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Morphometric analysis of lungfish endocasts elucidates early dipnoan palaeoneurological evolution

Citation

Clement, Alice et al. (2022), Morphometric analysis of lungfish endocasts elucidates early dipnoan palaeoneurological evolution, Dryad, Dataset, https://doi.org/10.5061/dryad.2rbnzs7p8

Abstract

Lungfish (Dipnoi) are lobe-finned fish (Sarcopterygii) that have persisted for over 400 million years from the Devonian Period to present day. They are the extant sister group to tetrapods and thus have the ability to provide unique insight into the condition of the earliest tetrapods as well as their own evolutionary history. The evolution of their dermal skull and dentition is relatively well understood, but this is not the case for the central nervous system. While the brain itself has very poor preservation potential and is not currently known in any fossil lungfish, substantial indirect information about it and associated structures such as the inner ears can be obtained from the cranial endocast. However, before the recent development of X-ray tomography as a palaeontological tool, these endocasts could not be studied non-destructively, and few detailed studies were undertaken. Here we describe and illustrate the endocasts of six Palaeozoic lungfish (Iowad ipterus halli, Gogodipterus paddyensis, Pillararhynchus longi, Griphognathus whitei, Orlovichthys limnatis, and Rhinodipterus ulrichi) from tomographic scans. We combine these with six previously described digital lungfish endocasts (4 fossil and 2 recent taxa) into a 12-taxon data set for multivariate morphometric analysis using 17 variables. We find that the olfactory region appears to be more highly plastic than the hindbrain, and undergoes significant elongation in several taxa. Further, while the semicircular canals covary as an integrated module, the utriculus and sacculus of the inner ear instead vary independently of each other. Functional interpretation suggests that olfaction has remained a dominant sense throughout lungfish evolution, and that changes seen in the labyrinth system may potentially reflect a change from a nektonic niche in older marine lungfish to more near-shore environments over time. Phylogenetic implications propose that endocranial form fails to support the monophyly of the 'chirodipterids'. Those with elongated crania similarly fail to form a distinct clade, suggesting that these are two paraphyletic groups that have converged either towards head elongation or truncation driven by constraints other than phylogeny.

Methods

Griphognathus whitei (NHMUK PV P 56034) was scanned at the Natural History Museum, UK, using a Nikon Metrology HMX ST 225 micro-CT scanner. The specimen was scanned with the following parameters: Energy 210 KeV, voxel size 102 microns, 180 degrees, 3142 proj, angle step 0.1 deg/proj, exposure time 0.5 sec/proj, object to detector 1170 mm (filters: 2.5 mm Cu). The resulting image stack was rendered and segmented manually using Drishti and Drishti Paint 2.6.1 to produce a virtual 3D endocast.

Orlovichthys limnatis (PIN (3725/110) was scanned at the Palaeontological Institute, Moscow, in three sections using a Skyscan 1172 micro-CT scanner. Each section was scanned with the following parameters: Energy 100 KeV, voxel size 34.1 microns, 180 degrees, 514 proj, angle step 0.7 deg/proj, exposure time 0.79 sec/proj, object to detector 344 mm (filters: 1 mm Al). The resulting image stacks of all specimens were combined using ImageJ and the subsequent image stack was rendered and segmented manually using Drishti and Drishti Paint 2.6.1 to produce a virtual 3D endocast.

Funding

Australian Research Council, Award: DP160102460

Australian Research Council, Award: DP200103398

Flinders University, Award: Visiting International Research Fellowship

Knut och Alice Wallenbergs Stiftelse

Callidus Services Ltd UK