edition, 1859), “From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, . . . .”  (emphasis added)  (Darwin, unlike Prothero, acknowledges an arbitrariness to the concept.)

But at the turn of the century, the typological concept, wrote Mayr, gave way to the biological species concept, a concept centered on whether discrete populations could interbreed successfully.  It was a concept and change which Mayr championed.  In his account, the shift was prompted by a growing awareness of two species phenomena that profoundly challenged the typological species concept.  The first of these were cryptic or sibling species – different extant species with virtually indistinguishable taxonomic characteristics that can be found living in the same geographic areas, but which do not interbreed.  Mayr described such species as “remarkably common.”  Based on morphology alone, a clutch of more than one cryptic species would be deemed a single species.

The second of these phenomenon to undermine the typological concept is the reverse of the first, called by some polymorphic species.  (See, for example, Prehistoric Life:  Evolution and the Fossil Record by Bruce S. Lieberman and Roger L. Kaesler, 2010.)  These populations are different in terms of their typological characteristics but they do interbreed; hence, despite their divergent physical appearances, they are not different species.  Assessing just the morphology of a polymorphic species would lead someone to identify two separate species, when, in fact, there is only one.

And the morphology of organisms is what paleontology has to go on, and, indeed, only some of those physical attributes of extinct organisms is often all there is to work with in the effort to distinguish among species.  Knowledge of interbreeding is beyond its reach.  So, regardless of how much the reality of cryptic and polymorphic species may put the lie to some of the species identified in the fossil record, such errors may be the inevitable price that has to be paid.

I’ve found Prothero’s treatment of the same topics an enlightening counterpoint to Mayr, principally for the strength of its advocacy of the typological concept.  He indicts the biological species concept, not because it’s wrong, but because it is “so impractical” (i.e., observing mating among even extant populations is highly problematic), and observes that even “most biologists define species on morphological criteria alone.”  But he is careful to ensure that the crucial, ultimate reality inherent in the biological definition is not lost.
[O]rganisms recognize each other as members of the same species, especially when they are choosing a mate.  If the species differences have meaning to the organisms themselves, then biologists view them as the “real thing” that we are trying to approximate when we name species based on morphological features.  (p. 36)
Here then is the essence of my current understanding.  The identification of species based on differences in morphology, which is all we can do with fossils, serves to approximate the biological reality.  Well and good.  But my new appreciation of cryptic and polymorphic species is salutary in this context, suggesting that the physical attributes captured in fossils that are purported to identify species, sometimes may not.  That’s important to keep in mind.  As will be made clear below, I've found the work of paleontologist Alton Dooley to be immeasurably helpful in shaping my thoughts.

Two stories highlight some of the sources of my frustration and interest in this issue, and show that this issue isn’t just academic.  One is about extant squirrels, the other about Miocene sharks.

The Squirrels

Alton Dooley, paleontologist at the Virginia Natural History Museum, posted recently on his blog about the fascinating work he and the Museum’s mammalogist have been doing.  (Updates From the Paleontology Lab, February 29, 2012.)  His posting highlights some of the findings from a paper the two of them have published, identifying a novel way to distinguish the skeletal remains of the eastern fox squirrel, Sciurus niger, from those of the eastern gray squirrel, S. carolinensis.

Separating these two species when all that you have are skeletal remains is exceedingly difficult because of the overlap of some characteristics, such as size, and variation in other characteristics within each species.  Though a tell-tale difference between the two species is a tiny premolar present only in the S. carolinensis, that portion of the upper jaw is often not preserved, rendering that marker problematic.  Instead, in a clever approach, Dooley and Nancy Moncrief capitalized on the fact S. niger has a genetic condition which causes an enzyme to build up in the animal’s bones and teeth.  As a result, the skeletal remains of the S. niger fluoresce under ultraviolet light.  Those of S. carolinensis do not.  Despite several limitations to this test, Dooley demonstrated its utility in distinguishing between the two species in a set of 7,000 year old skeletal remains.

So, without use of an ultraviolet light, a mixture of ancient skeletal remains from these two species would in all likelihood be incorrectly considered representative of a single species.

The Sharks

On a hunt for fossils on the beach at the base of the Calvert Cliffs, I came upon this small block of gray clay that had fallen from the cliff face.

Here’s the tip of jewel circled.

Stumbling on this fossil precipitated one of those moments where time’s arrow pauses.  After several moments, I had the presence of mind to remove my gloves and reach for my camera.  Later, at home, I carefully cleaned away the matrix exposing a relatively large Miocene fossil shark tooth with a nearly pristine set of serrations.

So, what do I have?  Well, that’s a bit problematic.  Though there’s been a simmering debate over the genus name that might be applied to teeth like this (a debate which has everything to do with whether this line of sharks can claim the wonderful, extant great white), I’m persuaded by the camp (seemingly in the majority now) that believes the line this tooth belongs to went extinct in the Pliocene, so no great white on its résumé.  I’m going with the genus name Carcharocles.  (The alternative genus name, Carcharodon, is used by those seeing this line leading to the great white, Carcharodon carcharias.)  (For a useful, though rambling discussion of the competing schools of thought, see Megalodon:  Hunting the Hunter, by Mark Renz, 2002.)

Actually, it’s the species name that raises the real questions in my mind about this tooth and about species designations in general.  My evident choices from the middle Miocene (generally the period producing teeth at this stretch of the Calvert Cliffs - and I cannot be more precise because I'm not sure which bed this tooth eroded from) are C. chubutensis and C. megalodon.  The latter is the holy grail for many fossil hunters, the source of the largest of large fossil shark teeth.  C. chubutensis is generally thought to have preceded C. megalodon; in the words of one source, it was a “short-lived chronospecies,” or a “bridge” from a previous Carcharocles species to megalodon.  (See Carcharocles by David Ward and Jim Bourdon on the web.)

The confusion arises because the factors used to distinguish these two species, including relative size and the presence of cusplets (small peaks in crowns on either side of the main cusp), are not dispositive.  In the picture below, the cusplets on my tooth are circled.  There’s nothing faint or doubtful about them.

Regarding size, although the biggest megalodon teeth (largest on record is nearly 7 inches) surpass those of chubutensis (maximum is at least 5 inches), there is significant overlap, particularly given that these sharks’ teeth increase significantly in size from juvenile to adult.  Adult chubutensis or juvenile megalodon?  Take your pick.

The presence or absence of cusplets is often treated as conclusive – if the tooth is adorned with cusplets, it must be a chubutensis . . . except, according to those in the know, during this time period, the teeth of juvenile megalodons sported cusplets.  That certainly complicates matters.

The treatment of some of the differences between these two species by Bretton Kent in Fossil Sharks of the Chesapeake Bay Region (1994) captures the dilemma confronting the befuddled amateur fossil collector.
The teeth of C. megalodon differ from those of C. chubutensis in having a slightly broader crown and in lacking cusplets.  However, neither of these characteristics is absolutely definitive.  There are teeth known that are intermediate in form between these two species, such as having very weak, vestigial cusplets.  The understanding of these intermediate forms is problematical, since we lack sufficient information to decide between different possible interpretations of their significance.  Based on the available evidence, which is admittedly meager, they may simply indicate that C. chubutensis and C. megalodon are evolutionarily very closely related, but nevertheless distinct, species.  (p. 69)
The challenge in identifying a tooth like mine is that it falls in the gray zone – where neither size nor cusplets are clear evidence one way or the other.  Were it in the 4 to 5 inch range with cusplets, it would clearly be chubutensis; a similar size or larger without cusplets, megalodon.  The whole process is made incredibly complicated by natural variation, evolutionary changes, growth and change as the organism matures, overlapping ranges and time periods, and the limited physical features of these animals in the fossil record (primarily just teeth).

Closing Thoughts

In both of these stories, there was relatively little success when the physical or morphological attributes of the animals’ remains, fossilized or not, were used to differentiate between species.  And that’s about all that the taxonomist has to go on when dealing with fossils.

I asked Alton Dooley whether his work on the two squirrel species has anything to say about the biological validity of the often subtle morphological distinctions used to differentiate among species in the fossil record.

His answer was incredibly informative and has guided my thinking.  It merits some quoting.  (His full response is in the comment section of the squirrel posting on his blog.)

Paleontologists, he notes, are
using morphological differences as a proxy for reproductive isolation.  While this seems to work pretty well (using extant taxa as a model to test it), there are reasons for suspecting that this is not always reliable. . . .  Even if morphospecies don’t exactly correlate with biological species, they are something we identify and measure, and track through time, so they’re probably good proxies on average.
But given that such morphospecies are proxies for a biological phenomenon, he asks,
[W]hat is a biological species, anyway?  The species problem has been debated with no satisfactory answer probably since Linnaeus.  Darwin spent significant time discussing it in ‘Origin of Species’.  Ernst Mayr spent most of his career attempting to address the issue.  In spite of what high school biology students are taught, coming up with a clear-cut definition of species that applies across all organisms seems to be impossible.  It’s similar to the struggles to define “life”; no one can really define it, but we all know it when we see it.  Usually.  Except when we disagree.  And then there’s this one case that just doesn’t seem to fit . . . .
And so I resume my quest, armed with a bit more understanding and some direction from very capable guides.  I also head forth with a final bit of commentary from Dooley (so relevant to the shark story I just told).
I think most of the fossil shark taxa below the genus level are junk!

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