What kind of evo neuroscience blog would this be if I didn’t blog on this?
Paleontologists found some fossils in Kansas which turned out to be actual fossilized brains, a extremely rare event in paleoneurology. Usually, paleoneurological data come from cranium endocasts, which are thought to be reasonable proxies for real brains. In this case, however, the whole brain from the extinct chimaeroid fish Sibyrhynchus denisoni, a cartilaginous fish closely related to sharks and ratfishes ¹.
(Image credit: P. Janvier)

The relatively good degree of conservation of this fossil brain allowed rough morphometric conclusions to be taken. First, watch this video
As can be seem from that video, the fossilized brain has an intact midbrain, medulla and cerebellum, and some cranial nerves can also be seen: the optic nerve, composed of the axons which connect retinal ganglion cells to central visual areas, can be seen projecting to very large optic tecta; oculomotoris nerves, which project from the optic tectum to the eye muscles, can also be seem. This species seemed to have big eyes (consistent with the size of their tecta), as inferred from the dimensions of their eye sockets. On the other hand, the regions of its brain which are responsible for auditory processing are rather small; according to Mo", this is consistent with
“previous studies of the inner ear, which have shown that S. denisori and closely related organisms had semicircular canals which were grouped together in the horizontal plane, rather than in three different planes, as in modern mammals. This suggests that S. denisori could detect lateral, but not vertical, water movements. Conspicuous by its absence, however, is the forebrain; instead, there is a vague blade-shaped structure which protrudes from the fron of the brain on only one side.”
We know very little about the organization of the auditory system of Inopterygii and Holocephali (the sister-group for Inopterygians), but cladistic analyses suggested that the state of the trait that is found in S. denisori is indeed ancestral (see references ² and ³). A lot could be also said about the conspicuosness of the forebrain, and this will probably be done as research on the field progresses and this article delivers its impact on the scientific community – especially among evolutionary neuroscientists and neuroichtyologists (I confess that I just made that word up, but it kinda describes what I do). I predict that lots of things will be said about how the brains of primitive fish were lacking forebrains, and everything else. Of course, this comments will simply ignore the hierarchical nature of evolution: Inopterygii are thought to be sister-grouped with modern Holocephali; as such, they are part of a bigger taxonomic group (class), Chondrichtyes, which diverged from jawless fish at about 468 millions of years ago, as I’ve learned from "paleoDB"http://paleodb.org. As lampreys and hagfishes both possess forebrains which are not that big, this is to be expected in an early cartilaginous fish. Inopterygii are an outgroup for Symoriidae, the clade which includes Holocephali – that is, chimeras (or ratfishes). Their outgroup, on the other hand, is composed by Elasmobranchii, true sharks.
Do their brains resemble that of chimerae? Of course, their brains resemble that of ALL vertebrates – that is what is called a bauplan. But let’s just focus on the optic tectum – a recent interest of mine. The optic tectum is a laminated structure which receives all of the retinal input in fish, as well as non-visual sensory input. The optic tectum of the extinct species is huge because it receives a lot of retinal input – Deacon’s “large-equals-well-connected” rule [4,5]. This suggests that those animals lived in places in which vision was rather important (probably shallow waters) and a good degree of illumination (which suggests that they were active during the day).
Not very much else can be said about these brains. As interesting as they are, fossilized brains can account for macroanatomy and morphometric variables, but what is interesting – at least as far as optic tecta go – is microcircuitry such as lamination patterns. We need to wait and see what will come from further research on the brain of this inopterygian, then…