Saturday, June 19, 2021

Chronospecies ~ Interesting But Perhaps Not Much Help

 In which the blogger grapples with a concept “new” to him, avoids several issues requiring deeper thinking than he’s capable of, and revisits a tooth to see if it can be identified this time around.

Very recently I encountered the term chronospecies and thought it new to me.  I took this to be  a reflection of how poorly read I am in the paleontological literature.  Subsequently, I found that the term had appeared in a quotation I used in a post nine years ago.  This is even more damning, revealing how inattentive I can be with what reading I do.  (See, Questing After Species:  Tales of Squirrels and Sharks, March 26, 2012).  This current post remedies that oversight (perhaps).

The term chronospecies is, I think, one that can come with an agenda.  That is, it reflects a perspective on how evolution has functioned for the species lineage to which it is applied.  This usage alone says nothing about whether the user believes that this evolutionary mode is pervasive or relatively infrequent, but it should trigger that question.  Chronospecies is defined nicely and quite succinctly by science historian John S. Wilkins as:

Arbitrary anagenetic stages in morphological forms, mainly in the paleontological record.  (Summary of 26 Species Concepts, 2002.)

Actually, this is Wilkins’ definition of successional species, of which he identifies chronospecies as a synonym.  Unpacking this a bit -- a chronospecies is a member of a lineage in which the transition from one physical stage (species) to another occurs through anagenesis, that is, by a slow process over time with one ancestor flowing into a descendant without speciation through splitting or branching.  Given how this evolutionary change apparently occurs, separating the lineage into individual species is an arbitrary act.  It’s unclear where one chronospecies ends and the next begins.

First challenging topic being avoided in this post.  The relative frequency or commonality of anagenesis evolution is at the heart of the discussions about what the fossil record reveals regarding the mechanisms through which speciation occurs.  Much simplified, the contrast is between slow, incremental change (anagenesis), and relatively sudden bursts with branching that follow long periods of little change (punctuated equilibrium).  Paleontologist Donald R. Prothero provides a nice overview of The Species Problem in Paleontology (Bringing Fossils to Life:  An Introduction to Paleobiology, 1998, p. 39-41.)  This is one of those difficult topics I’m avoiding in this post.

My most recent encounter with chronospecies came when paleontologist Victor Perez used it in response to a question during a Zoom presentation on his new and powerful methodology for estimating the body length of Otodus megalodon (the focus of all megatooth shark obsessives).  Critical to testing his approach is utilizing associated O. megalodon dentitions and knowing whether the teeth belonged to mature or immature individuals, and whether the dentitions belonged, in fact, to O. megalodon in the first place.  With chronospecies, this can be challenging if it can be done at all.  (For a detailed look at Perez’s methodology, see Body Length Estimation of Neogene Macrophagous Lamniform Sharks (Carcharodon and Otodus) Derived From Associated Fossil Dentitions, Palaeontologia Electronica, Volume 24, Number 1, 2021, 4-5.)

Additional challenging topics being avoided.  This post is not about ways of estimating how big the biggest shark was (for that, read Perez’s paper cited above).  I’m also not wading into the taxonomic morass that lurks beneath the application of the single generic name Otodus to the lineage of megatooth sharks that reaches back to the Paleocene and earlier and ends with the extinction of O. megalodon in the Pliocene.  In this post, I use Otodus for a simple and very unscientific reason:  I stumble over the pronunciation and spelling of the generic name applied by many paleontologists to most of the species in this lineage, Carcharocles.

Further, I am not in a position to weigh the relative merits of the different hypotheses advanced for the competing generic names.  Again, Perez’s 2021 paper provides a quick overview of this taxonomic debate.

What this post is about.  This post is about my effort to understand the concept of a chronospecies and how it bears on the tooth that was a subject of the 2012 post referenced earlier.  While walking the beach near Randle Cliff Beach along the Calvert Cliffs in 2012, I found the tooth shown below in a small block of material that had fallen from the cliffside.

The tooth’s cusplets are circled because that is one of the critical morphological elements used by paleontologists to distinguish two chronospecies in the Otodus lineage:  O. chubutensis and O. megalodon (below I refer to these two species by only their species name).  In the post from nearly a decade ago, I tried and failed to identify the species associated with it.  Will things be better a decade later?

It’s important to have a sense of the lineage of which these purported chronospecies are a part.  Here is a very helpful graphic showing this lineage which appeared in a paper by paleontologists Catalina Pimiento and Meghan A. Balk (Body-Size Trends of the Extinct Giant Shark Carcharocles Megalodon:  a Deep-Time Perspective on Marine Apex Predators (Paleobiology, Volume 41, Number 3, 2015, Figure 1, p. 480):

Pimiento and Balk summarize this lineage, as depicted in this figure, as follows:

It has been proposed that the megatooth sharks comprise a series of chronospecies (i.e., a group of species that evolve via anagensis and that gradually replace each other in a evolutionary scale) that are distinguished from each other in the fossil record by the morphological changes of their teeth.  These changes include the loss of lateral cusplets; broadening of tooth crowns; and, of most relevance to this study, size increase through geologic time.  (p. 480, citations omitted.)

In another article, Perez and a different set of colleagues tackled the question at the heart of this post.  Their piece is titled:  The Transition Between Carcharocles chubutensis and Carcharocles megalodon (Otodontidae, Chondrichthyes):  Lateral Cusplet Loss Through Time (Journal of Vertebrate Paleontology, Volume 38, Number 6, 2018).  These authors note that, although all chubutensis teeth apparently had cusplets, that’s not a sufficient criterion upon which to distinguish these two species because some members of megalodon also had cusplets, including some juveniles as well as some adults who retained cusplets as vestigial attributes.  Their study was focused on verifying statistically the cusplet changes in these fossil teeth as the sources of specimens along the Calvert Cliffs become geologically younger.  They demonstrate that the Miocene-aged formations exposed along these Chesapeake beaches cover the time period during which the transition from the one cuspleted species to the next uncuspleted species occurred.  That said, Perez et al. assert categorically that “definitive separation” of these two species “is impossible because a complex mosaic evolutionary continuum appears to characterize the transformation from cuspleted to uncuspleted teeth.”

But Perez et al. don’t come away empty handed in this taxonomic challenge.  They conclude that the change from cuspleted chubutensis to uncuspleted megalodon took place in a 2.4 million stretch of time, roughly 16.4 million to 14 million years ago.  Thus, go back further than 16.4 million years ago (in the Calvert Formation) and the likelihood that what’s at hand is chubutensis is very significantly increased; come forward to younger than 14 million year ago (into the Choptank and St. Marys Formations) and a cuspleted specimen is highly likely to be a megalodon (juvenile or with vestigial cusplets)  But, find yourself in that gray zone where the chubutensis to megalodon transition appears to have occurred and all bets are off.

I feel that this is, in some ways, a flipping of the script in the relationship between formation and fossil.  Relevant here is the concept of an index fossil, that is, a fossil of a species with a relatively broad geographical distribution but a relatively brief geological and temporal appearance.  Such a fossil can be used to identify geologic formations.  In essence, if you find the index fossil you know which formation you’re dealing with.  The flow is from fossil to formation, the first can identify the second.  With a chronospecies, the flow is the opposite, moving from formation to fossil.

In the end, because my tooth has cusplets, the odds favor it being a chubutensis.  But, and it’s a critical but, I do not know the bed in the formation that cradled this tooth for the millions of years after it was lost from a shark’s mouth.  Above the beach along which this tooth was found, a fairly broad range of formations is exposed, including the beds that encompass that 2.4 million year long period where the cusplet transition was principally occurring.  Absent that information and, after all of this sound and fury (well, curriebuction, at least) in this post, I’m left standing here with fossil in hand and still I do not know if this is a chubutensis or a megalodon tooth.

Ah, chronospecies, disappointing. 

No comments:

Post a Comment

Nature Blog Network