Is this the Missing Link to Sawskates of the WIS? Onchopristis - Ischyrhiza

[Mikrogeophagus]

Still hard at work on my North TX shark guide which has grown to be quite the behemoth over the past year or so. As I've continued writing, I've learned all sorts of things both in looking at the past literature and making discoveries of my own. To keep the guide more manageable for casual reading, I won't be adding every thought that passes through my mind. This topic, however, is something I think to be particularly intriguing, so I'm giving it a post of its own. It intersects with my old Central TX shark guide which I will link here in case people are not familiar with it.

 

 

Ischyrhiza and Onchopristis are two genera of sawskates that are known in the Texas Cretaceous from the Albian all the way into the Maastrichtian. As shown in the phylogenetic tree below, their respective families, Ischyrhizidae and Onchopristidae, are considered to be sisters and therefore closely related.

 

From: http://dx.doi.org/10.13140/RG.2.2.12811.41765

 

Currently in my collection from TX, the chronological record of these two genera appear as follows:

 

Late Albian: Onchopristis dunklei (possible Onchopristis dunklei praecursor)

Middle Cenomanian: Onchopristis dunklei dunklei

Late Cenomanian: ?Onchopristis sp. rostrals (possibly new species)

Late Turonain: Ischyrhiza schneideri

Early/Middle Campanian: Ischyrhiza mira

Maastrichtian: Ischyrhiza mira

 

*NOTE: Perceptive readers may notice a lack of mention of Ischyrhiza avonicola and similar species that are known only by short rostral teeth. I will dive deeper into these in my guide. I do not think they are important to talk about here.

 

Onchopristis dunklei is a species of sawskate strongly represented in the Woodbine of North TX, where it was also originally described. This organism commonly manifests as oral teeth and rostral teeth. The rostrals are iconic for their multi-barbed distal margins, which is a key identifier that is never seen in any species of Ischyrhiza. Their heterodont oral teeth are quite distinct from those of Ischyrhiza as well. Their lateral shoulders are much shorter and the tooth as a whole is comparatively mesiodistally compressed, presenting a top-heavy appearance.

 

Onchopristis dunklei rostral from Woodbine

 

Onchopristis dunklei oral from Woodbine (Cappetta and Case 1999)

 

Skipping over to the Eagle Ford/Austin Chalk of North TX (Turonian-Coniacian), we see the appearance of Ischyrhiza schneideri. This species was at one point considered synonymous with the similar Ischyrhiza mira (Campanian-Maastrichtian). Despite the identical nature of their rostral teeth, the morphology of the orals for both species are very clearly different. I. schneideri typically has skinnier lateral shoulders, a thinner and longer labial apron, and shorter root height. There is also a co-occurring species called Kiestus texanus that is very similar to I. schneideri and I suspect may be synonymous by representing different positions in the same mouth, but for brevity's sake I won't dive into this.

 

I. schneideri orals and rostral (Cappetta and Case 1999)

 

I. mira rostrals and orals from NSR watershed

 

Nestled between the temporal occurrences of the Middle Cenomanian O. dunklei and Turonian/Coniacian boundary I. schneideri lies a strange association of sawskate orals and rostrals from the Late Cenomanian Lower Bouldin Flags in Central TX. I have not seen something like this reported in the literature, and such a perfect transitional state of sawskate evolution feels almost too good to be. Shown below are rostral teeth that bear prefect resemblance to those which belong to the older O. dunklei and in co-occurrence are oral teeth in close resemblance to the younger I. schneideri. In that same deposit I did not find any O. dunklei-like orals or I. schneideri-like rostrals.

 

?Onchopristis sp. rostrals

 

?Onchopristis sp. orals

 

?Onchopristis sp. could be a "missing link" that appears to stitch together a continuous lineage of sawskates across the Texas stratigraphic column! The piecewise nature of their evolution seems to allow the puzzle pieces to simply fall in place. The shared rostrals of O. dunklei and ?O. sp. connect the two, as do the shared orals of ?O. sp. to I. schneideri, and finally the shared rostrals of I. schneideri and I. mira. 

 

Evolution can sometimes occur in rapid bursts. Perhaps, ?O. sp. is a rare glimpse at a snapshot taken midway within the quick transition from one genera to another. This brings the arbitrary nature of taxonomic distinctions into the equation when trying to define such an organism. I am leaning towards calling this a member of Onchopristis because of the very distinct barbs on the rostral teeth, however it is fair to argue for these as belonging to Ischyrhiza too. Perhaps these two iconic genera could even be synonymized... what a headache! And of course, it is possible I am looking at two different, but co-occurring species. I am doubtful of this, however only more screening in the Lower Bouldin Flags could truly clarify this matter.

 

Curious to hear what y'all think...

 

Thanks for reading!

[Jared C]

A piece of the puzzle to add in:

 

In North Texas, the upper cenomanian/lower Turonian are represented well in the Britton member of the eagle ford. However, in central Texas the Eagle Ford as you know is a lot thinner, and the members have provided some challenges to geologists. 

I was trying to solve this puzzle myself, and by all accounts of recent research I read, it appears there is an unconformity between the bouldin flags and south bosque member that completely cuts out the upper cenomanian-lower Turonian. 
 

All that is to say that the bouldin flags is most likely not late cenomanian, but actually middle cenomanian. The south bosque picks up in the lower middle Turonian it seems. 

Edited Wednesday at 03:38 PM by Jared C

[Mikrogeophagus]

2 hours ago, Jared C said:

All that is to say that the bouldin flags is most likely not late cenomanian, but actually middle cenomanian. The south bosque picks up in the lower middle Turonian it seems.

That's pretty interesting! I'm admittedly not well versed with the local geology, so I'll have to look more into that. Despite the shift to the Middle Cenomanian from the Late Cenomanian, I'm thinking the Bouldin Flags would still be younger than the Woodbine (Rush Creek Member specifically) seeing as the Pepper Shale of Central TX correlates with the Woodbine and is stratigraphically lower than the Bouldin Flags. Thanks for the correction!

[Jared C]

17 minutes ago, Mikrogeophagus said:

That's pretty interesting! I'm admittedly not well versed with the local geology, so I'll have to look more into that. Despite the shift to the Middle Cenomanian from the Late Cenomanian, I'm thinking the Bouldin Flags would still be younger than the Woodbine (Rush Creek Member specifically) seeing as the Pepper Shale of Central TX correlates with the Woodbine and is stratigraphically lower than the Bouldin Flags. Thanks for the correction!

It's definitely younger, and I think you're on to something. You can probably write a book at this point with your north Texas shark teeth project. The Roger and Farrish field guide is a tad outdated and is still the bible for sharks across the US. I saw a copy in prep lab up in Bowman, North Dakota where the Hell Creek is the main focus. I suspect the people are hungry for another shark book