Ceratotrichia and dorsal fin shape in arthrodires

Cocco and Dunk

Traditional artwork of arthrodires with shrinkwrapped dorsal fins. Top: Coccosteus cuspidatus by Stanton Fink, from Wikimedia Commons (CC BY-SA 3.0). Bottom: Dunkleosteus terrelli by Hugo Salais, from Ferrón et al. (2017). 

Arthrodiran placoderms like Coccosteus and Dunkleosteus are commonly depicted in paleoart with short, rounded dorsal fins. This is based on fossils of Coccosteus that have part of the dorsal fin preserved. However, most paleoartists have neglected the fact that not all of the fin is preserved. In reality, the soft tissue outline of the dorsal fin was likely much larger than the extent of the preserved skeletal elements. This is indicated by skeletal similarities with chondrichthyans and by an exceptional specimen of Dunkleosteus, both of which will be discussed shortly.

Here I’ve created a new reconstruction of the dorsal fin of Coccosteus cuspidatus based on modern sharks, particularly orectolobiforms which are similar in ecological niche. Reconstructing the fins of arthrodires based on sharks has been done before by Ferrón et al. (2017), but they only used this method for the caudal fin of Dunkleosteus and not the dorsal. While this post is focused on the dorsal fin, the same logic used here applies to restoring all the other fins. It is time to stop shrinkwrapping arthrodires!

Dunkleosteus and Coccosteus

Top: Dunkleosteus terrelli (CMNH 8982) pectoral fin with preserved pterygiophores and ceratotrichia/soft tissue outline (indicated by arrows), from Carr et al. (2010). Bottom: Coccosteus cuspidatus (NHT D4) dorsal fin with preserved pterygiophores and faint soft tissue outline, from Trewin (1986).

In chondrichthyans, the fins (minus the caudal) have two main skeletal components: pterygiophores and ceratotrichia. Pterygiophores are cartilage elements that form the base of the fin, while ceratotrichia are thin fibers of elastoidin (a type of collagen) that extend from the pterygiophores and form the fin’s overall shape. Chondrichthyan fins are divided into two types based on the proportions of the pterygiophores and ceratotrichia. Aplesodic fins are <50% pterygiophores and >50% ceratotrichia, while plesodic fins are >50% pterygiophores and <50% ceratotrichia. Aplesodic fins are more flexible and found in slow-swimming benthic species, while plesodic fins are more rigid and found in fast-swimming pelagic species (Compagno, 1999; Maia et al. 2012).

Preserved pterygiophores from the dorsal fin are known from multiple specimens of Coccosteus cuspidatus (Miles & Westoll, 1968). While none have preserved ceratotrichia, at least one specimen (NHT D4) has a faint soft tissue outline at the front of the fin (Trewin, 1986). Unfortunately, the photograph is too low-resolution to be informative for my reconstruction. One specimen of Dunkleosteus terrelli (CMNH 8982) has both preserved pterygiophores and ceratotrichia from one of the pectoral fins. The ceratotrichia are preserved as a carbonaceous film, which shows that the size of the fin greatly exceeded that of the pterygiophores, being aplesodic (Carr et al. 2010). These specimens demonstrate that arthrodires had a similar fin skeleton to chondrichthyans. They also demonstrate that arthrodires had aplesodic fins with much larger soft tissue outlines than most reconstructions.

Coccosteus fin

Comparison of the Coccosteus dorsal fin reconstruction from Miles & Westoll (1968) with my reconstruction based on orectolobiform sharks.

To reconstruct the dorsal fin of Coccosteus cuspidatus, I first had to examine its ecology. Coccosteus inhabited an ancient freshwater lake known as Lake Orcadie in Scotland during the Middle Devonian. Due to a lack of juvenile remains from Lake Orcadie, it has been suggested that at least part of its life was spent in the ocean (Trewin, 1986). This is also supported by specimens found in marine sediments in Estonia (Newman et al. 2015). It has been proposed that Coccosteus was a benthic ambush predator capable of short bursts of speed, evidenced by its low-angle, heterocercal caudal fin (Miles & Westoll, 1968). Preserved stomach contents contain scales from fast-swimming acanthodians as well as slower lungfish (Davidson & Trewin, 2005).

The ecological niche of Coccosteus is most similar to orectolobiform (“carpet”) sharks. Orectolobiforms are also benthic ambush predators with low-angle caudal fins adapted for acceleration. While all extant orectolobiforms are marine, some extinct species did live in freshwater. I referenced the first dorsal fins of small (~1 m TL) orectolobiform sharks as illustrated in Goto (2001). My fin outline for C. cuspidatus is an intermediate between the whitespotted bamboo shark (Chiloscyllium plagiosum) and the Japanese wobbegong (Orectolobus japonicus). The skeletal elements of the fin are redrawn from Miles & Westoll (1968), with the ceratotrichia being added. Overall, my reconstruction represents a more plausible soft tissue outline for the dorsal fins of arthrodires.

Coccosteus fin 2

Skeletal diagram of my reconstruction of the dorsal fin of Coccosteus.

Addendum (10/20/2022)

The new arthrodire Amazichthys trinajsticae from the Late Devonian of Morocco is the first with a complete dorsal fin (Jobbins et al., 2022). It demonstrates that the soft tissues of the fin extended well past the cartilaginous base, just as I predicted in this post.

Amazichthys

The nearly-complete holotype of Amazichthys trinajsticae, from Jobbins et al. (2022).

References

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