The oldest three-dimensionally preserved vertebrate neurocranium

Abstract

The neurocranium is an integral part of the vertebrate head, itself a major evolutionary innovation1,2. However, its early history remains poorly understood, with great dissimilarity in form between the two living vertebrate groups: gnathostomes (jawed vertebrates) and cyclostomes (hagfishes and lampreys)2,3. The 100 Myr gap separating the Cambrian appearance of vertebrates4–6 from the earliest three-dimensionally preserved vertebrate neurocrania7 further obscures the origins of modern states. Here we use computed tomography to describe the cranial anatomy of an Ordovician stem-group gnathostome: Eriptychius americanus from the Harding Sandstone of Colorado, USA8. A fossilized head of Eriptychius preserves a symmetrical set of cartilages that we interpret as the preorbital neurocranium, enclosing the fronts of laterally placed orbits, terminally located mouth, olfactory bulbs and pineal organ. This suggests that, in the earliest gnathostomes, the neurocranium filled out the space between the dermal skeleton and brain, like in galeaspids, osteostracans and placoderms and unlike in cyclostomes2. However, these cartilages are not fused into a single neurocranial unit, suggesting that this is a derived gnathostome trait. Eriptychius fills a major temporal and phylogenetic gap in our understanding of the evolution of the gnathostome head, revealing a neurocranium with an anatomy unlike that of any previously described vertebrate.

Colonization of the ocean floor by jawless vertebrates across three mass extinctions

“…† Gilpichthys greenei , known from numerous poorly preserved specimens from the Mazon Creek, from all of our analyses. Recent phylogenetic analyses place † G. greenei as an indeterminate jawless vertebrate [ 37 , 49 , 66 , 67 ] or as the most stemward hagfish [ 38 , 53 , 73 ]. Because † G. greenei is classically united with Myxiniformes based on the absence of features like mineralized teeth, the placement of this taxon among early vertebrates might be biased by the phenomenon of stemward slippage, whereby decomposition of the body can lead to erroneous interpretations of a fossil’s phylogenetic affinities [ 102 , 103 ].…”

The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes

“…Comparison to other heterostracans suggests that while this feeding ecology may not be shared by all members of the group, the anatomy that underlies it is present in pteraspids and the oldest and earliest well-known heterostracan [74]; as such it may be plesiomorphic for the clade. In other armoured stem-gnathostomes (osteostracans, galeaspids, thelodonts, pituriaspids, anaspids, arandaspids and astraspids) there is little information on the anatomy of their feeding apparatus [1,71,84,[88][89][90][91], but marked variation in their anatomy suggests a range of ecological roles. Meanwhile, evidence that filter feeding in ammocoete larval lampreys represents an independent evolutionary innovation [23] (although see [92]) suggests that suspension feeding has evolved separately amongst jawless vertebrates at least once.…”