Entelognathus identifies enigmatic lungfish skull bones

Updated Nov 21, 2019
with new bone identities for Entelognathus.

The Devonian lungfish (order: Dipnoi),
Howidipterus (Fig. 1) has a patchwork of bones that experts identify with letters and numbers, not traditional fish or tetrapod names. That’s because lungfish appear to subdivide several of their skull bones.

Figure 1. The Middle Devonian lungfish, Howidipterus, with subdivided skull bones colorized here to match those in the placoderm Entelognathus (Fig. 2).

Figure 1. The Middle Devonian lungfish, Howidipterus, with subdivided skull bones colorized here to match those in the placoderm Entelognathus (Fig. 2).

The closely related Silurian placoderm,
Entelognathus (Fig. 2) has a more simplified and relatable set of skull sutures. By applying this blueprint to the skull of Howidipterus, one can see how several bones were split into several more, here (Fig. 1) color-coded to match Entelognathus.

Figure 2. The placoderm, Entelognathus, is widely considered the outgroup to the crossopterygians, the stem tetrapods. Compare the skull bones to those of Polypterus (Fig. 1) and Tinirau (Fig. 3). The posterior is unknown.

Figure 2. The placoderm, Entelognathus, is widely considered the outgroup to the crossopterygians, the stem tetrapods. Compare the skull bones to those of Polypterus (Fig. 1) and Tinirau (Fig. 3). The posterior is unknown.

Entelognathus primordialis (Zhu et al. 2013; Late Ludlow, Silurian, 419 mya; IVPP V18620) is a genus of placoderm fish with dermal marginal jaw bones (premaxilla, maxilla and dentary), features previously restricted to Osteichthyes (bony fish). Here all the tetrapod skull bones are identified on this fish (Placoderm and bony fish bones often have different names). Purported sclerotic rings are circumorbital bones (prefrontal, postfrontal, jugal). The purported jugal is the dorsal maxilla. The opercular is the quadratojugal. The pineal plate is the parietal. The central plate is the postparietal. The rostral is the nasal. The marginal plate is the supratemporall. The anterior paranuchal plate is the tabular.

Howidipterus donnae (Long 1992; NMV P181884; Middle Devonian) is a primitive lungfish medial fins (pectoral and pelvic fins unknown) that nests with Polypterus in the LRT. Note the many subdivided bones of the skull, typical of lungfish. The lateral teeth are absent. The pterygoid developed a tooth plate. The dentary is not visible laterally, where the splenial, angular and surangular overlap it. The squamosal is tiny.

The Nile bichir, Polypterus, is a related taxon in the LRT. Like lungfish, the bichir has a short maxilla, along with a suite of other shared traits.


References
Long JA 1992. Cranial anatomy of two new Late Devonian lungfishes (Pisces: Dipnoi) from Mount Howitt, Victoria. Records of the Australian Museum 44:299-318
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.

wiki/Entelognathus
wiki/Polypterus
wiki/ 
Howidipterus

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