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In Protopterus and Neoceratodus the premaxilla, maxilla and dentary disappear

Friday, January 14, 2022 14:42

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If you study fish morphology, don’t start with these two.
The lungfish Protopterus (Figs. 1, 2) and Neoceratodus (Fig. 3) are the most recent additions to the large reptile tree (LRT, 2041 taxa). Reisz and Smith 2001 report, “Lungfish… are unique in that adults lack marginal teeth and have to rely on palatal dental plates for crushing food.” Not only do they lack marginal teeth, they also lack the marginal bones that carry those teeth.

This can be confusing for PAUP
because vertebrate taxa without tooth-bearing bones go back to Chondrosteus and sturgeons. Extant lungfish have a long lineage documented by Early Devonian lungfish. Bichirs join lungfish in the LRT, distinct from all prior studies that separated them before testing.

Figure 3. Neoceratodus skull with dermal bones colored here. Note the circumorbital bones, the premaxilla and maxilla, all missing from the smaller Criswell 2015 skull. Note the lateral excavation of the nasals.

Neceratodus forsteri
(Kreft = Krefft 1870; 150cm) is the extant Australian lungfish. Here (Fig. 3) the smaller skull from Criswell 2015) includes no premaxilla, maxilla, quadrate and circumorbital bones. But note, the larger skull (Fig. 3) includes all these bones, sometimes to a lesser extent than seen in other fish.

Figure 4. The Middle Devonian lungfish, Howidipterus, with subdivided skull bones colorized here to match tetrapod homologs. Lungfish have greatly subdivided dermal skulls, distinct from the skull of Protopterus.

Howidipterus donnae
(Long 1992; NMV P181884; Middle Devonian) is a primitive lungfish with large medial fins. The pectoral and pelvic fins are not known. Howidipterus nests with Polypterus in the LRT. Note the many subdivided bones of the skull, typical of lungfish. The lateral teeth are absent. The pterygoid and prearticular develop a tooth plate during maturation. The splenial, angular and surangular take the place of the dentary. The squamosal is tiny.

Figure 5. Polypterus nests with lungfish. Note the fusion of the preoperucular with the squamosal along with the retention (or return) of marginal teeth and tooth-beaing marginal bones.

Criswell 2015 wrote:
“Lepidosirenidae is a clade of freshwater lungfishes that include the extant South American Lepidosiren paradoxa Fitzinger, 1837 and African species of the genus Protopterus. These genera have been geographically separated since the break-up of Gondwana in the Early Cretaceous, but they display similar biology and morphology.”

Lungfish are ancient with basal representatives, like Powichthys (here updated in Fig. 6), Uranolophus and tiny Janusiscus radiating in the Early Devonian.

Figure 6. Early Devonian Powichthys, dorsal and palatal views. Colors added here. This image updates prior attempts at deciphering the myriad of subdivided skull bones.

Figure 7. Strunius is basal to Powichthys and lungfish in the LRT.

Criswell 2015 wrote:
“The ancestry of the lepidosirenids remains poorly understood.”

In the LRT Powichthys (Fig. 6) and Strunius (Fig. 7) are proximal ancestral taxa. Deeper ancestors (Fig. 8) extend to coelacanths, spiny sharks, sharks, jawless fish and basal chordates.

Figure 8. Subset of the LRT focusing on the lobe-fin half of the bony fish. Lungfish are in green. *Powicthys*, the basalmost and earliest lungfish, is Early Devonian. So are *Janusiscus* and *Uranolophus*. Others are either recent of Middle Devonian.

References
Boulenger GA 1900. A list of the batrachians and reptiles of the Gaboon (French Congo), with descriptions of new genera and species. Proceedings of the Zoological Society of London 1900: 433–456.
Criswell KE 2015. The comparative osteology and phylogenetic relationships of African and South American lungfishes (Sarcopterygii: Dipnoi). Zoological Journal of the Linnean Society, 174, 801-858.
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.
Reisz R and Smith MM 2001. Lungfish dental pattern conserved for 360 Myr. Nature 411: 548.

wiki/Howidipterus
wiki/Dipnorhynchus
wiki/Dipterus
wiki/Spotted_lungfish-Protopterus
wiki/Neoceratodus

Source: https://pterosaurheresies.wordpress.com/2022/01/15/in-protopterus-and-neoceratodus-the-premaxilla-maxilla-and-dentary-disappear/

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