How Many Species of Titanichthys are there?

One thing I’ve been puzzled by ever since I first started learning about Titanichthys is that there are currently five named species from the Appalachian Basin alone (T. agassizi, T. clarki, T. attenatus, T. rectus, and T. hussakofi). There are also two more species described, one from Poland (T. kozlowskii) and one from Morocco (T. termieri). However, only three of the species T. agassizi, T. clarki, and T. termieri are described from even remotely complete remains with the others being nomen dubia from the early 20^th century. While nobody has done any work to test the theory of filter-feeding in this genus from what I’ve seen all the evidence points towards that position so I’ll stick with that assumption of life habit for this post. While there are quite a few modern-day mega-planktivores (basking shark, whale shark, megamouth shark, all the baleen whales) it seems suspicious that there were five species of a single filter-feeding genus in the relatively small Appalachian Basin. Especially given that most modern mega-planktivores are very widespread and easily distinguished from gross morphology that I would think would be visible in the fossil record. So are those five species really valid and if not how many species of Titanichthys are there really?

Whale Shark (Rhincodon typus) Face portrait of an 8 meters individual filter feeding plankton at night, Thaa Atoll, Maldives

Figure 1. Modern giant filter-feeders. Whale shark (left, CCBY-SA by Arturo de Frias Marques). Megamouth shark (right, CCBY-SA by FLMNH Ichthyology).

            The main difference between the two well-known species from the Appalachian Basin (T. agassizi and T. clarki) is the form of the lower jaw (inferognathal). The image below shows the two morphs from Newberry’s 1889 monograph quite well. Essentially, agassizi has a thin rod-like inferognathal without much of a ventral bend at the anterior edge. In contrast, clarki has a more robust inferognathal with a developed posterior blade and a distinct ventral bend at the anterior edge. While a complete inferognathal of T. termieri has not been published a casting company appears to have a complete specimen and the Wyoming Dinosaur Center also appears to have a specimen with jaws. Assuming these specimens do represent Moroccan specimens this species inferognathal corresponds strongly to the clarki morph.

Figure 2. Inferognathal of T. agassizi (top) and T. clarki (bottom). From Newberry 1889 (Public Domain). Note that the inferognathals are upside down in this plate because Newberry thought the anterior portion projected upward and the groove faced dorsally.

            But there is more variance in remains attributed to Titanichthys than the original descriptions imply. There are inferognathals that are simply long rods with no apparent anterior bend, essentially an extreme agassizi form (Fig. 3), which then grades towards a more robust form, the clarki type, and finally the most extreme morph has a well-developed posterior blade with an extremely exaggerated anterior curvature that does not correspond to any named or published material that I’ve dubbed the “tusk” morph (Fig.4). Beyond the inferognathals of the well-known species the rest of skeleton is not actually that different. There are some minor differences in the shapes of plates such as the pineal and paranuchal between species but the holotype and paratype of T. termieri have differently shaped paranuchals. And although we don’t have many (published) samples of North American taxa to look at intraspecific variability studies from the extremely impressive Gogo Reef fauna have shown lots of variability in plate shapes. The combination of gradation between inferognathal shapes, lack of other distinguishing characters, and the known degree of intraspecific variability are, in my opinion, a strong argument that Titanichthys has been taxonomically oversplit maybe representing only two or maybe even just one species.

Figure 3. Extreme T. agassizi rod-like morph, left inferognathal from Dunkle & Bungart 1942.

Figure 4. Extreme T. sp. tusk morph, CMNH 7412. Partial left inferognathal. CCBY-Image by author

If we accept that some of the variation in inferognathal shape is within species what might explain the range of forms? There are a couple of possibilities including 1) anagnetic evolution, 2) ontogeny, or 3) intraspecific variability/plasticity.

Anagenetic Evolution

By anagenetic evolution I mean that the range of inferognathal shapes could be a single lineage evolving over time with the T. agassizi form transitioning to the clarki form and then finally the ‘tusk’ form or vice versa. If this was true we should not find any of the different forms to overlap each other in time, at least we shouldn’t find extreme agassizi in the same strata as clarki morphs. Unfortunately, there has not been much published work on stratigraphy within the Cleveland Shale where most Titanichthys material is from so we aren’t able to resolve this as a possibility currently. I suspect people from the Cleveland Museum or other who have collected material from the Cleveland Shale would have some sense of this.

 

Ontogeny

Another possibility is that the different morphs represent different stages of growth with the jaw changing shape as the animal grew. It wouldn’t be the first time several species of placoderm were found to be ontogenetic morphs. The famous Dunkleosteus used to contain half a dozen species, all in the Appalachian Basin based on inferoganthal shape, but were found to all fall into a simple growth curve and thus synonomized (Hlavin 1976). An ontogenetic explanation would predict that inferognathal forms would be correlated with size, small inferognathals would all be one shape and all large ones a different shape. Now this hypothesis, I think, can be rejected from the collections in the Cleveland Museum because all three morphs (agassizi, clarki, and ‘tusk’) occur at large sizes. It is possible that ontogeny could still explain some variation if there really are several valid species, but the inferognathals in collections really need to be measured to detect this.

Intraspecific Variability/Plasticity

I think this is the most likely explanation for much of the variation in inferognathals of Titanichthys. But this is also the hardest to test because there isn’t an obvious way to test it. Anagenetic evolution and ontogeny both have specific predictions but this explanation does not. To test this hypothesis we would need to have much better descriptions of the species of Titanichthys with many examples of each. Then examine if there are consistent differences between and gaps in morphology between specimens, and whether they correspond to previously designated species. Basically, you really need a monograph of the genus to sort this out.

 

So in the end I don’t know how many species of Titanichthys there really are but I have serious doubts that there are five (or six including the ‘tusk’ morph). Based on my time in the collections of the Cleveland Museum of Natural History I’d guess there are between one and three species. It might be that T. agassizi, T. clarki, and the ‘tusk’ morph are all valid but I’m very skeptical still. It is because of all this uncertainty in species identification that I left the recently described specimen of Titanichthys in open nomenclature as Titanichthys cf. clarki (Boyle & Ryan 2017). It is worth noting that the new specimen shares characteristics with all three well-known species, including the Moroccan one. In fact, Robert Carr (2009) has suggested that T. termieri occurs in the Appalachian Basin and conversely that Dunkleosteus terrelli occurs in Morocco. This is from a conference abstract several years ago, but it does suggest that material to resolve questions about Titanichthys are already available in museum collections and while I would love to get around to working on this issue myself it seems unlikely any time in the near future. So if anyone wants to jump on this go for it, placoderms need some more attention!

 

References

Boyle, J. and M.J. Ryan. 2017. New information on Titanichthys (Placodermi, Arthrodira) from the Cleveland Shale (Upper Devonian) of Ohio, USA. Journal of Paleontology 91:318-336.

Carr, R.K. 2009. A big fish story: new links between the Appalachian Basin and Morocco in the Late Devonian. Cincinatti Museum Center Scientific Contributions 3:204.

Dunkle, D.H. and P.A. Bungart. 1942a. The infero-gnathal plates of Titanichthys. Scientific Publication of the Cleveland Museum of Natural History 8:49-59.

Hlavin, W.J. 1976. Biostratigraphy of the Late Devonian black shales on the cratonal margin of the Appalachian geosyncline, Unpublished D. Phil. Thesis, Boston University.

Newberry, J.S. 1889. Paleozoic Fishes of North America. US Geological Survey: Monographs 16.