DOI: 10.5061/dryad.2rbnzs7p8
title: Morphometric analysis of lungfish endocasts elucidates early dipnoan palaeoneurological evolution

description: 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.
description: <em>Griphognathus whitei</em> (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. <em>Orlovichthys
 limnatis</em> (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.