Extant Hexanchus nakamurai (Bigeye Sixgill Shark)

[MarcoSr]

Here is a Hexanchus nakamurai (Bigeye Sixgill Shark) jaw from my collection.  The jaw is 8 inches wide and 5.25” inches high.  There are two extant Hexanchus species, Hexanchus griseus (Bluntnose Sixgill Shark) and Hexanchus nakamurai (Bigeye Sixgill Shark).  H. nakamurai is a much smaller adult shark than H. griseus with a max size of about 180 cm versus a max size of at least 482 and probably 550 cm for H. griseus Ebert 2013.

 

The bigeye sixgill shark has also been called Hexanchus vitulus by researchers like Compagno and Bass in publications from the 1970s and 1980s.  This is what Compagno said in 1984:

 

 

 

More modern researchers like Ebert and Fowler now use H. nakamurai as the name for the bigeye sixgill shark.

 

Note this jaw had some features that made identification to the species a bit tricky.  Looking at publications from two major shark researchers, Compagno 1984 and Bass 1975, H. griseus have 6 lower lateral teeth on each side of the symphyseal tooth and H. vitulus (aka nakamurai) only have 5.  Note this jaw has only 5 lower lateral teeth.

 

The below two pictures are from Compagno 1984 "VOL. 4   SHARKS OF THE WORLD An Annotated and Illustrated Catalogue of Shark Species Known to Date Part 1 - Hexanchiformes to Lamniformes:"

 

H. griseus:

 

 

 

Note Adnet 2006 states (see below) that this dentition shown is an incorrect representation for H. griseus because of the lower symphyseal tooth shown.

 

H. vitulus (aka nakamurai):

 

 

 

I have enclosed in a red box below the jaw/tooth differences between H. griseus and H. vitulus (aka nakamurai) from Bass 1975 V “The families Hexanchidae, Chlamydoselachidae, Heterodontidae, Pristiophoridae, and Squatinidae.”

 

 

 

Also pl. 3 B from Bass 1975 below for H. vitulus (aka nakamurai) matches the picture below of the upper parasymphyseal teeth from this jaw.

 

 

Below are also the dentitions of a H. griseus and H. vitulus (aka nakamurai) from Bass 1975:

 

 

So based upon the 5 lower lateral teeth and the four parasymphyseal teeth bulging markedly forward from the line of the rest of the jaw, the jaw in this post would be a H. vitulus (aka nakamurai) per Bass.

 

That being said Compagno 1984 stated the below:

 

 

 

So per Compagno a H. griseus can also have only 5 lateral lower teeth.  Also, some of the tooth features of the upper teeth and the total number of teeth of this jaw more closely match H. griseus.  This jaw was sold as a H. griseus by a reputable Taiwan shark dealer which adds to the confusion.   I see lots of jaws misidentified by sellers.  I see line drawings and grainy dentition pictures by the major shark researchers like I’ve shown in this post which don’t really show adequate tooth detail for species differentiation.

 

Taking all of the above into consideration, I’m going with an id of Hexanchus nakamurai (Bigeye Sixgill Shark) for this jaw. It is really sad that the published information by shark researchers on extant shark jaws/teeth for a good number of species is confusing, sometimes contradictory and often inadequate to positively id extant shark jaws to a species from it.  Also it is very difficult to find out what publications have been written on a shark genus/species (I use google searches, shark reference websites and references in applicable papers) and to get copies of them (a lot of the papers and especially the books are expensive to buy).  You really need to see the actual shark that the jaw came from to be able to make a more positive id to a species in a lot of cases.  The large number of misidentified teeth and jaws on the web only amplifies the id difficulty.

 

Here is the overall jaw (8 inches wide and 5.25” inches high):

 

 

 

Note that the cartilage in this jaw is very thin compared to the jaws of other species.

 

 

If you believe Welton 1978 and the discussion by Adnet 2006 "Biometric analysis of the teeth of fossil and Recent hexanchid sharks and its taxonomic implications" below the median erect cusp on the lower jaw symphyseal tooth below indicates a H. nakamurai shark.  Adnet also states that the line drawing by Compagno that I show above for H. griseus  is an incorrect representation showing such a tooth.

 

 

Lower jaw symphyseal tooth:

 

 

 

The upper jaw has parasymphyseal teeth.  Here is the upper jaw symphysis where you can see the two pairs of parasymphyseal teeth:

 

 

 

Continued in the next reply:

 

Marco Sr.

[MarcoSr]

Hexanchus nakamurai have very non-descript commissural teeth.

 

Upper jaw left and right side commissural teeth:

 

 

 

Lower jaw left and right side commissural teeth:

 

 

 

Below are pictures of the upper jaw lateral teeth (labial views).

 

Upper jaw UL 1&2 and UR 1&2 teeth:

 

 

 

Upper jaw UL 3&4 and UR 3&4 teeth:

 

 

 

Upper jaw UL 5&6 and UL 5&6 teeth:

 

 

 

Upper jaw UL 7&8 and UR 7&8 teeth:

 

 

 

 

The lower jaw has 5 lateral teeth left and 5 lateral teeth right of the symphyseal tooth in addition to the non-descript commissural teeth.

 

Below are pictures of these lower jaw lateral teeth (labial and lingual views) left and right side.  Note per Adnet 2006  “The main results suggest that the cusp number ratio (number of cusps per mm) is preferable to width of the lower tooth for inference of total body size, at least in species of Hexanchus. The presence of a serrated edge or an enlarged acrocone appears to depend on ontogeny and care must be taken when using these as taxonomic characters.”

 

Lower jaw LL 1 and LR 1 teeth:

 

 

 

Lower jaw LL 2 and LR 2 teeth:

 

 

 

Lower jaw LL 3 and LR 3 teeth:

 

 

 

Lower jaw LL 4 and LR 4 teeth:

 

 

 

Lower jaw LL 5 and LR 5 teeth (Note the LR5 tooth is pathological):

 

 

 

Continued in the next reply:

 

Marco Sr.

[MarcoSr]

Here are pictures that show the number of rows of teeth in this jaw:

 

I can see 5 rows of upper symphyseal and commissural teeth and 3 rows of upper teeth in the rest of the jaw:

 

 

 

I can see 4 rows of lower teeth:

 

 

 

The upper jaw looks like it has 17 tooth files including 2 parasymphyseal, 8 lateral and 7 commissural files on the left side and 18 tooth files including 2 parasymphyseal, 8 lateral and 8 commissural files on the right side of the symphysis.

 

The lower jaw looks like it has 12 tooth files including 5 lateral files and  7 commissural files left and 10 tooth files including 5 lateral files and 5 commissural files right of the symphyseal tooth.

 

Marco Sr.

[doushantuo]

Good post,as always

[MarcoSr]

48 minutes ago, doushantuo said:

Good post,as always

 

Thank you.  I went into a lot of detail in this post because my research showed that this jaw was misidentified by the seller.  I see many misidentified shark jaws on e-bay and other sites on the web.  This jaw was from a reputable seller who sells lots of shark jaws on e-bay.  It is disturbing because in a lot of cases you have to rely on the seller's id because the jaw/teeth features/characteristics themselves don't allow an independent check of the species id given by the seller.  The only way to be sure of a lot of shark jaw species ids is to see the shark that the jaw came from and compare the shark features/characteristics in addition to the jaw/teeth features/characteristics to the literature.  Unfortunately the literature can be inadequate in some cases to even do this especially if you are not a shark expert.

 

Marco Sr.

[doushantuo]

about to post something you might like,BTW

 

"Diversity of dermal denticle structure in sharks"

@MarcoSr

[ynot]

I really like what You do with these extant shark jaw threads.

Please keep them coming.

[Tidgy's Dad]

These really help put our fossil shark teeth into the context of how they fit into the creature's mouth. 

[MarcoSr]

2 hours ago, ynot said:

I really like what You do with these extant shark jaw threads.

Please keep them coming.

 

Tony

 

My Luxo magnifying lamp went out on me last week so I couldn't look through matrix and used the time to do some research and take some pictures of a few of my extant shark jaws.  I'm supposed to get a new magnifying lamp in the mail tomorrow.  If it works OK, I'll be back to searching VA matrix for the VA fauna studies that I help with.

 

Marco Sr.

[MarcoSr]

2 hours ago, Tidgy's Dad said:

These really help put our fossil shark teeth into the context of how they fit into the creature's mouth. 

 

I really believe that to understand and be able to accurately id a lot of different fossil shark teeth you need to understand and be familiar with the jaws and teeth of extant shark species.  So I have bought and continue to buy as many different species of extant shark jaws/teeth that I can find/afford.

 

Marco Sr.

[Tidgy's Dad]

24 minutes ago, MarcoSr said:

 

I really believe that to understand and be able to accurately id a lot of different fossil shark teeth you need to understand and be familiar with the jaws and teeth of extant shark species.  So I have bought and continue to buy as many different species of extant shark jaws/teeth that I can find/afford.

 

Marco Sr.

I utterly concur.