Palaeontologists have been interested in the endocasts of vertebrate skulls (moulds of the interior of the skull made by sediment) since at least the nineteenth century, due to the possibility that these can reveal details of the animal’s brain. Unfortunately the brains of some vertebrates are very different to the skull cavities, with those of Sharks and Lungfish occupying as little as 1% of the available space, though in other groups, such as Birds and Mammals, the endocast records the shape of the brain far more accurately. Early Ray-finned Fish, Actinopterygians, are thought to fall into the latter category, and their endocasts have been studied extensively since the early 20th century, although this has been problematic, as the best method available for most of that time has been to make serial sections of the skull, a destructive technique that can only be used on the most abundant of fossils, and which leaves only the palaeontologist’s notes and interpretation for examination by future generations of scientists.
In a paper published in the Journal of Paleontology in July 2014, Sam Giles and Matt Friedmann of the Department of Earth Sciences at the University of Oxford present the results of a study of two early Ray-finned Fishes made using computed tomographic X-ray scanning to build up a picture of the cranial endocasts without destroying the original skulls.
The first Fish selected was Mimipiscis toombsi, from the Late Devonian Gogo Formation of Western Australia. This species had its endocast studied by traditional methods by Brian Gardiner of King's College, London in the 1980s. The specimen chosen for this study came from the collection of the Natural History Museum in London, and comprises a 25 mm long by 15 mm with specimen, which was removed from its original matrix via acid preparation, and now has its dorsal surface preserved in a thick resin block. Although some original sediment remains and is preserved in the resin along with the skull, the brain cavity is largely devoid of infilling, creating a void space which shows up well in CT scans.
Photograph of the Mimipiscis toombsi used in the study as it is preserved. Giles & Friedmann (2014).
Giles and Friedmann found that the endocast of Mimipiscis toombsi was long and narrow, with elongate olfactory tracts. The forebrain comprises only 20% of the total length (25-30% is more typical) and is separated from the midbrain by a constriction. The forebrain is narrow, barely wider than the olfactory tracts, which diverge almost immediately after the olfactory bulbs, unlike the situation seen in most early Ray-finned Fish, where the two olfactory nerves share a common canal for most of their length. The midbrain is about twice the width of the forebrain, with poorly developed optic lobes. The hindbrain takes up about 60% of the length of the skull (50% is more typical).
Reconstruction of the endocast of Mimipiscis toombsi, in ventral view. (1) Three-dimensional rendering of endocast; (2) interpretive drawing of endocast. Anatomical abbreviations — bhc, buccohypophysial canal; b.oc.a, canal for branch of occipital artery; ccc, communication between cranial cavity andnotochordal canal; mcv,canal for middle cerebral vein; oof, otico-occipital fissure; opt.f, optic fenestra; spio, canal for spinooccipital nerve; s.v, saccus vasculosus; v.fon, vestibular fontanelle; I, canal for olfactory nerve; IV, canal for trochlear nerve; V, canal for trigeminal nerve; Vprof, canal for profundus nerve; VI, canal for abducens nerve; VII, canal for facial nerve;VIIlat, canal for lateralis branch of facial nerve; IX, canal forglossopharyngeal nerve; X, canal for vagus nerve. Giles & Friedmann (2014).
The second Fish chosen was Kentuckia deani, from the Early Carboniferous Stockdale Formation of Kentucky. Two specimens were examined, both from the collection of the Museum of Comparative Zoology at Harvard University. The first is 30 mm in length and preserved within a nodule, which has been broken in half to reveal the internal structure of the skull. The second is 15 mm in length and also within a nodule, which has been prepared so as to expose the upper surface of the skull.
(2) Kentuckia deani, 15 mm specimen; (3) Kentuckia deani, 30 mm specimen, part; (4) Kentuckia deani, 30 mm specimen, counterpart. Giles & Friedmann (2014).
Giles and Friedmann found that both endocasts of Kentuckia deani were less slender than that of Mimipiscis toombsi, with proportions closer to those of other, more familiar, ray finned fish. The forebrain makes up about 25% of the total length, with a single tract carrying both olfactory nerves for most of their length. The midbrain is twice the width of the forebrain, with well-developed optic lobes; the optic nerves also share a single opening. The hindbrain makes up about 50% of the total length.
Reconstruction of the endocast of Kentuckia deani, in left lateral view. (1) Three-dimensional rendering of endocast; (2) Interpretive drawing of endocast. The pocket of the lateral cranial canal has been ‘cut’ to avoid obscuring parts of the labyrinth. Anatomical abbreviations —acv, canal for anterior cerebralvein; aur, cerebellar auricle; c.c,crus commune; die, diencephalon; ica, canal for internal carotid artery; l.c.c, lateral cranial canal; sac, sacculus; s.su, sinus superior; s.v, saccus vasculosus; I, canal for olfactory nerve; III, canal for oculomotor nerve; IV, canal for trochlear nerve; V, canal for trigeminal nerve; VI, canal for abducens nerve; VII, canal for facial nerve; VIIlat, canal for lateralis branch of facial nerve. Dotted lines infigure indicate unresolved regions. Giles & Friedmann (2014).
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