Oxford Clay Metriorhynchid Teeth

[PointyKnight]

Hey everyone!

 

Continuing from the other ID thread, I’d like to hear your opinions on another recent acquisition from the Oxford Clay: a group of associated metriorhynchid teeth.

 

 

Now, there are several metriorhynchid taxa described from the Oxford Clay Formation: Gracilineustes leedsi, Ieldraan melkshamensis, Suchodus brachyrhynchus, Suchodus ?durobrivensis, Thalattosuchus superciliosus, and Tyrannoneustes lythrodectikos. Pretty much all of them have decent descriptions of their dentition available, so comparing these teeth to the literature facilitates the ID process quite a bit.

 

First off: We’re examining 3 teeth, none of which are complete (all lacking the apex), but are seemingly unworn and thus have some distinctive diagnostic features preserved. They display weak to moderate mediolateral compression and some lingual curvature as well as free, smooth, and opposed carinae. The basic texture of the enamel is slightly rough or pitted on both lingual and labial surfaces.

 

Tooth A is the larger of the more complete teeth. It preserves about half of the crown as well as part of the root. No constriction is apparent at the transition between root and crown. The tooth displays slight lingual and distal curvature. Slight ridges overlay the basic enamel texture. Labially, these striae are long, continuous, and become less numerous apically. Lingually they are shorter, more numerous, and show no apical reduction in the part preserved. On both surfaces the striae are similar in strength and separate (non-fusing and non-anastomosing).

 

Tooth B is the smaller of the more complete teeth. It preserves a more apical portion of the crown than tooth A and no root. Again, the tooth displays slight lingual and distal curvature. There are no apparent striations on the labial surface. On the lingual surface, striae are distinctly apparent without magnification. Among these, some are continuous while the majority is singular or discontinuous. The striae are longest medially, some spanning the entire preserved length of the crown, and become shorter and less dense towards the carinae. Again, all striae are separate.

 

Tooth C only preserves one anteroposterior half of the crown, the exact position of which is indeterminable through curvature. Similar to tooth A, parts of the root are preserved and show no constriction at the transition towards the crown. There are no conspicuous striations present on either surface.

 

Since the teeth are relatively small, excluding taxa based on size alone isn’t possible. Therefore, we can best approach this ID by looking at the characteristic morphologies of each taxon:

 

 

Gracilineustes leedsi ANDREWS 1913

The teeth of G. leedsi are conical, unfaceted, and display moderate to strong mediolateral compression and lingual curvature. Ornamentation occurs in the form of short, low apicobasal ridges that are barely visible with the naked eye. These are more abundant and strongly developed lingually, most numerous basally and become reduced apically, but do not interact with the carinae (YOUNG et al. 2013, 2015).

Overall, G. leedsi appears to be a poor fit, as the striae in one tooth are not only very strongly developed, but also continuous along the length of the crown.

 

 

Ieldraan melkshamensis FOFFA et al. 2017

The teeth of I. melkshamensis exhibit a unique morphology among metriorhynchids, in which the labial surface is divided into three distinct apicobasal planes. The central plane is distinctly fluted by five strongly developed troughs, a feature that is found in all known teeth of the species. Additionally, the teeth are ornamented by overlaying apicobasal striae. These are short, discontinuous, and occur on both lingual and labial surfaces, giving the enamel a rough appearance. Again, the ornamentation does not interact with the carinae (FOFFA et al. 2017).

Like Gracilineustes, I. melkshamensis has to be considered a poor fit: The typical dental morphology of this species is not present, and the ornamentation differs quite a bit from what we see in these teeth.

 

 

Suchodus brachyrhynchus EUDES-DESLONGCHAMPS 1867

The teeth of S. brachyrhnychus show a varying degree of mediolateral compression, weak in some teeth and strong in others (unspecified whether this is positional). Enamel ornamentation consists of weakly developed apicobasal ridges of unspecified extend on the lingual side, while the labial side is described to lack conspicuous ornamentation entirely (FOFFA et al. 2017, YOUNG et al. 2013).

Again, as enamel striations in S. brachyrhynchus differ substantially from the ones seen in these teeth, this species seems to be an unlikely fit.

 

 

Suchodus ?durobrivensis LYDEKKER 1890

The validity of this species has been disputed in the past, certain authors regard it as synonymous with S. brachyrhynchus, and I couldn’t find many references to it in more recent papers. ANDREWS 1913 calls this species ‘Metriorhynchus durobrivense’ – his description of the teeth as possessing a nearly smooth enamel with only slight ornamentation in the form of ‘extremely’ fine apicobasal ridges of unspecified extend certainly sounds similar to the general attributes of S. brachyrhynchus teeth. Varying descriptions of the teeth as mediolaterally compressed (sometimes this compression is specified to be ‘weak’) align with this as well (ANDREWS 1913, FOFFA et al. 2017).

Regardless of whether S. brachyrhynchus and S. durobrivensis represent the same species, neither of their teeth show the morphology we are looking for and are a poor fit for this ID.

 

 

Tyrannoneustes lythrodectikos YOUNG et al. 2013

The teeth of T. lythrodectikos are moderately to strongly mediolaterally compressed. The enamel lacks distinct ornamentation on both labial and lingual sides, except for the very base of the crown, where low, well-spaced, and short apicobasal ridges are present (FOFFA et al. 2017, YOUNG et al. 2013).

T. lythrodectikos exhibits strongly reduced ornamentation, a condition not found in these teeth, and can therefore be disregarded as a possible ID.

 

 

Thalattosuchus superciliosus YOUNG et al. 2020

Lastly, we must examine Thalattosuchus superciliosus. The YOUNG et al. 2020 paper on the description of the genus is unfortunately not openly accessible, yet detailed descriptions of the dentition exist in prior literature from when it was still assigned to ‘Metriorhynchus’ superciliosus. The teeth of this species are conical and curve lingually, sometimes with a certain degree of mediolateral compression (this condition is reportedly more apparent in newly erupted teeth). The ornamentation is strong, with short to moderately long apicobasal ridges that are distinctly visible on the lingual side, but less developed labially (newly erupted teeth feature strong ornamentation on both surfaces, suggesting that differences in ornamentation might be caused by wear). Ornamentation does not interact with the carinae. (ANDREWS 1913, VIGNAUD 1997, YOUNG et al. 2013).

Overall, T. superciliosus appears to be a good ID for these teeth: The most extreme example of ornamentation observed in tooth B excludes all other taxa, but fits well with the typical dentition of this species. The variation between the teeth is easily explained by the natural variation that can be observed in individual metriorhynchids due to tooth position and wear. T. superciliosus has a relative abundance of specimens assigned to it, especially when compared to some of the other taxa, and thus our understanding of its dental characteristics is fortunately rather extensive.

 

 

Considering everything, I'm rather confident in this ID. Still, I’d very much like to hear your thoughts! Especially if anyone has a PDF of the YOUNG et al. 2020 paper on Thalattosuchus, I’d really appreciate it!

 

Thank you for your help!

 

 

ADAMS-TRESMAN, S.M. (1978): The Callovian (Middle Jurassic) Marine Crocodile Metriorhynchus From Central England

ANDREWS, C.W. (1913): A Descriptive Catalogue Of The Marine Reptiles Of The Oxford Clay - Part II

FOFFA, D., YOUNG, M.T., BRUSATTE, S.L., GRAHAM, M.R. (2017): A New Metriorhynchid Crocodylomorph From The Oxford Clay Formation (Middle Jurassic) Of England, With Implications For The Origin And Diversification Of Geosaurini

VIGNAUD, P. (1997): La Morphologie Dentaire Des Thalattosuchia (Crocodylia, Mesosuchia)

YOUNG, M.T., DE ANDRADE, M.B., BRUSATTE, S.L., SAKAMOTO, M. (2013): The Oldest Known Metriorhynchid Super-Predator: A New Genus And Species From The Middle Jurassic Of England, With Implications For Serration And Mandibular Evolution In Predatious CLades

YOUNG, M.T., BEATTY, B., STEEL, L. (2015): First Evidence Of Denticulated Dentition In Teleosaurid Crocodylomorphs

[pachy-pleuro-whatnot-odon]

Sorry it took me so long to take a look at this, Mien!  But you definitely did your research. Very useful overview you compiled here!

 

Unfortunately, I won't be able to add too much to your own conclusions, except for that a lot of the marine crocodile material from the Oxford Clay I've seen is available commercially (vertebrae) are attributed to M. superciliosus, now apparently T. superciliosus. Therefore, at the risk of it being a bit of waste basket taxon, it does appear this is the most common species found. And seeing as tooth morphology matches, I'd say it's likely that T. superciliousus is your prime suspect (what a practically chosen name, by the way, "Thalattosuchus superciliosus" - absolutely not at risk of confusion with the overarching group of thalattosuchia! ).

 

For what it's worth, feel free to take a look at my thread here, where I've looked into a marine reptile tooth that was a bit of a mystery to me when I first received it. From what I understand from the above, however, I now feel confident in ascribing it to T. superciliousus.

 

 

 

 

This same thread included the following images, which, though self-identified by the authors of their respective sources, may be useful for comparative purposes, as descriptive texts can otherwise be rather abstract. The last two figures you probably already known from Foffa et al. (2017), but are included here for convenience of others...

 

Metriorhynchus brachyrhynchus, Marnes des Dives, Upper Callovian, Villers-sur-Mer (source)

 

 

 

 

 

 

 

 

 

Cf. Tyrannoneustes lythrodectikos, Callovian, Peterborough member of the Oxford Clay (source)

 

 

 

 

 

 

 

 

 

 

Ieldraan melkshamensis. Figure 4 from Foffa et al., 2017. A new metriorhynchid crocodylomorph from the Oxford Clay Formation (Middle Jurassic) of England, with implications for the origin and diversification of Geosaurini

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A. Ieldraan melkshamensis; B. 'Metriorhynchus' brachyrhynchus; C-E. indeterminate geosaurin. Adapted from figure 5 from Foffa et al. (ibid.)

[PointyKnight]

@pachy-pleuro-whatnot-odon Thanks Alexander! Thalattosuchus indeed seems to be the most common metriorhynchid from the formation, though not by as wide a margin as I thought - each species has a number of skulls attributed to them, only a few are still catalogued under the wrong identity. Yet I have to correct myself on the discussion above as well: After reviewing some of the material and going through YOUNG's literature again, I must revise the description of Gracilineustes:

 

Prior descriptions are apparently based on worn teeth, something that's a major roadblock in metriorhynchid ID due to how easily the enamel ridges wear with the lack of underlying dentine support. The teeth of Gracilineustes and Thalattosuchus are very similar, seeing how they're the most closely related among all these taxa. The unworn ornamentation of Gracilineustes is strong as well, similar to that seen in Thalattosuchus. The point of distinction here seems to be the continuity of the ridges along the crown and the extent of each ridge type:

 

1. Long, continuous ridges along the crown without short accessory ridglets → Gracilineustes

 

2. Continuous ridges along the crown dominant with some short accessory ridglets → likely Gracilineustes

 

3. Only isolated continuous ridges, short accessory ridglets dominant → Thalattosuchus

 

4. No continuous ridges, labial ornamentation present → Thalattosuchus

 

5. No continuous ridges, labial ornamentation absent → Suchodus

 

Like mentioned before, since the enamel wears so easily the absence of certain characters must always be taken with a big grain of salt - on the other hand though, the presence of an unworn enamel type should give a pretty direct indication of identity. Seeing how the lingual ridges on your tooth are all rather continuous, I'm inclined to believe your tooth belongs to Gracilineustes! As for the cf.Tyrannoneustes tooth, I haven't yet spoken to Jacob about it, but I think it's a better fit with Suchodus - in Tyrannoneustes, ornamentation should be confined to the very base of the tooth, and the lingual side of this specimen shows discontinuous ridges along the entire crown, in contrast to the very smooth labial side. I should have two teeth arriving soon which show some interesting morphology, I'll probably add their discussion under here or link it in a new post. Hopefully they'll work to support these descriptions or maybe add new aspects to them!

[pachy-pleuro-whatnot-odon]

@PointyKnight Wow, Mien! That seems like quite the fine-grained distinction between the teeth of these species - especially with the enamel getting damaged so easily...! I would indeed not know whether my specimen would classify as Thalattosuchus or Gracilineustes in that case But seeing as the high degree of labiolingual compression, strong lingual ridges and presence of what appear to be worn sporadic ridges labially, I'd say that - in light of this new, more precise distinction - you're probably right, and my specimen belongs to Gracilineustes leedsi. Thanks for pointing that out!

 

As to the cf. Tyrannoneustes tooth, I'm not quite sure what you mean by not having spoken to Jacob yet. Do you actually know who owns this tooth, then? I don't think I know Jacob...

 

Very interested in seeing these teeth you think might have different morphology, by the way. I don't think I've seen any available for sale that clearly stood out from any of the others I saw... Not that I really paid attention to this before, though...

Edited April 30, 2021 by pachy-pleuro-whatnot-odon

[PointyKnight]

@pachy-pleuro-whatnot-odon Indeed, this is also noted by YOUNG et al. in their amended diagnoses for the different taxa. Having minimal wear on these teeth can be decisive for ID!

 

Also yes, this one is Jacob's tooth, he found it himself in the Kings dyke Reserve. He's very active on Instagram and regularly posts about his OC finds.

 

I'll make a post about the new teeth once they get here, I'll let you know!