Tristychius is a small Carboniferous whale shark in the LRT, not a hybodont, despite the spines

I misunderstood Tristychius earlier,
and so did other workers who considered this odd shark a hybodontid. Those large dorsal spines (Fig. 2) turn out to be not so important phylogenetically. They are not a ‘key’ trait. After analysis (Fig. 6), Tristychius dorsal spines turn out to be only convergent with those of hybodontid and other ‘spiny’ sharks.

Figure 1. Tristychius skull from Coates et al. 2019 with new identities assigned based on tetrapod homolog colors. Note what appears to be the orbit does not tightly enclose the eyeball. Rather phylogenetic bracketing indicates the eyeball was on a long optic nerve, as in the whale shark, Rhincodon (Fig. 3).

For too long Tristychius was a bad fit in the LRT.
In the LRT bad fit = bad scores. When I finally put the skull of Tristychius together with that of Rhincodon, the whale shark (Fig. 3), I understood things that had escaped me previously. For instance, prior to this weekend I had no idea a small eyeball could lie far outside what looked like the orbit, supported by long optic muscles and a long optic nerve (Fig. 3).

Figure 1. Tristychius, a basal shark from the Early Carboniferous,
Figure 2. Tristychius, a basal shark from the Early Carboniferous. Despite the large dorsal spines, this taxon nests closer to Rhincodon, the whale shark, in the LRT. Note the mistaken underslung jaws of this reconstruction.

In 1978
JRF Dick described Tristychius (Fig. 2), a shark known from Early Carboniferous iron nodule concretions.

From the Dick 1978 abstract:
“Tristychius arcuatus sensu stricto is a medium-sized hybodont shark with a short gape, a functionally heterocercal tail and narrow-based, tribasal pectoral fins. Its most unusual feature is a well developed opercular gill cover composed of long hyoid rays. Evidence suggests that this character was present in several Palaeozoic sharks, although it is absent in all Recent elasmobranchs. It is not clear whether it was primitively present in chondrichthyans or evolved separately in several lineages.”

Coates et al. 2019 disagreed with Dick 1978 about those opercular gill covers. Coates et al. 2019, wrote, The hyoid rays are well preserved but, in contrast with previous reconstructions (Dick 1978), are much too short to form an opercular flap.” Sister taxa in the LRT do not have opercular gill covers composed of long hyoid rays… BUT only one node apart, ancestral taxa also with jaws lacking teeth (Chondrosteus and Stronglyosteus) and sturgeons do have opercular gill covers. We talked about gill covers and sharks earlier here.

In 1978 Dick was unable to use software to perform a phylogenetic analysis. So he did what everyone did back then, he made a judgement call based on a few traits (= “Pulling a Larry Martin“).

From the Dick 1978 abstract (continued):
“Hybodonts and ctenacanths are recognised as separate, specialised shark radiations, neither of which can be directly ancestral to Recent sharks. Of the two, hybodonts appear to be more closely related to Recent forms, although the presence of typical hybodont finspines in Tristychius arcuatus indicates that they had diverged from ancestral euselachians before the beginning of the Carboniferous.”

In the LRT hybodonts are basal to bony fish (= recent forms). Tristychius is close to extant whale sharks.

More recently, Coates et al. 2019
considering Tristychius a suction feeder 50 million years before the bony fish equivalent. The authors also report, “The labial cartilages are large and comparable to examples known in Mesozoic hybodontids and modern suction feeding elasmobranchs such as nurse sharks (genus Ginglymostoma).” (see short video above).

The question of suction feeding depends on two things:
1) how fast the mouth opens and closes, and 2) whether or not the mouth cavity is capable of expanding rapidly to produce the inrush of water. Nurse sharks have perfected this technique (see video above). Whale sharks are more primitive. They simply open their mouths and engulf without a rapid inrush. So which type of feeding to Tristychius employ? Phylogenetic bracketing should tell us. Coates et al. did not mention either Rhincodon or Loganeillia, but did mention Ginglystoma, the nurse shark.

Figure 3. Comparative anatomy: Rhincodon on left and Ginglymostoma on right. Compare both to Tristiychius in figure 1, which has larger labial cartilages, a smaller lacrimal and jaws similar in length, like the whale shark.

In the LRT Tristychius is more closely related to whale sharks than nurse sharks
(Fig. 5), so we are free to consider Tristychius as more passive, open-mouth feeder than a quick suction feeder. Coates should have included more comparative taxa after a valid phylogenetic analysis, and not just cherry-pick a favorite or two.

Figure 4. The whale shark, Rhincodon, slowly opens its gaping mouth, the opposite of the nurse shark, Ginglymostoma, the vacuum cleaner in the video above.

Tristychius arcuatus
(Agassiz 1837; Dick 1978; Coates et al. 2019; Early Carboniferous; 24 cm est based on the skull in figure 1, 60cm max est.) was considered a Hybodus relative, but here nests with the whale shark, Rhincodon (Figs. 3, 4). Tristychius has a shorter torso, large pectoral and pelvic fins and large dorsal spines. Like the whale shark and similar basal sharks (like the nurse shark, the manta, etc.) the nares point anteriorly, slightly ventrally. Teeth are nearly absent with only a few, no longer than a millimeter, present. The postfrontal is strongly developed here, as in the whale shark. Note the large anterior gill bars (= labial cartilages in red) that restrict jaw depression and create lateral walls for the open jaws. The hyomandibular is tall and slender in Tristychius and the parietal is split medially, traits distinct from Rhincodon. The ceratohyal has less of a lateral exposure here than in the whale shark or nurse shark.

Figure 5. New tracing of Longanellia. This is a Rhincodon and Tristychius ancestor from the Early Silurian. The transition from micro scales to micro teeth around the jaw margins had its genesis with this taxon.

Coates et al. 2019 reported,
“The jaws of Tristychius (Fig. 2, A to C, G, and I) differ in numerous respects from the standard jaw morphology found in most Paleozoic chondrichthyans.” That’s the sort of clue that should have made Coates et al. expand their taxon list.

Examination indicated by phylogenetic bracketing
reveals that Early Silurian Loganellia (Fig. 5) is the basalmost taxon with labial cartilages, reducing the gape of its jaws while providing shark cheeks. If Loganellia were suction feeding in the Early Silurian, then Chondrosteus-types (Fig. 7) were suction-feeding in the Ordovician. So that sets the clock back on the shark-sucking hypothesis of Coates et al. (regarding Early Carboniferous Tristychius) another 180 million years.

Figure 6. Newly revised subset of the LRT focusing on basal vertebrates. Here Tristychius nests with Rhincodon as t basal position on the shark family tree, nowhere near Hybodus.

Coates et al. estimated the oral volume changes
of Tristychius through the bite cycle, but did not make comparisons to the voluminous oral cavity of Rhincodon. Coates et al. did not provide a phylogenetic analysis because their paper focused on functional morphology. Even so, an analysis is always called for. Otherwise you’ll be comparing whale sharks to hybodontids.

Figure 7. Chondrosteus is a late survivor of an earlier radiation that also fed by suction, extending the jaws while depressing the ceratohyal (gray shape) to expand the oral cavity. This, in turn, improves on the similar method of feeding in sturgeons.

Given the nature of sister taxa,
Tristychius was more likely a planktonic filter feeder with a slow-moving jaw opening mechanism. Fast opening and biting jaws first appeared in the vertebrate recored n the very next clade in the LRT (Fig. 6), currently occupied by the nurse shark, Ginglymostoma (Fig. 3), a traditional relative of whale sharks, which it greatly resembles.

References
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72.
Coates MI, Tietjen K, Olsen AM and Finarelli JA 2019. High-performance suction feeding in an early elasmobranch. Science Advances 5 (9): eaax2742
Dick JRF 1978. On the carboniferous shark Tristychius arcuatus Agassiz from Scotland.
Earth and Environmental Sciences Transactions of the Royal Society, Edinburgh. 70: 63–108.

wiki/Whale_shark
wiki/Tristychius

Publicity:
uchicago.edu/news/3d-reconstructions-show-how-ancient-sharks-found-alternative-way-feed

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