Jakob Vinther

Brachiopods appeared early in the Cambrian radiation and represent an important clade for understanding the origin of animal body plans and the evolution of biomineralized structures. A robust phylogenetic framework of the living representatives is needed to answer persistent questions about the evolution of brachiopod mineralogy, shell morphology, and identification of the brachiopod sister group among Lophotrochozoa. To the best of our knowledge, we present the first phylogenomic analysis of Brachiopoda and related lophotrochozoan lineages using a comprehensive dataset encompassing all five extant brachiopod orders and using a custom-developed data-refining strategy, compositionally heterogeneous substitution models, and a novel implementation of the parametric-bootstrap likelihood ratio test. We recover monophyly of Lophophorata, with strong support for Brachiopoda as sister to Bryozoa plus Phoronida. Within Brachiopoda, we recover the clade of inarticulated phosphatic Linguliformea sister to the inarticulated calcitic Craniiformea, with both sister to the articulated calcitic Rhynchonellata, with miniaturized Thecideida unexpectedly nested within Terebratulida. Our phylogeny implies that at least one evolutionary transition in primary mineralogy occurred in Brachiopoda, a phenomenon not recognized in any other metazoan group.

Modern cephalopods are unique among molluscs in ancestrally possessing a chambered, gas- and fluid-filled shell, the internal septa of which are penetrated by a siphuncle. Combined with jet propulsion, the siphuncle enables cephalopods to engage in a nektonic and mostly predatory lifestyle through long-term regulation of their bouyancy¹. The origin of this key innovation remains unknown, as major gaps exist between the earliest accepted cephalopod, Plectronoceras cambria² from the late Cambrian era, and molecular clock estimates placing the divergence of the lineage from other molluscs in the early Cambrian^3,4. Here we report Eoceras shaanxiense gen. et sp. nov., a millimetre-sized cephalopod with an orthoconic shell from the Shuijingtuo Formation (Cambrian Stage 3) of South China. Internally, the shell exhibits multiple septa along with a peripherally situated segmented tube that appears to bridge the septa through minute canals, indicating that the soft body migrated aperturally to form successive chambers during growth while maintaining contact with earlier chambers via the tube. Collectively, these features identify the segmented tube in E. shaanxiense gen. et sp. nov. as a candidate primordial cephalopod siphuncle, thus extending the known range of stem cephalopods back to the early Cambrian and revealing the early stages in the evolutionary assembly of a chambered phragmocone employed in buoyancy regulation.

The evolutionary origin of nervous systems in animals remains elusive and is largely hidden from the fossil record. Ctenophores, one of the earliest-branching animals possessing neurons, are instrumental to our understanding of nervous system origin, and a few rare ctenophore fossils preserve traces of nervous tissue as carbonaceous remains. Cambrian ctenophores appear to exhibit a more diverse neuroanatomy than that of modern species, suggesting secondary loss in extant ctenophores. However, much remains unknown about the origin and ontogeny giving rise to the structural organization of modern ctenophore nervous systems. Here, by investigating the neural anatomy of the ctenophore Mnemiopsis leidyi during development, we identified a ladder-like nerve net (LNN) beneath the comb rows that converges into condensed neurites and connects to the aboral organ. Examination of carbon-rich areas of Ctenorhabdotus capulus , an extinct ctenophore from the Burgess Shale, reveals a pattern similar to that of M. leidyi , consistent with a shared neural organization. Furthermore, M. leidyi exhibits a condensed comb nerve, resembling the longitudinal nerve preserved in the Cambrian ctenophore Fasciculus vesanus and the giant axon of extant Euplokamis dunlapae . Our study reveals conserved evolutionary constraints shaping nervous system architectures linked to locomotory organs and indicates that the different modes of nervous system organization observed in Cambrian ctenophores are variably retained in modern species.

Vertebrate vision is mainly accommodated by a pair of lateral image-forming camera-type eyes and is supplemented in non-mammalian vertebrates by a dorsal pineal complex (pineal and parapineal organs) functioning as photoreceptive and/or endocrine organs¹. The pineal complex shares a common genetic and embryological basis with the lateral eyes, both derived from evaginations during the development of diencephalon². Despite being widely heralded as the ‘third eye’ in crown vertebrates³, the nature of the pineal complex and its presumed visual capability in early vertebrates² remain unknown. Here we describe two pigmented features situated between the lateral eyes in two species of myllokunmingids, the earliest known fossil vertebrates (approximately 518 million years ago), and interpret these as pineal/parapineal organs. In both myllokunmingid species, the pineal complex contains abundant melanin-containing melanosomes identical to those in the retinal pigment epithelium in the lateral eyes, together with a distinctive, regularly ovoid structure interpreted as a lens. Our results indicate that the lateral eyes and pineal complex in myllokunmingids probably functioned as camera-type eyes capable of image formation. Thus, we propose that the four camera-type eyes represent an ancestral vertebrate character, corroborating hypotheses about the deep homology between the eyes and pineal complex.

Vertebrate vision is mainly accommodated by a pair of lateral image-forming camera eyes and is facilitated in non-mammalian vertebrates by a dorsal pair of photoreceptive organs, the endocrine pineal complex (pineal and parapineal organs)1. The pineal complex shares a common genetic and embryologic basis with the lateral eyes, both of which are derived from evaginations during the development of diencephalon2. Despite widely heralded as the third eye in crown vertebrates3, the nature of the pineal complex and hypothesised visual capability in early vertebrates2 remains unknown. Here we describe in two species of myllokunmingids, the earliest known fossil vertebrates (c.a. 518 Million Years Ago, Mya), two pigmented features situated between the lateral eyes and interpret these as pineal/parapineal organs. In both myllokunmingid species, the pineal complex bears abundant melanin-containing melanosomes that are identical to those in the retinal pigment epithelium (RPE) in the lateral eyes, and a distinctive, regular ovoid structure that is interpreted as a lens. Our results reveal the lateral eyes and pineal complex in myllokunmingids likely functioned as camera eyes capable of image formation. As such we propose that four camera eyes are a vertebrate ancestral character, corroborating hypotheses regarding the deep homology between eyes and pineal complex.

The characteristic cardinal spines of Isoxys, a cosmopolitan bivalved arthropod, have been focal to understanding its role in Cambrian ecosystems. It has been proposed that the spines had either a hydrodynamic function, to aid buoyancy, or a defensive function, to protect against predators. Here, we demonstrate that the unusually elongated cardinal spines in Isoxys volucris (by far the most abundant taxon in the lower Cambrian Sirius Passet Lagerstätte, North Greenland) had a primary defensive function. Spine measurements of 85 specimens show a negative allometry during ontogeny, with the ratio of cardinal spine length to carapace length decreasing from >3.2 to 0.9. Negative allometric growth is inconsistent with a hydrodynamic function since larger carapaces would require spines which are proportionally at least as long (isometry or positive allometry). Instead, the negative allometry provides evidence for a defensive adaptation comparable to that seen in modern lower‐trophic organisms, in which elongated spines increase the overall size of juveniles to deter predators. Isoxys volucris was the dominant food source for higher‐trophic benthic and pelagic predators in the Sirius Passet biota, as revealed by the gut contents of arthropods, lobopods, palaeoscolecids and stem‐chaetognaths. Its long spines therefore indicate similar adaptive responses to extremely high predation pressures in both modern and early Cambrian ecosystems.

Nectocaridids are enigmatic Palaeozoic animals with a controversial phylogenetic position. Previous hypotheses have placed them in their own phylum, chordates, molluscs (specifically cephalopods), or radiodont panarthropods. We describe here a nectocaridid, Nektognathus evasmithae gen. et sp. nov. from the early Cambrian (~519 million years) Sirius Passet Lagerstätte of North Greenland. Key specimens preserve paired, phosphatized arcuate structures consistent with preservation of a ventral ganglion, a feature characteristic of extant and fossil chaetognaths, including the amiskwiid Timorebestia koprii also from Sirius Passet. Nektognathus shares a gnathostomulid-like jaw apparatus, lateral fins, subterminal anus, and large antennae with Timorebestia and Amiskwia , placing nectocaridids in the chaetognath stem lineage. The complex sensory anatomy of nectocaridids, which is partially shared with other extinct amiskwiids, highlights a more dynamic predatory lifestyle much higher in the trophic food chain during early chaetognath evolution.

Pelage coloration, which serves numerous functions, is crucial to the evolution of behavior, physiology, and habitat preferences of mammals. However, little is known about the coloration of Mesozoic mammaliaforms that coevolved with dinosaurs. In this study, we used a dataset of melanosome (melanin-containing organelle) morphology and quantitatively measured hair colors from 116 extant mammals to reliably reconstruct the coloration of six Mesozoic mammaliaforms, including a previously undescribed euharamiyidan. Unlike the highly diverse melanosomes discovered in feathered dinosaurs, hairs in six mammaliaforms of different lineages and diverse ecomorphotypes showed uniform melanosome geometry, corresponding to dark-brown coloration consistent with crypsis and nocturnality. Our results suggest that the melanosome variation and color expansion seen in extant mammals may have occurred during their rapid radiation and diversification after the Cretaceous-Paleogene extinction.

Mollusks encompass enormous disparity, including familiar clams and snails alongside less familiar aculiferans (chitons and vermiform aplacophorans) with complex multicomponent skeletons. Paleozoic fossils trace crown mollusks to forms exhibiting a combination of biomineralized shells and sclerites (e.g., scales, spines, and spicules). We describe a shell-less, Cambrian stem mollusk, Shishania aculeata gen. et sp. nov., with conical, hollow chitinous sclerites and a smooth girdle, broad foot, and mantle cavity. The sclerites have a microstructure of narrow canals consistent with the impressions of chaetal microvilli found in annelids and brachiopods. Shishania sclerites provide a morphological stepping stone between typical chaetae (chitinous bristles) and the external organic part of aculiferan sclerites that encloses a mineralized body. This discovery reinforces a common origin of lophotrochozoan chaetae and the biomineralized aculiferan sclerites, suggesting that the mollusk ancestor was densely covered with hollow chitinous chaetae.