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).
See also…
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