Sea horse evolution back to large Cretaceous predators

Another series of taxa pulled from the LRT
focusing on phylogenetic miniaturization (PM) in the lineage of sea horses (Fig. 1). PM starts with 60cm-long Early Cretaceous Notelops and similar extant Scomberoides, the queenfish (Fig. 1), which is also (quite obviously) basal to mackerel and tuna.

Figure 1. Seahorse evolution back to Notelops (Early Cretaceous).

Figure 1. Seahorse evolution back to Notelops (Early Cretaceous).

Less obviously,
in the large reptile tree (LRT, 1806+ taxa) another descendant of Scomberoides is the 10x smaller zebra fish (Danio, Fig. 1).

Here’s where it gets interesting…
The sagittal crest present in Scomberoides (Fig. 1) is absent in Danio and the parietals return to meet each other medially, as in basal bony fish like Amia and Prohalecites. This phylogenetic reversal makes creating a cladogram more difficult, due to convergence, but all the more challenging. Danio descendants remains tiny and crestless. I have no data if Scomberoides hatchlings have crests or not. If so that would be a case of neotony leading to Danio.

Relative to Notelops
larger eyes are first seen, not in tiny Danio, but in big Scomberoides (Fig. 1), prior to PM. That increase in orbit size comes at the cost of a reduction in cheek plates that never comes back in descendant taxa. In Scomberoides the circumorbital ring actually overlaps the preopercular (light yellow) and hyomandibular (dark green). That’s a rare trait that makes it a bit difficult to score.

The jugal
(cyan color) in Danio (Fig. 1) is still large, though disconnected from the circumorbital ring where Gregory 1933 labels it the symplectic. According to Wikipedia, the symplectic is “an additional bone linking the jaw to the rest of the cranium.”  That also makes that bone difficult to score. Seeing this bone in a variety of taxa led to the conclusion that it was homologous to the jugal. Starks 1901 listed several synonymies used by various authors for bones of the fish skeleton. None synonymized the jugal and symplectic. That may have changed in the 120 years since. Let me know, if so.

Stickleback stickles
readily seen in Gasterosteus, are first seen in Scomberoides (Fig. 1), though lost in Danio.

Jaw joint migration from behind the orbit
to way out in front of the orbit in this series of taxa starts with Scomberoides, documents a mid-point in Danio, and reaches a conclusion in Gasterosteus (Fig.1).

Figure x. Rayfin fish cladogram. This one represents the latest subset of the LRT.

Figure x. Rayfin fish cladogram. This one represents the latest subset of the LRT.

That’s the utility of the LRT
and the ready-at-your-fingertips online data with all bones colorized using DGS.


References
Starks EC 1901. Synonymy of the fish skeleton. Proceedings of the Washington Academy of Sciences 3:507-539. PDF here.

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