Campanian Ischyrhyza

[readinghiker]

I am working on a new shark fauna from the Campanian, located to the southwest of Cuba, New Mexico.  This is the youngest Cretaceous shark fauna

known from this state.  I am assuming that the pictured oral teeth are from  Ischyrhyza cf. mira, but there are some anomalies that puzzles me.

As you can see, one tooth has ornamentation on both labial shoulders of the main cusp, while the other has ornamentation on the

lingual shoulders.  Is this due to ontological reasons, sexual dimorphic reasons, or something else?  All oral teeth from Ischyrhyza

mira from the Turonian and Coniacian from New Mexico that I have seen lacks this ornamentation, lingually or labially.  Does anyone

have any suggestions as to what is going on here?  Thank you!

[JamieLynn]

While I cannot help you, I will be curious to see what others say, as I too have found various tiny sawfish teeth that seem a bit different, with ornamentation that others from the same local to not have.

[Al Dente]

Some of the Maastrichtian ones have ornamentation. Here's a tooth found in the Mt. Laurel Formation of NJ that I photographed for Jerry Case a number of years ago.

 

 

[readinghiker]

So did Jerry Case call these teeth Ischyrhyza mira?  If not, do the ornamentations lead to a new species?  If so, why did I. mira develop ornamentation in the late Cretaceous?  Was this species so cosmopolitan that they could be found in both the New Jersey area and the Western Interior Seaway?  So many questions!

[FossilDAWG]

Ischyrhiza mira has been reported from the Dinosaur Park Formation in Alberta (DOI: https://doi.org/10.1017/S0022336000028006) Ischyrhiza cf mira has been reported from the Favel Formation (Late Cenomanian - Mid Turonian) of Manitoba (Kilmury AA, Anderson A, Wijesinghe DS, Verstraete AF, Ezeana W, Anderson AE, Brink KS. 2023. Microvertebrate faunal assemblages of the Favel Formation (late Cenomanian-middle Turonian) of Manitoba, Canada. PeerJ 11:e15493 https://doi.org/10.7717/peerj.15493)

So yes, the taxon occurs in the Western Interior Seaway far to the north of New Mexico.

 

Don

[Al Dente]

1 hour ago, readinghiker said:

Jerry Case call these teeth Ischyrhyza mira?

Yes.

 

1 hour ago, readinghiker said:

If so, why did I. mira develop ornamentation in the late Cretaceous

No idea. There were other changes too. The size of the rostral teeth increased dramatically by the mid to late Maastrichtian.

[readinghiker]

Thank you, Don and Al Dente.  It seems that Case was a lumper, putting taxa with similar morphologies together.  I would tentatively put myself in this category, but I wonder if I am being too conservative.  Some sharks, such as Scapanorhynchus texanus were so successful that there was very little morphological change, if any, for literally millions and millions of years.  However, it is generally agreed that species can mutate into a new animal after around two million years.  The Coniacian Ischyrhyza teeth that I have collected had no ornamentation displayed (nor did the Santonian Ischyrhyza teeth).  The difference in years between the Coniacian and Campanian sites is at least three to four million, which would put a new species of Ischyrhyza well within a  possibility.  Would it be a leap to become a splitter with these teeth (there are many, so these are not anomalies)?  And would the labial versus lingual ornamentations be a result of sexual dimorphism?  Thanks to all who reply to this thread, there is so much knowledge here!