Shark tooth ID

[Andy123]

Hi guys I have an angustidens tooth (on the left) and a Chubutensis Tooth (on the right) I am finding it hard to separate the 2 as different species as they look very similar, the only difference I notice is the roots are different, are there any key factors to look out for with identifying the 2. Thanks  

[sixgill pete]

Really the only way to differentiate to be sure is knowing the age of the sediments. C. angustidens is an Oligocene species and C. chubutensis is early Miocene. The roots can be different due to tooth position, along with width of the tooth. 

[digit]

Such is the problem with chronospecies where a lineage evolved over time with one "species" morphing over time into the next in a succession leading (in this case) to the megalodon shark (which grabs all the attention and movie rights).

 

It helps to keep in mind that nature has no comprehension of "species" which is a concept entirely created by humans so that we could pigeon-hole individuals into nice neat compartments in order to speak about them or study them. I had a friend who was a taxonomist that worked on bromeliads (the new world tropical plant family that includes forms as diverse as Spanish Moss and pineapples). In a former life I had a collection of many hundreds of species and varieties of this plant family. Many years ago I once asked him about two plants that I had of a particular Brazilian species that looked nothing alike but which had been recently synonymized into a single species. One had short green leaves and the other had long strappy burgundy leaves. I began my question by stating the obvious that these two plants looked nothing alike and I could not wrap my head around how these two plants could represent the same species. We were taking a tour at the time of the collection house at Selby Gardens in Sarasota, FL and so he wandered into the collection and pulled out half a dozen or so plants and arranged them on an empty potting table. They formed a transition from the short green leaved form on one end and the strappy burgundy form on the other. He stated that plant collectors back in the 1950s had encountered and collected both of the forms on the ends and found them desirable forms to bring into cultivation. People had been growing and propagating those two forms which had been described by taxonomists as totally separate species. In later years additional waves of collections to the region where these two "species" were found had turned up several more additional forms which were for a time thought to be just varieties of the initial two distinctive species. Over time the gaps were filled and a continuous spectrum had been created between one "species" and the other. He asked me to look at the line-up and decide where one species ended and the other began which was of course as impossible as asking where black changes to white in a gray scale.

 

 

In the end the two species had to be synonymized with the first described species taking precedence over the other which became the junior synonym. Plant growers are free to use cultivar names to help distinguish the various forms of a species when they are quite distinctive--like calling black and white "very dark gray" and "very light gray".

 

Harry's concrete example of why two particular plant species needed to be synonymized also taught me a broader lesson about the nature of "species" and how not to think of them as discrete entities rigidly defined and immutable and more as convenient (sometimes temporary) labels that allow science to happen.

 

You can find examples of "classic" or prototypical specimens that represent the named stages along the development of this particular lineage of sharks as they evolved from a smaller species having teeth without serrations but with well developed side cusps to a larger species having huge serrated bladelike teeth without (in most cases) side cusps. There will always be forms along the continuum where you can point to something like a very "chub-like" angie where the categories get rather blurry. This is just the nature of a chronospecies and why it is helpful to know the age of the formation in which it was found.

 

Here's an article about a friend of mine who's worked on this transition with a link to his paper on the topic.

 

https://www.floridamuseum.ufl.edu/science/megalodons-teeth-evolved-into-the-ultimate-cutting-tools/

 

 

Cheers.

 

-Ken

[FranzBernhard]

Very interesting, @digit

Little bit of topic, but I have a question:

10 minutes ago, digit said:

He stated that plant collectors back in the 1950s had encountered and collected both of the forms on the ends and found them desirable forms to bring into cultivation.

Was there never any (accidental) interbredding between the two forms during cultivation?

Franz Bernhard

[sharkdoctor]

You may want to supplement your reading from @digit with Brett Kent's amazing shark chapter from the Calvert volume: 

https://smithsonian.figshare.com/articles/book/The_Geology_and_Vertebrate_Paleontology_of_Calvert_Cliffs_Maryland_USA/9761762

 

 

[digit]

43 minutes ago, FranzBernhard said:

Was there never any (accidental) interbredding between the two forms during cultivation?

With collectors who had extensive collections and may have had both forms of a plant there is always the chance of accidental (or intentional) cross-breeding between plants. Bromeliads propagate very readily by vegetative offsets known as 'pups' and thus they were generally spread to other collectors as vegetative clones of the parent plant.

 

Plants (and sharks) may evolve by an interesting process called reticulate evolution.

 

https://en.wikipedia.org/wiki/Reticulate_evolution

 

As a simple example, picture a plant species that grows in the tropics along a mountain ridge and in the valleys to either side. Let's say it likes warm moist shady conditions to thrive and the bottom of the leaves contain a burgundy pigment. There are few photons of light that filter through the tree canopy and reach the plant. Chlorophyll uses photos in the reddish end of the spectrum and reflect back those in the green range (making the leaves appear green to our eyes). If the photon does not hit the chlorophyll molecule on its first pass it may be reflected by this burgundy pigment and give the chlorophyll a second chance at snagging the photon. (This is a strategy used by a diverse assortment of plant families that grow in low light conditions.)

 

         

 

The climate for the moment is much the same across its range including the forested mountain ridge and a single species with little blue flowers and discolor (burgundy underside) leaves covers the region. Then the climate climate goes through a cooler drier period (say a period of glaciation at the higher latitudes). The forest on the mountain ridge disappears and it becomes a rocky zone inhospitable to our little plant. The populations on either side are cut off from each other. Because of a rain shadow effect one valley changes from a tropical forest to a more arid grassy plain.

 

https://en.wikipedia.org/wiki/Rain_shadow

 

Most of the population of our little plant struggles to survive and adapt in these new conditions but some evolve to thrive in this new habitat. They no longer need their burgundy pigments because the canopy is gone and they are growing in open savanna. No need to produce those burgundy pigments anymore and since there is plenty of sunlight the plants long strappy leaves change over time to become much shorter. The pollinating insects that were attracted to the little blue flowers are no longer to be found on this side of the ridge and over time the plants are pollinated more and more by bats which use odors to locate their source of nectar. The flowers change over time from small blue petals to large white petals and become very fragrant.

 

Another climatic shift brings an end to our hypothetical ice age (quick wasn't it?) and conditions return to much the way they were at the beginning of this thought experiment. Where there was once one species there are now two distinctive species each adapted to its own environment. The white-flowered bat-pollinated "newer" species is now limited to growing in a few areas of grassy savanna being encroached upon by the returning forest. This forest brings with it the ancestral form of the species with the strappy discolor leaves and the little blue flowers. If the pollinators are now so different that these plants will never cross pollinate again then the two lineages live on as independent species. Let's complicate matters and suppose a species of moth that is equally happy drinking nectar from the little blue flowers and the larger smelly white flowers. These two plants were once a single species and so let us say that they have not diverged enough that cross-pollination is a problem. A natural "hybrid" form is created with medium length leaves and just a hint of color on the bottom surfaces. The flowers are medium size and a light blue. This hybrid form that was created by an instance of reticulate evolution may then back-cross with either of the two diverged "parent" forms to create a whole spectrum of intermediate forms similar to a hybrid swarm.

 

https://en.wikipedia.org/wiki/Hybrid_swarm

 

Evolution is the adaptation to the current local environment and as environments are often complicated and ever-changing evolution is usually messy and rarely straightforward and linear. Did the evolution of Otodus obliquus in the Paleocene through many named stages to Carcharocles megalodon in the Mid Miocene occur in a simple linear fashion or was it complicated and turbulent? Likely the latter. These mega-toothed sharks ranged far and wide globally and likely there were isolated pockets where sub-populations evolved to meet the needs of dispatching local prey items that were plentiful. Shark fossil evolution is more problematic than for other taxonomic groups since we are usually only left with isolated teeth and no other skeletal elements (except in rare cases) to work with and makes understanding the difference between extinct shark species much more difficult.

 

Looking at your two teeth that started this discussion I'd be hard-pressed to ID the second larger tooth on the right as C. chubutensis out of context for the age of the formation it was found. Florida is mainly megaldon country with very few of the more ancestral cusped forms showing up. I did have the opportunity to collect a few "chubs" when I was up diving in North Carolina but have not yet had the pleasure of collecting inland in the Carolinas or other areas where C. auriculatus or C. angustidens may be found. To my eye your larger tooth seems to be on the early end of the evolution from angustidens to chubutensis (others with more experience may disagree).

 

 

Cheers.

 

-Ken

[Andy123]

The angi in the left was oligocene epoch 28mil years ago approx found chandler bridge formation SC and the chub in the left is early miocene epoch  20 million years ago approx chilcatay formation Peru, but like I said it’s hard to tell the difference between the 2  thanks for the info !