Tilt the skull of Kolymaspis to understand it better

Today’s post expands on
an earlier post that reported on the introduction of Early Devonian Kolymaspis (Fig 1), which nested with Diandongpetalichthys (Fig 1), a taxon known in the literature from a diagram of the skull in dorsal view.

Simply tilting the skull of Kolymaspis
helps in understanding its role as a transitional taxon between Diandongpetalichthys and the Bothriolepis clade (Fig 1). At an elevated angle Kolymaspis demonstrates the elevation of the posterior skull and the migration of the eyes from a lateral position in Diandongpetalichthys to a median position in Bothriolepis and likely conjoined position in Microbrachis (Fig 2). This clade also developed armored pectoral fins, distinct and apart from the origin of pectoral fins in other fish, contra Brazeau et al 2023.

This exercise also helped to identify
the skull bones of related taxa. Understanding the two-part parietal and two-part tabular in this clade helped identify the other skull elements, many of them fused.

Kolymaspis sibirica
(TsNIGR 7656 Brazeau et al 2023; Early Devonian, Siberia, 410 mya, Fig 1). Note the transitional dorsal placement of the orbits. According to Brazeau et al 2023, reporting in Science News, an online publication, “The fossil shows signs that one pair of gill arches, structures which support the gills in fish, gradually transitioned from helping fish to breathe into a ‘hinge’ at the back of the head. Over millions of years, this structure eventually became the shoulder. As the head and body separated, further adaptation would have allowed parts of the skeleton to become the basis of early fins.”

This hypothesis is not supported by the LRT, as described earlier (see links below).

References
Brazeau MD et al (6 co-authors) 2023. Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle. Nature https://doi.org/10.1038/s41586-023-06702-4

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