Devonian fish heads

In a quest for understanding
the origins of everything reptilian, today we’ll take a look at the skulls of three Devonian fish with skull bones homologous with those of reptiles.

Cheirolepis trailli (Agassiz 1835; Middle Devonian, 390 mya; 30-55?cm in length; Fig. 1) is considered one of the earliest actinopterygian (ray-finned) fish with ‘standard’ dermal skull bones. Those bones were homologous with those of coeval and later sarcopterygian fish, like Eusthenopteron (Fig. 2) and Osteolepis (above right, Fig. 3), and tetrapods, like Ichthyostega. That series of small rostral bones will fuse to become the nasal in tetrapods. The maxilla will become much shallower, the rostrum will lengthen.

In Cheirolepis (now the basal taxon in the large reptile tree, LRT, 955 taxa) the orbit is far forward. The jaws opened up to right angles to form a large gape. The pineal opening pierced the inter-frontal (originally the singular frontal). Note the great depth of the maxilla on the cheek. That doesn’t last in tetrapods. Bony gill covers were present. Those disappear, too. The pectoral fins were lobed and muscular, but the hind fins were not. The pelvis and dorsal fins were large and broad-based. The tail was heterocercal, like a shark’s tail. The body was deep and broad anteriorly, narrower posteriorly. The lumbar  region becomes wider and less streamlined in basal tetrapods.

The homologies of the tabular and postparietal
are questionable here, so two solutions are shown (Fig. 1). The one on the left is likely correct based on the random appearance of post parietals in Osteolepis (Fig. 3) from the random splitting of elongate parietals.

The homologies of the frontals and parietals
are reidentified here for Cheirolepis (Fig. 1) and Eusthenopteron (Fig. 2) based on tetrapod skull bones and the pattern in Osteolepis skull roofing bones (Fig. 3). In their original identity, the pineal opening pierced the parietals. Here the pineal opening migrates from the inter frontal to the frontals, heading toward the parietals in basal tetrapods. The pineal basically follows the lateral eyes as they also migrate posteriorly.

Eusthneopteron foordi (Whiteaves 1881; Late Devonian, 385 mya; 1.8m in length) was one of the first fish genera known to share a long list of traits with basal tetrapods.

Distinct from Cheirolepis, 
Eusthenopteron had choanae (palatal openings for the passage of air, internal nares). The posterior maxilla was not so deep. The orbits were smaller and set further posteriorly. The mandible bones were more like those of tetrapods. Limb bones appear within the pectoral and pelvic fins, but no distinct wrist, ankle, metapodial or digit bones are yet present. Fin rays remain. The jaws were rimmed with tiny teeth. The palate had several large fangs. The tail was not so heterocercal, but stretched out more or less in line with the vertebral column.

Not as visible in these figures…
While all fish have anterior and posterior external naris for odor-laden water to enter and exit, in Eusthenopteron and Osteolepis the posterior naris has migrated to the orbit to become the tear duct. Now, that’s a clue that these fish were spending time poking their eyes above the water and perhaps not gulping air like a lungfish, but breathing through its new choanae (internal nares).

Variation in the skull shield of Osteolepis (Fig. 4)
is shown above (Fig. 3). Bones originally labeled intertemporals are here considered supratemporals. Original supratemporals are here considered tabulars. Note the random splitting of the parietals, originally considered anterior parietals (APa) and parietals (Pa). Here those bones are parietals and post parietals based on tetrapod homologies. In one Osteolepis specimen (Fig. 3 lower right) extra bones (supernumeraries – sa) appear, but do not appear in related taxa.

References
Agassiz JLR 1835. On the fossil fishes of Scotland. Report of the British Association for the Advancement of Science, British Association for the Advancement of Science, Edinburgh.
Graham-Smith W 1978. On the Lateral Lines and Dermal Bones in the Parietal Region of Some Crossopterygian and Dipnoan Fishes. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 282 (986):41-105.
Schultze H-P 1984. Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (Osteolepiform rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada. Journal of Vertebrate Paleontology 4: 1-16.
Whiteaves JF 1881. On some remarkable fossil fishes from the Devonian rocks of Scaumenac Bay, in the Province of Quebec. Annals and Magazine of Natural History. 8: 159–162.

Cheirolepis fossil images
wiki/Cheirolepis
wiki/Eusthenopteron