The cranial endocast of the Middle Devonian dipnoan Dipterus valenciennesi and a fossilized dipnoan otoconial mass

Abstract

The well known Middle Devonian (Eifelian–Givetian) lungfish Dipterus valenciennesi from Scotland, UK, has been studied for more than one hundred years though our understanding of the neurocranium and cranial cavity is incomplete. Micro‐CT scanning demonstrates that the internal cast of the cranial cavity, the endocast, possesses a mix of primitive and derived characters. The olfactory bulbs are sessile, as in the derived extant Lepidosirenidae. However, Dipterus valenciennesi possesses the primitive condition of a shallow telencephalon unlike Upper Devonian forms such as Rhinodipterus kimberleyensis and extant lungfish taxa. Further information revealed by micro‐CT scanning has allowed coding for characters pertaining to the neurocranial cristae, previously unobservable in Dipterus valenciennesi, allowing hypotheses of relationships between Devonian Dipnoi and the position of Dipterus valenciennesi to be reassessed. New analyses do not refine the phylogenetic position of Dipterus valenciennesi but do increase support for most recently established Devonian dipnoan phylogeny. The first record of a three‐dimensionally preserved fossilized otoconial mass from the utricular recess in a fossil dipnoan is also described. Comparison of morphometric data of the endosseous and soft tissue manifestations of the labyrinth system in extant dipnoan taxa demonstrates that there is largely good correspondence between the two. When compared to Dipterus valenciennesi, extant taxa exhibit semi‐circular canals that are reduced in length. Furthermore, compared to Dipterus, in extant dipnoan taxa the size of the otoconial mass has increased relative to the utricular and sacculolagenar pouches containing them. The functional implications for these observations suggest that the Dipnoi have evolved towards a more sedentary lifestyle and behaviour at least since the Middle Devonian.

The cranial endocast of the Upper Devonian dipnoan Chirodipterus australis

“…6 & 8). This canal likely represents that for the Jiddle cerebral vein as suggested by Stensiö (1963) and Miles (fig 47, 1977) rather than the canal for the abducens nerve nVI as interpreted by Challands (2015) for a siJilar structure in Dipterus. FurtherJore, in NHMUK PV P56038, a separate canal also eJerges froJ the Jain body of the endocast posterior and ventral to nerve VII and the canal for the Jiddle cerebral vein.…”

“…Endocasts of fossil Devonian dipnoans available for coJparison with Chirodipterus australis include Chirodipterus wildungensis (Säve-Söderbergh, 1952), Dipnorhynchus sussmilchi (CaJpbell & Barwick, 1982;CleJent et al, 2016), Rhinodipterus kimberleyensis (CleJent & Ahlberg, 2014), Dipterus valenciennesi (Challands, 2015). CoJparison of the endosseous labyrinth is possible with Griphognathus whitei (Miles, 1977, fig.…”

Electroreception in early vertebrates: survey, evidence and new information

“…Alternatively, Cheng () proposed that the tubuli were a part of the lateral line system, in part because they have a similar histological structure to the lateral line canals. However, the rostral tubuli appear to carry nerves: specifically, the profundus, superficial ophthalmic and buccal nerves (Miles ; Challands ). Campbell et al .…”

“…However, given the clear connections with the neurocranium and sensory line canals (Campbell et al . ; Challands ), we assume that at least part of the rostral tubuli carried nerves for our interpretation of the pore‐group pits (see below).…”

“…Rostral tubuli communicate with the lateral lines and presumably carried nerves (Miles ; Campbell et al . ; Challands , but see discussion on rostral tubuli above), and it is likely that lateral line nerves also innervated the pore‐group clusters via the rostral tubuli. Finally, although the depth and diameter of the pore group pits is larger than the values reported for living lungfish (Roth & Tscharntke ; Kemp ), this might be expected given that the fossils dealt with here are marine species (see above for explanation of differences between electroreceptors in marine and freshwater species).…”