Remarkable preservation of brain tissues in an Early Cretaceous iguanodontian dinosaur

Abstract

It has become accepted in recent years that the fossil record can preserve labile tissues. We report here the highly detailed mineralization of soft tissues associated with a naturally occurring brain endocast of an iguanodontian dinosaur found in c. 133 Ma fluvial sediments of the Wealden at Bexhill, Sussex, UK. Moulding of the braincase wall and the mineral replacement of the adjacent brain tissues by phosphates and carbonates allowed the direct examination of petrified brain tissues. Scanning electron microscopy (SEM) imaging and computed tomography (CT) scanning revealed preservation of the tough membranes (meninges) that enveloped and supported the brain proper. Collagen strands of the meningeal layers were preserved in collophane. The blood vessels, also preserved in collophane, were either lined by, or infilled with, microcrystalline siderite. The meninges were preserved in the hindbrain region and exhibit structural similarities with those of living archosaurs. Greater definition of the forebrain (cerebrum) than the hindbrain (cerebellar and medullary regions) is consistent with the anatomical and implied behavioural complexity previously described in iguanodontian-grade ornithopods. However, we caution that the observed proximity of probable cortical layers to the braincase walls probably resulted from the settling of brain tissues against the roof of the braincase after inversion of the skull during decay and burial.

Advanced analytical techniques for studying the morphology and chemistry of Proterozoic microfossils

“…The high-resolution form of X-ray CT used for fossils is known as X-ray microtomography (mCT) and has been utilized in palaeobiology for almost two decades (Rowe et al 2001;Sutton 2008). It is routinely applied to Phanerozoic vertebrate and invertebrate fossils, ranging from echinoderms (Rahman & Zamora 2009) to dinosaurs (Brasier et al 2016) and from plants (Spencer et al 2013) to arthropods (Garwood & Sutton 2010). The study of microfossils using CT has become viable in recent years with the use of synchrotron-based systems in which more intense, monochromatic X-rays result in improved contrast and greater spatial resolution (Donoghue et al 2006;Huldtgren et al 2011).…”

Multiphase progenetic development shaped the brain of flying archosaurs

“…Because brain tissue itself rarely fossilises 24,25 , palaeoneurology resorts to studying the cranial endocast that reflects the surface geometry of the endocranial cavity in which the brain was housed during life 12,17 . Non-avian dinosaurs generally exhibit an overall brain shape resembling the crocodilian brain more strongly than the modern avian brain 26,27 .…”

Volume of the crocodilian brain and endocast during ontogeny

“…However, in the case of extinct animals, it is difficult to determine these characteristics because the brain is entirely composed of soft tissue and, therefore, the chances of fossilisation are extremely rare [2]. Hence, the brain proportions and morphology of some extinct vertebrates are usually inferred from their endocasts only [3–4].…”

“…Although the correlation between brain size and behavioural complexity remains ambiguous, brain size, brain dimension compartmentalisation and endoneurocranial space are widely used as measures of behavioural complexity and sensory-locomotive capacity in vertebrates [ 1 ]. However, in the case of extinct animals, it is difficult to determine these characteristics because the brain is entirely composed of soft tissue and, therefore, the chances of fossilisation are extremely rare [ 2 ]. Hence, the brain proportions and morphology of some extinct vertebrates are usually inferred from their endocasts only [ 3 – 4 ].…”