Author Archives: jamesboy2013

About jamesboy2013

Starting my 5th year as a PhD student at the University at Buffalo under Chuck Mitchell. Working on graptolite community structure and evolutionary dynamics.

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 20th 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?

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.

Newberry1889-Agassizi-Clarki

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.

Dunkle&Bungart1942_Fig1a

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

7412-Inferognathal-Tusk

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.

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GeoRange: an R package for calculating geographic range

One thing that I wasn’t expecting going into grad school was the amount of coding I would do. I took an Intro to Programming in Java during my 2nd year in undergraduate, which I enjoyed, but otherwise never thought much about it. That changed during my master’s work when I had to write some of my own code in the R Programming Language for some specialized tests. I’ve since taken a course on data analysis in R and another focusing on Python and written A LOT of R code for research. Even though I don’t consider myself any kind of coding expert in R I’ve gotten to the point where I think other people might actually find some of the functions I’ve written to be useful. So, I’ve written and submitted to CRAN a package dubbed GeoRange which should be available for download shortly if it isn’t already.

 

Quick thank you to Dave Bapst for his advice and encouragement in publishing R packages. The whole process was made much easier by this online tutorial.

 

GeoRange is for calculating and analyzing six different methods of geographic range from point occurrence data (i.e. latitude and longitude). It was born from my interest in geographic range as it relates to extinction risk. Very quickly, all else being equal a species that is more widespread across the Earth is less likely to be wiped out by stochastic events than one with a small range (Jablonski 2005). For example, a species confined to a single island in the Caribbean might be killed by a single hurricane season whereas it’s nearly impossible to wipe out all individuals of a species that occurs across the Atlantic. Pretty much all the geographic range measures in the package can be done (in some cases more efficiently) within ArcGIS but I didn’t do use it because I am not really a fan of ArcGIS to put it mildly. I can use it but the system always seems very glitchy and opaque for my tastes and data analysis can be a hassle. I was going to end up doing some analyses in R anyway so I figured I might as well do everything in R.

 

The actual measures of geographic range include the convex hull area, maximum pairwise distance, latitudinal range, longitudinal range, X x X degree cell count, and minimum spanning tree distance. The first five are fairly standard measures that are commonly used in extinction analyses but the minimum spanning tree (MST) may be unfamiliar to people, even those that study extinction. Essentially, the MST finds the most cost-effective way to connect all points without ever creating a loop, a problem similar to the Traveling Salesman Problem. Originally the MST was used to find the most efficient ways to lay down power-lines with the cost between points corresponding to the cost of building. In terms of geographic range the cost is the great circle distance between points and thus the MST represents the minimum distance a species must have traveled to have reached all points. That might include crossing impassable terrain and is unlikely to represent the actual path or distance traveled but it still seems to be an excellent correlate of extinction risk, especially after accounting for sampling (Boyle et al. 2017). Not sure why this is yet except that it better captures other factors, like abundance and fragmentation, that are associated with extinction risk.

MST&CH_100pts_UShape

Figure 1. Horseshoe-shaped distribution (thick black outline) with 100 random points generated. Showing the minimum spanning tree (thin black lines) and convex hull (blue outline) showing the stark difference in methods for certain shapes.

            For analyses of multiple taxa GeoRange is set up to work with capture matrices and can work directly with data from the Paleobiology Database via the downloadPBD function in the velociraptr package.

There are some known limitations with this package that I’m looking to fix in some future updates. A major issue is that calculating the MST takes a long time for more than 1000 points. The PlotMST function doesn’t account for points connecting around the prime meridian so that creates off lines that jump across the plot. The CellCount function isn’t equal area cells, so high latitude cells are stretched compared to equitorial ones and similarly the random point generation functions RandRec and RandHorseShoe don’t account for the stretching of lat/long area with latitude. There are lots of little tweaks to increase user options and expand functionality but for now I’m happy to get some feedback on the work and keep up my coding skills.

References

Boyle, J. 2017. GeoRange: Calculating Geographic Range from Occurrence Data. R Package version 0.1.0. https://CRAN.R-project.org/package=GeoRange

Jablonksi, D. 2005. Mass extinctions and macroevolution. Paleobiology 31:192-210.

New Data on Titanichthys At Last

Since the last time I posted here I’ve managed to have two new papers published. One in Paleobiology on graptolite extinction risk (Boyle et al. 2017), which I’ll summarize in another post, and one in the Journal of Paleontology (Boyle & Ryan 2017) on the placoderm Titanichthys. The work on Titanichthys was part of my undergraduate capstone research that started waaaay back in 2010 with Dr. Ryan at the Cleveland Museum casually remarking that a new nearly complete skeleton had just been prepared and somebody needed to describe it. So today I’ll be summarizing the results of that paper and where I’d like to get more done on this taxon in the future.

To start, a little background on Titanichthys. It is an old taxon named in 1885 by John Strong Newberry (an interesting man for a number of reasons worthy of his own post) and also a very large animal. Ballpark estimates are a total length >5 meters, making it larger than its more famous contemporary Dunkleosteus and possibly the largest vertebrate animal that had evolved up to that time. The genus is confined to the latest Devonian (Frasnian?-Fammennian) making it one of the last arthrodire placoderms before the entire group perished at the end of the Devonian. The remains of Titanichthys are the second most common taxon after Dunkleosteus but are commonly isolated and fragmentary making its overall morphology somewhat mysterious. Of the 7 currently recognized species only 3 (T. agassiziT. clarki, and T. termieri) are known from at least partially complete specimens. While the description of T. termieri (Lehman 1956) is pretty good by modern standards the other two well-known species were described in the late 19th or earliest 20th century without many modern morphologic conventions and without designation of a holotype. In fact, the effective “holotype” of T. clarki described by Eastman (1907) is a composite of several individuals and has been partially restored with plaster making interpretation of the figures somewhat dubious.

So prior to this new paper the most recent publication on Titanichthys was over 50 years old and we lacked definitive reference material to work from. The new specimen of Titanichthys (CMNH50319) is unique because it is articulated and nearly complete, only lacking parts of the thoracic shield and the ventral shield (Fig. 1). This has allowed for the first description of five new plates (rostral, postmarginal, postsuborbital, submarginal, and posterior superognthal; see Placoderm primer for plate info) and updated descriptions for every other plate that’s been previously described for the genus, with exception of the anterior dorsal lateral. So now we have a very nice specimen to compare new material to and could start looking at the range of variation in morphology. The posterior superognathal was perhaps the most interesting because it is tiny relative to the lower jaw (inferognathal) especially when compared to other arthrodires. This, along with the animals overall large size, the flattened body shape, and elongated lower jaws without anything resembling teeth are all strongly convergent with large filter-feeding vertebrates that are known today such as the Whale Shark, Basking Shark, and Megamouth Shark as well as the fossil teleost fishes Pachycormids. The suggestion that Titanichthys was a filter-feeder wasn’t a novel idea but the new data does lend a lot more support to this hypothesis which could now be tested explicitly.

Figure 2_2col

Figure 1. Reconstruction of CMNH50319 (Titanichthys cf. clarki) in lateral view. Lightly shaded regions are portions of plates that are overlapped. Dark shaded regions are empty space. IG = Inferognathal. Scale Bar = 10cm.

One other important feature we were able to sort out from this specimen was the orientation of inferognathal, which has a deep groove and an anterior tip that bends noticeably from the rest of the jaw. Originally, the groove was thought to be the dorsal surface and to have held tooth plates, making the anterior tip projecting dorsally as well. However, Dunkle and Bungart (1942) presented evidence that the groove faced ventrally making the anterior tip project ventrally as well. When Lehman (1956) described T. termieri he favored the original orientation. Because CMNH50319 had the left inferognathal preserved and articulated with the cheek shield we can now definitively say that the Dunkle and Bungart’s hypothesis is correct. That ventrally projecting tip is odd among arthrodires and can be very strongly developed (Fig. 2) based on observations in the Cleveland Musueum collections. This ventral curve makes the most sense, in my opinion, as an adaptation to increase gape size for filter-feeding.

Tusk Jaw

Figure 2. Reconstruction of undescribed left inferognathal of Titanichthys from the Cleveland Museum of Natural History showing the extreme condition of the ventrally projecting anterior tip. I refer to this as the tusk morph.

Despite all the anatomical detail we were able to report, when it came to assigning CMNH50319 to a particular species we ran into a problem. It had a mix of features from the well-known species and because all the species are poorly known (sample size of 1 or 2) we don’t really have any idea whether those differences are variable within or between species. We did have the inferognathal which is the easiest way to distinguish between T. agassizi and T. clarki, CMNH 50319 was definitively closer to the T. clarki form (it’s worth noting that the inferognathal of T. termieri is not known). So in the end we decided to classify CMNH50319 as Titanichthys cf. clarki, the cf. stands for confers and means close to. We probably could have just called it T. clarki without any complaints but I thought that would mask the uncertainty and variation within the genus.

We were able to perform a phylogenetic analysis with the specimen expanding on previous analyses (Carr 1991, Trinajstic & Dennis-Bryan 2009, Carr & Hlavin 2010,  Zhu & Zhu 2013, and Zhu et al. 2015). We added CMNH50319, T. agassiziBungartius perissus, and Tafilalichthys lavocati to the analysis with 121 characters. The resulting consensus tree (Fig. 3) is not very robust in all honesty. Too many taxa for the number of characters and A LOT of missing data. However, Titanichthys was recovered as a basal aspinothoracid in a monophyletic clade with Tafilalichthys and Bungartius. It will be interesting to see if that clade holds up in the long run because Tafilalichthys and Bungartius have both been associated with taxa that have crushing dentition. That would be a lot of variation in jaw morphology within a single group. I’m convinced the key to resolving arthrodire relationships are the Wildungen taxa from west Germany that seem perfectly placed in time and space to bridge the phylogenetic gap that currently exist.

Phylogeny

Figure 3. Strict consensus of 2769 trees from a PAUP analyis.

Looking ahead to more research in this area several questions are clear. First, we can explicitly test the hypothesis that Titanichthys was a filter-feeder. Second, I gleaned over the geographic distribution of the species but as of right now there were 5! species of Titanichthys coexisting in the Appalachian Basin of North America. For giant filter-feeding organisms that seems incredibly unlikely to me and I suspect that most of the species should actually be synonomized. Looking through the Cleveland Museum collections for this research revealed a large amount of variation in Titanichthys jaws and I wouldn’t be surprised if the different morphs grade into one another or are ontogenetic. There’s plenty of material to work with to explore this project and I’d like to get to it sometime in the future, but if somebody beats me to it I’d be happy just to know that it’s being worked on.

References

Boyle, J. & M.J. Ryan, 2017, New Information on Titanichthys (Placodermi, Arthodira) from the Cleveland Shale (Upper Devonian) of Ohio, USA. Journal of Paleontology, v. 91, p. 318-336.

Carr, R.K., 1991, Reanalysis of Heintzichthys gouldii, an aspinothoracid arthrodire: Zoological Journal of the Linnean Society, v. 103, p. 349–390.

Carr, R.K., and Hlavin, W.J., 2010, Two new species of Dunkleosteus Lehman, 1956, from the Ohio Shale Formation (U.S.A., Famennian) and the Kettle Point Formation (Canada, upper Devonian) and a cladistic analysis of the Eubrachythoraci (Placodermi, Arthrodira): Zoological Journal of the Linnean Society, v. 159, p. 195–222.

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

Eastman, C.R., 1907, Devonic fishes of the New York formations: New York State Museum Memoir, v. 10, p. 1–235.

Lehman, J.-P., 1956, Les Arthrodires du Dévonien superieur du Tafilalet (sud Marocain): Notes et Mémoires du Service Géologique du Maroc, v. 129, p. 1–114.

Trinajstic, K., and Dennis-Bryan, K., 2009, Phenotypic plasticity, polymorphism and phylogeny within placoderms: Acta Zoologica, v. 90, p. 83–102.

Zhu, Y.-A., and Zhu, M., 2013, A redescription of Kiangyousteus yohii (Arthrodira: Eubrachythoraci) from the Middle Devonian of China, with remarks on the systematics of the Eubrachythoraci: Zoological Journal of the Linnean Society, v. 169, p. 798–819.

Zhu, Y.-A., Zhu, M., and Wang, J,-Q., 2015, Redescription of Yinosteus major (Arthrodira: Heterostiidae) from the Lower Devonian of China, and the interrelationships of Brachythoraci: Zoological Journal of the Linnean Society, v. 176, p. 806–834.

Placoderm Nurseries

Many sharks and bony fishes make use of protected shallow habitats for their young to grow in today. Typically these habitats provide protection from predators and abundant resources that rush downstream from the eroding highlands. This allows the maximum number of offspring to grow quickly before reaching adult size where they are less vulnerable to predation and can migrate out to deeper waters. Given how common this is today it would be expected that extinct organisms would have done this as well. Unfortunately, even if they were common we still might not be likely to find evidence of nurseries in the fossil record for two reasons. First, nurseries tend to be in shallow environments such as rivers and estuaries which are likely to be eroded as sea level rises and falls or continents collide erasing the environment. Second, identifying a nursery requires finding a location that is dominated by juvenile organisms and in most nurseries the juveniles are either eaten (no fossils left) or successfully reach adulthood and leave the nursery. So it’s only in the rare circumstance where there is a mass death of juveniles due to some catastrophe (at least from the organisms’ perspective) and then up to luck that the sediments aren’t eroded away for millions of years before they are discovered and sampled by paleontologists.

Despite these unlikely circumstances there are a number of likely nursery site of chondrichthyans (sharks and their relatives) in Triassic rocks of Kyrgyzstan (Fischer et al. 2011) and Pennsylvanian rock of Illinois (Sallan & Coates 2014). The longer ago sediments were formed the more likely they will have been destroyed by geological processes so it was exciting news this past week when a new study in interpreted a site in Belgium as a placoderm nursery (figure 1) from the Late Devonian (bonus points for being placoderms!). I’ll detail some of the bits I find the most interesting here. The study (Olive et al. 2016) was published in PLoS One and thus is open-access so I encourage everybody to go read the full paper for themselves, it’s a light read at nine pages.

journal.pone.0161540.g002

Figure 1. From Olive et al. 2016 showing a reconstruction of the Strud nursery site. Scale bar =2cm. Illustration by J. Jacquot Hameon (MNHN, Paris).

 

The locality in Strud, Belgium would have been along the edge of the paleocontinent of Laurussia/Euramerica, a combination of North America and northern/western Europe, in the Late Devonian. It most likely represents the river deposits on an alluvial plain, a relatively calm environment, except in the case of flooding events. The authors report the recovery of 105 fragments of placoderms comprised of three species all of which were relatively small and showed other morphological correlates of juveniles based on previous studies of closely related placoderms (Werdelin & Long 1986; Deaschler et al. 2003). So what happened to these young placoderms that they weren’t eaten and yet they didn’t survive into adulthood? At Strud the most likely explanation is that these juveniles were in a pond or small tributary that dried up and became isolated from the main channel. So bad luck for the placoderms, good luck for paleontologists. What’s even more interesting though is that the Strud locality is the second known placoderm nursery of the Late Devonian. The other is in Tioga County, Pennsylvania (Downs et al. 2011) with similar species. In that case the hundreds of specimens are more complete and tend to be oriented in the same direction. Again, the explanation for their remains is that they were isolated in a shrinking body of water that also slowly became anoxic (aiding preservation) leading to a mass kill.

Both of these placoderm nurseries had large numbers of similarly aged individuals, and in the case of the Pennsylvania site there’s strong evidence that they represent a life assemblage, rather than one which has been time-averaged. So these sites probably show us that these placoderms had large numbers of offspring, though how exactly is up for debate. Some placoderms are known to have had live offspring (Ptyctodonts; Long et al. 2008) while there is questionable evidence of egg-laying behavior in other (Ritchie 2005; figure 2). Because of the number of similarly aged juveniles both nurseries most strongly support an egg-laying behavior in the species found there (primarily antiarchs). Given the position of placoderms as the outgroup to the rest of the jawed vertebrates their reproductive strategies can help us chart the evolution of reproductive strategies in Earth’s early history. It will be exciting to see if more of these nursery sites appear in other parts of the world outside of Luarussia and even earlier in fossil record. There are also tantalizing hints that large placoderm species (e.g. Dunkleosteus) might have used nearshore habitats for their juveniles as well (Daeschler & Cressler 2011)!

Cowralepis egg

Figure 2. Possible egg case of the plcaoderm Cowralepis. From Ritchie 2005. 

 

References

Daeschler, E.B. and W.L. Cressler III. 2011. Late Devonian paleontology and paleoenvironments at Red Hill and other fossil sites in the Catskill Formation of north-central Pennsylvania. Geological Society of America Field Guide 20:1-16.

Daeschler, E.B., A.C. Frumes, and C.F. Mullison. 2003. Groenlandaspid placoderm fishes from the Late Devonian of North America. Records of the Australian Museum 55:45-60.

Downs, J.P., K.E. Criswell, and E.B. Daeschler. 2011. Mass mortality of juvenile antiarchs (Bothriolepis sp.) from the Catskill Formation (Upper Devonian, Famennian Stage), Tioga County, Pennsylvania. Proceedings of the National Academy of Science Philadelphia 161:191-203.

Fischer, J., S. Voigt, J.W. Schneider, M. Buchwitz, and S. Voigt. 2011. A selachian freshwater fauna from the Triassic of Kyrgyzstan and its implication for Mesozoic shark nurseries. Journal of Vertebrate Paleontology 31:937-953.

Long, J.A., K. Trinajstic, G.C. Young, and T. Senden. 2008. Live birth in the Devonian period. Nature 453:650-652.

Olive, S., G. Clement, E.B. Daeschler, and V. Dupret. 2016. Placoderm assemblage from the tetrapod-bearing locality of Strud (Belgium, Upper Famennian) provides evidence for a fish nursery. PLoS One 11:e0161540.

Ritchie, A. 2005. Cowralepis, a new genus of phyllolepid fish (Pisces, Placodermi) from the late Middle Devonian of New South Wales, Australia. Proceedings of the Linnean Society of New South Wales 126:215-259.

Sallan, L.C. and M.I. Coates. 2014. The long-rostrumed elasmobranch Bandringa Zangerl, 1969, and taphonomy within a Carboniferous shark nursery. Journal of Vertebrate Paleontology 34:22-33.

Werdelin, L. and J.A. Long. 1986. Allometry in the placoderm Bothriolepis canadensis and its significance to antiarch evolution. Lethaia 19:161-169.

Placoderm Primer

This post has been a long time in coming but hopefully I’ll be able to start posting regularly after this and get a steady stream of updates going. Here, finally, is a primer on placoderms since I’m likely to be posting about them more.

Placoderms are an entirely extinct group of armoured fishes that were most common in the seas of the Devonian Period (419-459 mya). They were an impressively diverse group of organism containing both ray-like bottom feeders (Rhenanids) and giant predators (Dunkelosteus). Placoderms also include some of the smallest known vertebrates (Minicrania lirouyii ) [1] with a head and thoracic shield of only 20mm) and the largest vertebrates to have evolved up to the Devonian (Titanichthys; probably maxed out at over 5 meters long). Despite their diversity and dominance for over 40 million years no placoderms survived past the end of the Devonian for reasons unknown. So this is a whole branch of life that originated, diversified, and went extinct just as vertebrates were starting to move onto land!

There are nine orders of placoderms currently recognized [2] Stensioellida, Pseudopetalichthyida, Petalichthyida, Ptyctodontida, Acanthothoraci, Rhenanida, Antiarchi, Phyllolepida, and Arthrodira with the first two being poorly known. They were long thought to be a monophyletic group but recent discoveries have turned the old relationships on its head, especially the discovery of a mid-Silurian vertebrate, Entelognathus primordialis, that has a combination of characters seen in placoderms and bony fishes (osteichthyes) [3]. I might do a separate post just about that strange beast sometime. Anyway, the different orders of placoderms are now thought to be paraphyletic with Antiarchs as the most basal, arthrodires as intermediate, and ptyctodontids as the sister-group to the remaining jawed vertebrates [4], figure 1 although this is still in flux.

In terms of morphology placoderms as a whole have an ossified dermal skeleton, that is the bones form from the dermal layer near the surface of the skin rather than internally like most of the bones in our own bodies (Interestingly most of our skull bones are dermal in origin). This external skeleton is commonly referred to as armor and it protects the soft internal body of the fishes including the internal skeleton which is rarely preserved and appears to be largely composed of cartilage. The armor of the placoderm is typically broken down into four groups of armor plates that are tightly connected to each other: head, cheek, thoracic, and ventral armors (figure 2). In antiarchs the pectoral fins are enclosed by jointed appendages reminiscent of arthropods.

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The head shield in most arthrodires consists of seven paired plates (postnasal, central, preorbital, postorbital, paranuchal, marginal, and postmarginal) and three median plates (rostral, pineal, and nuchal). In some primitive arthrodires the rostral and pineal plates are fused (Buchanosteidae) to form a rostropineal and in other forms the postnasals appear to have been lost. The head shield is connected to the thoracic shield by a hinged contact between the paranuchal and the anterior dorsal lateral. The thoracic shield is composed of five to seven plates in arthrodires (median dorsal, anterior dorsal lateral, anterior lateral, posterior dorsal lateral, posterior lateral, interolateral, and spinal). The interolateral and spinal plates are lost in independent lineages, coccosteids and aspinothoarcids (surprise surprise) respectively. Depending on the species the thoracic armor is articulated to varying degrees with the ventral armor which would have protected the soft underbelly. This part of the armor consists of four plates (anterior median, posterior median, anterior ventral lateral, and posterior ventral lateral) which are typically found in isolation or fragmented. Finally, the cheek shield is connected to the head shield either loosely with the preorbital or intimately with both the preorbital and lateral portion of the head shield. The cheek shield consists of three plates (suborbital, postsuborbital, and submarginal).

I’ll stop there for now. I’ve updated the placoderm occurrence database so I might do something about placoderm diversity next. I also have a paper on Titanichthys under review so maybe more on that soon too!

References

1. Zhu, M. & P. Janvier. 1996. A small antiarch, Minicrania lirouyi Gen. et sp. nov., from the Early Devonian of Qujing, Yunnan (China), with remarks on antiarch phylogeny. Journal of Vertebrate Paleontology 16:1-15.

2. Denison, R.H. 1978. Handbook of Paleoichthyology: Placodermi. Gustav Fischer Verlag,             Stuttgart, New York, 128 pp.

3. Zhu, M., X. Yu, P. E. Ahlberg, B. Choo, J. Lu, T. Qiao, Q. Qu, W. Zhao, L. Jia, H. Blom, and Y. Zhu. 2013. A Silurian placoderms with osteichthyan-like marginal jaw bones. Nature 502:188-193.

4. Davis, S.P., J.A. Finarelli, and M.I. Coates. 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486:247-250.

5. Dunkle, D.H. and P.A. Bungart. 1947. A new genus and species of arthrodiran fish from the Upper Devonian Cleveland Shale. Scientific Publication of the Cleveland Museum of Natural History 8:103-117.

What is a fish?

Since a lot of what I’ll be posting will have to do with placoderms and other marine animals I thought it would be a good idea to explore what the word ‘fish’ actually means. What makes something a fish? The question is not as simple as it first appears, we all know in general what we mean when we refer to fish in conversation but this is partially due to the fact that the overwhelming majority of fish today belong to the actinopterygians (ray-finned fishes) which includes everything from gars to goldfish. All these fish share a similar body with paired fins, a tail fin, gills, a bony skeleton, and swim bladders. So let us take the idea that fish are water-dwelling organism with fins, scales, gills, and who are poikilothermic (their body temperature fluctuates with the temperature of their environment). This covers most of the organisms you would refer to as fish in normal conversation but what about eels that don’t have the paired fins or scales? Or the even stranger (and uglier) lampreys and hagfishes that also lack jaws in addition to scales and fins? There are even some fish that can regulate their body temperatures both by more active circulation (Salmon sharks)1 or by producing antifreeze proteins in their blood2. For every rule defining fish there are many exceptions to counter them. The reality is that the group we refer to as ‘fish’ is a grouping of human convenience for swimming organisms in the water, which usually possess scales and gills.

A quick side note I feel obligated by a former professor to pass on! Fish is the proper plural when referring to multiple individuals of the same species while fishes is used when referring to a group of individuals containing more than one species.

Today if we consider only the kinds of fish with scales, paired fins, and gills (teleosts) they are the most diverse group of vertebrates on the planet. If we look at the evolutionary history of marine organisms similar to teleosts there was an even greater diversity in the past. The figure below is from a review of fishes over the last 500 million years and I would highly recommend giving it a read if you have any interest in ichthyology (reference is at the end of the post). The first thing you might notice is that all terrestrial vertebrates (including you dear reader!) are descended from the sarcopterygians (lobe-finned fishes) and thus a technically a fish. Those swim bladders I mentioned earlier are primitively lungs so it is not just land animals that have lungs but the majority of vertebrates. In fact you are more closely related to a goldfish or salmon than either of those is to a shark.

Image

Figure 1. A phylogeny of fishes, extant and extinct from Friedman and Sallan 2012.

It is also probably clear that there a large number of entirely extinct lineages of fishes. Everything from the anapsida to Osteoraci, excluding the eel-like unarmored conodonts, are historically referred to as ostracoderms or agnathans. They are primitive armored fishes that lack any jaws and had their greatest diversity in the late Silurian to early Devonian (~420 mya) hundreds of millions of years before the dinosaurs walked the land and before any vertebrates walked the land for that matter. The many lineages of ostracoderms are a fascinating array of creatures that has only recently received renewed attention for understanding the evolution of all jawed vertebrates. Unfortunately, there is still relatively little known but they more posts about them will probably appear as new publications come out.

Moving into the gnathostomes (vertebrates with jaws) one of the earliest branches were the paraphyletic acanthodians (spiny sharks), which are known from mostly fragmentary remains. The lines including the acanthodians lead on to both chondrichthyes (sharks and rays) as well as teleosts (ray-finned fish, lobe-finned fish, and terrestrial vertebrates). The other branch led to the placoderms (armor skin) which were a highly diverse group of fishes with armor around their heads and part of the trunk. Below are reconstructions of the many forms of placoderms which dominated the Devonian seas before becoming extinct at the end of that period. Placoderms include the earliest known instance of live birth4 and included the largest vertebrates to have ever evolved up to the end of the Devonian likely greater than five meters in length.

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Figure 2. The placoderms Coccosteus from Wikipedia.

The next post will be a primer on placoderms before I really start to delve in on specific hypotheses, publications, or theories. I’ll attempt to answer reasonable comments in a timely manner and be glad to answer any questions.

References

1Goldman, K. J., S. D. Anderson, R. J. Latour, and J. A. Musick. 2004. Homeothermy in adult

salmon sharks, Lamma ditropis. Environmental Biology of Fishes 71:403-411.

2Fletcher, G. L., C. L. Hew, and P. L. Davies. 2001. Antifreeze proteins of teleost fishes. Annual

Review of Physiology 63:359-390.

3Friedman, M. and L. C. Sallan. 2012. Five hundred million years of extinction and recovery: a

Phanerozoic survey of large-scale diversity patterns in fishes. Palaeontology 55:707-742.

4Long, J. A., K. Trinajstic, G. C. Young, and T. Senden. 2008. Live birth in the Devonian Period.

Nature 453:650-652.

What am I doing?

Hello world!

I’ve been brainstorming about starting a blog for a while but kept putting it off for another day but no more! For most of my life I’ve been interested in paleontology and that’s what I’m going to write about for the most part on this blog. I can’t say I have a very specific focus on any particular group but there are a few that will probably be the majority of post to start with at least.  This blog was in large part inspired by SVPOW which has been an enormous amount of fun for me to follow over the past two years. 

I’m currently a master’s student working on evolutionary patterns in graptolites (if you don’t know what those are don’t worry I’m sure they’ll appear here from time to time) and will be continuing on to a PhD in the fall. My undergraduate career was spent working on placoderms from the Cleveland, Ohio area and I’ve really gotten fond of them but I don’t have much opportunity to work on them where I am now so I’m partially using this blog as a way to keep myself involved in that literature by trying to get other people interested in them as well.

It’s my hope that this blog will help me work on a couple things. First, helping me improve my scheduling and time-management which could definitely use some work. Second, to help me learn how to write and portray ideas more effectively. And third, to get some ideas bouncing around my head somewhere easily accessible so other people can learn, provide feedback, and maybe answer some of the questions I haven’t been able to.

I’ve already alluded to the fact that this blog will probably cover some graptolites and definitely placoderms. Both groups are from the Paleozoic and I’m expecting most of my posts on specific groups or organisms to be from that time. However, I also was introduced to paleontology like most people through dinosaurs and so they’ll creep into discussion too I have no doubt. But most things in the fossil record younger than the Cretaceous just don’t interest me as much for whatever reason and so this blog will almost exclusively be Before the Bolide.