Gogosardina: the genesis of the squamosal

Choo, Long and Trinajstic 2009 brought us
a small, Late Devonian actinopterygian, Gogosardina coatesi (Figs. 1, 2; holotype WAM 07.12.2) known from four crushed and incomplete specimens. One contains conodont elements lodged among the branchial arches.

Figure 1. Gogosardina from Choo, Long and Trinajstic 2009 shown at full size if shown on a typical 72dpi computer monitor.

Figure 1. Gogosardina from Choo, Long and Trinajstic 2009 shown at full size if shown on a typical 72dpi computer monitor. Gray areas indicate missing bones on skull.

Here
(Fig. 2) a few skull bones are relabeled according to their tetrapod homologs, as in all taxa entered into the large reptile tree (LRT, 1663+ taxa). The skull is nearly identical to coeval and similarly-size Mimipiscis with slightly rotated premaxilla, a straighter anterior maxilla, a higher naris and only a partial ‘razor back’ ridge anterior to the dorsal fin. The skull is proportionally larger as well. Both have a large pineal opening between the frontals (yes, the frontals), distinct from almost all fish. The excurrent naris is confluent with the orbit. This entire clade lacks postparietals.

Figure 2. Gogosardina soul from Choo, Long and Trinajstic 2009. New labels in red. The intertemporal anchors the large hyomandibular in all fish.

Figure 2. Gogosardina soul from Choo, Long and Trinajstic 2009. New labels in red. The intertemporal anchors the large hyomandibular in all fish.

Choo, Long and Trinajstic considered Gogosardina to be
a stem actinopterygian. No cladogram of relationships was published then. Wikipedia lists Gogosardina among the Palaeonisciformes (Hay 1902). In the LRT Gogosardina nests between Cheirolepis and Mimipiscis, all basal to the extant anchovy, Engraulis., which is not traditionally considered to be a paleonisciform.

Figure 3. Pteronisculus shows how the jugal splits to form a jugal and squamosal, a bone that will ultimately take over for the preopercular.

Figure 3. Pteronisculus shows how the jugal splits to form a jugal and squamosal, a bone that will ultimately take over for the preopercular in this clade.

Clade member, Pteronisculus
(Fig. 3) splits the jugal into four parts. The posterior two become the single squamosal in Strunius (Fig. 4), Onychodus (Fig. 5) and all later lobefins and ultimately tetrapods.

Figure 5. Strunius shows the next step in the enlargement of the squmosal and the two bones making up the preopercular.

Figure 4. Strunius shows the next step in the enlargement of the squmosal and the two bones making up the preopercular.

In Onychodus
(Fig. 5) the squamosal is beginning to take over the preopercular (= postsquamosal). Thereafter the postsquamosal is a vestige until it disappears in most tetrapods, only to reappear in a few basal tetrapods undergoing reversals.

Figure 1. Onychodus is typical of most fish having dual external nares strictly for olfactory sensing. Gill covers are part of the respiratory apparatus.

Figure 5. Onychodus continues the enlargement of the squamosal and the reduction of the preopercular (post squamosal) in our tetrapod lineage.

Wikipedia reports,
“The Palaeonisciformes (Hay 1902) are an extinct order of early ray-finned fishes (Actinopterygii) which began in the Late Silurian and ended in the Late Cretaceous. It is not a natural group, but is instead a paraphyletic assemblage of the early members of several ray-finned fish lineages.”

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

With regard to anchovies, Wikipedia reports, 
Clupeiformes (Goodrich 1909) is the order of ray-finned fish that includes the herring family, Clupeidae, and the anchovy family, Engraulidae.” 

The LRT nests few traditional clupeiformes,
but the wolf herring, Chirocentrus, nests elsewhere (at a more basal node, along with toothy Trachinocephalus) apart from the anchovy, Engraulis. So this seems to be a paraphyletic clade based on these two disparate taxa.

Putting related taxa in phylogenetic order
helps us visualize the less dramatic processes of evolution that no one ever talks about, like the origin of the squamosal. which ultimately creates the dentary-squamosal jaw joint in mammals and humans.


References
Choo B, Long JA and Trinajstic K 2009. A new genus and species of basal actinopterygian fish from the Upper Devonian Gogo formation of Western Australia. Acta Zoologica (Stockholm) 90 (Supp 1):194–210.

wiki/Gogosardina (not online yet)
wiki/Palaeonisciformes
wiki/Clupeiformes

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