As longtime readers know,
tetrapod bone colors were used here on fish skull photos and diagrams as they were added to ReptileEvolution.com (Figs. 1a, b) over the past two years. That was necessary in order to score several dozen new fish taxa for a growing online phylogenetic analysis that, until then, included only tetrapods.
Somehow it all worked out.
I was pleasantly surprised from the start at how readily tetrapod bone colors could be applied to fish skulls (Figs. 1a, b).
A few days ago, I learned that back in 2018,
Dr. David Marjanović (researcher, Museum für Naturkunde, Berlin) suggested fish workers do the same in an SVP abstract: “It is difficult to tease apart the homologies of bones across Osteichthyes, often even within Actinopterygii. For a long time, it seems, anatomists gave up the attempt; numerous separate—sometimes contradictory—nomenclatures were used in different decades for different taxa or by different authors. However, a flood of recent discoveries provides grounds for optimism.”
The time to do this is now. It is a great idea, a necessary idea.
“The tetrapod stem is much more densely sampled than 25 years ago, confirming
unambiguously that the large bones of the actinopterygian skull table—which lie in roughly
the same places as the frontal and parietal of crown-group tetrapods—are homologous to
the parietal (the “preorbital” of “placoderms”) and the postparietal. This affects the next
more lateral series as well: as recently proposed, the “dermosphenotic”/“infraorbital 5” is
the intertemporal (which participates in the orbit margin in a few early tetrapods), the
“dermopterotic”/“intertemporal” is the supratemporal and the “supratemporal” is the
Fish nomenclature can get confusing if you’re a tetrapod fan. Here (Figs. 1a, 1b, 2) I don’t identify fish nomenclature at all. I let colors tell the tale: pink for nasals, cyan for jugals, orange for postfrontals, yellow green for intertemporals (= prootics), etc. (Fig. 1). That way if bones split or appear as a result of a split, they can be identified in several views. Unfortunately many earlier tetrapods were colored in a more slipshod manner, not in accord with these standards. Over time these will be repaired.
“Further, the base of the tetrapod stem clarifies the original spatial relationships of other
bones: the bone dorsal of the (anterior) naris is plesiomorphically not the nasal, but the so called anterior tectal, and the one ventral to it is the so-called lateral rostral (apparently
homologous to the septomaxilla of crown-group tetrapods), making it likely that these are
the homologs of the actinopterygian “nasal” and “antorbital” respectively. Unlike in
tetrapods, the squamosal of many other sarcopterygians has a long contact with the maxilla and could be homologous to the (second) “supramaxilla”.
Beyond actinopterygians, tetrapod homologies must be extended back to all craniates and gnathostomes.
“Outside the tetrapod stem, the placoderm-grade animal Entelognathus has shown that some homologies can be traced beyond Osteichthyes.”
Nothing else can proceed unless a valid phylogenetic cladogram, like the LRT, has been established.
“I further propose that the unpaired “vomer” of various actinopterygians is the “prerostral plate” seen in “placoderms” and the Silurian osteichthyan Guiyu, the actual paired vomers being represented by the “vomerine toothplates”.
The present cladogram (subset Fig. 3) and the colors traced on taxa in ReptileEvolution.com documents the splitting and fusion of the tetrapod homologs of bones in various fish. Sometimes two or three bones represent the lacrimal. The squamosal and the quadratojual result from such bone splits. The maxilla and premaxilla appear on the lower rim of the lacrimal as new bones holding the marginal teeth. In the LRT (subset Fig. 3) Guiyu is a derived taxon close to coelacanths.
“The braincase remains underresearched even within crown-group tetrapods, and
neomorphic bones seem more common there than in the dermal skeleton; still, it seems
clear that the best candidates for homologs of the opisthotic are the “autopterotic” and/or
perhaps the “epiotic”/“epioccipital” of actinopterygians, not the “intercalary” sesamoid.”
“I propose further homologies throughout the skeleton based on ontogenetic data and the
rich fossil record, and hope to start a discussion on this promising field. Confidently
identified homologs would give a boost to phylogenetics and evolutionary biology.”
Marjanović D 2018. Yes, we can homologize skull (and other) bones of actinopterygians and tetrapods. Abstracts Society of Vertebrate Paleontology 2018.