Updated February 10 2021
with a some details modified in the morphology resulting in a shifting of this taxon to the base of the ray-fin fish clade (not the spiney/lobe-fin fish clade).
…and it still looks like Hybodus,
(Fig. 2) its 50x larger proximal ancestor.
A nice surprise today
as a phylogenetically miniaturized hybodontid shark with gill covers and ray fins, Prohalecites porroi (Figs. 1, 4, 5, Belloti 1857, Deecke 1889, Tintori 1990, MCSNIO P 348, Middle Triassic; 4cm), enters the large reptile tree (LRT, 1780+ taxa) as THE basalmost bony ray-fin fish.
Figure 1. Prohalecites porroi in situ from Arratia 2015, colors added. No dorsal spines here.
That makes Prohalecites close to a late-surviving
human, mammal, reptile, tetrapod and bony fish ancestor. Prohalecites needs to be in every paleo textbook from here on out.
No trace of scales is preserved
in any specimen. No neurocranial material is preserved. Hemichordacentra are present. The preoperular is so slender it is twig-like.
Figure 2. Hybodus fraasi fossil in situ is 50x larger than an adult Prohalecites, the basalmost bony fish.
Surprisingly little has been written
about Prohalecites. While Arratia 2015 considered it “the oldest of the Teleosti”, she did not mention Hybodus, its proximal ancestor in the LRT.
Tintori 1990 left Prohalecites as a Neopterygian incertae sedis,
“because its characters do not perfectly fit in any of these cited groups.”
Arratia and Tintori 1998 wrote,
“Prohalecites possesses an interesting mosaic of primitive and advanced chalacters, some of which have been previously interpreted as synapomorphies of Teleostei.”
“The election of the outgloup plays a significant role in the phylogenic position of Prohalecites and other neopterygians. Unquestionably, Prohalecites is not a Teleostei.”
Their cladograms nested Prohalecites between Amia and all higher bony fish. Neither sharks nor Hybodus are mentioned in the text. So taxon exclusion hampers an otherwise highly focused study.
Figure 1. Subset of the LRT focusing on ray-fin fish, their speed, niches and extant.
too much focus, not enough of a wide-angle view hampered prior workers. Whenever taxa are tested together that have never been tested together before, new relationships can be recovered. That’s why the LRT was created 10 years ago. You should have so many taxa in your cladogram that it tells you which taxa to include in your more focused study. Cherry-picking taxa has become outdated. That traditional practice leads to false positives and enigmas.
Figure 4. Prohalecites skull from Arratia 2015, colors added.
wrote on the history and current status of the fish clade Teleostei (Müller 1845).
Figure 5. Prohalecites diagram from Tintori 1990, colors added.
According to Arratia,
Müller defined the clade based on soft tissue traits not visible in fossils. Thus, the taxon content of the clade has changed several times over the last century.
From the Arratia abstract:
“The monophyly of the total group Teleostei, which now includes Triassic pholidophorids, is supported by numerous synapomorphies.” This, of course, would be “Pulling a Larry Martin“, which happens frequently out there. Remember, it is better to define a clade by establishing two taxa that recover a last common ancestor on a wide gamut, comprehensive cladogram. Don’t rely on a few or a few dozen traits. Convergence must be allowed in your hypothetical model, because convergence and reversal did happen.
Arratia also wrote,
“Prohalecites from the Ladinian/Carnian (Triassic; c. 240 Ma) boundary represents the oldest stem teleost.” That piqued my interest in this tiny fish only half as long as a human finger.
According to Arratia,
“during most of the last 170 years there has been a dichotomy in the treatment of teleosts, where fossil and living groups have been studied separately, including distinct classifications.”
Looking at the simple cladogram in figure 5 of Arratia 2015,
it looks like Teleostei includes Pholidophorus, Leptolepis, their last common ancestor and all descendants. Two other cladograms are shown in Arratia figure 8 based on earlier analyses. A full page cladogram is shown in Arratia figure 9 placing Amia and Lepisosteus as two of five outgroup taxa. Basal ingroup taxa include Pachycormus and Aspidorhynchus. Synapomorphies were listed for each node followed by a report on the pertinent traits. All this was for nought because the phylogenetic context was incomplete and invalid.
“The results demonstrate the importance of including fossil teleosts in the phylogenetic analysis, especially because some of their characters and combination of characters introduce a new perspective in understanding the origin and early radiation of the group, and indirectly provide a new scenario to interpret homologous characters.”
It goes without saying that Arratia 2015 did not include
placoderms, or lobefins in a teleost clade defined by Pachycormus and Aspidorhynchus in the LRT.
Prohalecites demonstrates, once again,
phylogenetic miniaturization at the genesis of a major clade, despite its late (Middle Triassic) appearance in the fossil record.
Was Prohalecites larger in the Silurian?
Maybe. It’s worth looking for. Or maybe bony fish began by neotony.
Earlier we looked at the origin of bone ‘islands’
on a cartilage substrate in the hatchlings and juveniles of the extant taxon, Amia. Prohalecites documents the origin of bone in the tiny adult bony fish descendants of hybodont sharks.
Arratia G and Tintori A 1999. The caudal skeleton of the Triassic actinopterygian †Prohalecites and its phylogenetic position, p. 121–142. In: Mesozoic Fishes 2—Systematics and Fossil Record. G. Arratia and H.-P. Schultze (eds.). Verlag Dr. F. Pfeil, München.
Arratia G 2015. Complexities of early Teleostei and the evolution of particular morphological structures through time. Copeia 103(4):999–1025.
Bellotti C 1857. Descizione di alcune nuove specie di pesci fossili di Perledo e di altre localtta lombarde. 419–432. In Sopani A (ed) Studi geologici sulla Lomabardia. Editore Turati, Milano.
Deecke W 1889. Über Fischea ùs verschiedenen Horizonten der Trias. Palaeontogaphica 45:97–138.
Müller J 1845. Über den Bau und die Grenzen der Ganoiden, und über das natürliche System der Fische. Physikalisch-Mathematische Abhandlungen der ko¨niglichen Akademie der Wissenschaften zu Berlin 1845:117–216.
Tintori A 1990. The actinopterygian fish Prohalecites from the Triassic of northern Italy. Palaeontology 33:155–174.