The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins.

Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic acanthodians are possibly the earliest gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution. Their relationships with modern gnathostomes have remained mysterious, partly because their un-mineralized endoskeletons rarely fossilized. Here I present the first-known braincase of an Early Devonian (approximately 418-412 Myr bp) acanthodian, Ptomacanthus anglicus, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes in exhibiting several plesiomorphic features shared with placoderms and some early chondrichthyans. Most striking is its extremely short sphenoid region and its jaw suspension, which displays features intermediate between some Palaeozoic chondrichthyans and osteichthyans. Phylogenetic analysis resolves Ptomacanthus as either the most basal chondrichthyan or as the sister group of all living gnathostomes. These new data alter earlier conceptions of basal gnathostome phylogeny and thus help to provide a more detailed picture of the acquisition of early gnathostome characters

Evolutionary origins of teeth in jawed vertebrates: conflicting data from acanthothoracid dental plates (‘Placodermi’)

© The Authors. Palaeontology published by John Wiley & Sons Ltd on behalf of The Palaeontological Association. doi: 10.1111/pala.12318 829. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.NHM Repositor

Early Gnathostome Phylogeny Revisited: Multiple Method Consensus

This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.A series of recent studies recovered consistent phylogenetic scenarios of jawed vertebrates, such as the paraphyly of placoderms with respect to crown gnathostomes, and antiarchs as the sister group of all other jawed vertebrates. However, some of the hylogenetic relationships within the group have remained controversial, such as the positions of Entelognathus, ptyctodontids, and the Guiyu-lineage that comprises Guiyu, Psarolepis and Achoania. The revision of the dataset in a recent study reveals a modified phylogenetic hypothesis, which shows that some of these phylogenetic conflicts were sourced from a few inadvertent miscodings. The interrelationships of early gnathostomes are addressed based on a combined new dataset with 103 taxa and 335 characters, which is the most comprehensive morphological dataset constructed to date. This dataset is investigated in a phylogenetic context using maximum parsimony (MP), Bayesian inference (BI) and maximum likelihood (ML) approaches in an attempt to explore the consensus and incongruence between the hypotheses of early gnathostome interrelationships recovered from different methods. Our findings consistently corroborate the paraphyly of placoderms, all `acanthodians' as a paraphyletic stem group of chondrichthyans, Entelognathus as a stem gnathostome, and the Guiyu-lineage as stem sarcopterygians. The incongruence using different methods is less significant than the consensus, and mainly relates to the positions of the placoderm Wuttagoonaspis, the stem chondrichthyan Ramirosuarezia, and the stem osteichthyan LophosteusÐthe taxa that are either poorly known or highly specialized in character complement. Given that the different performances of each phylogenetic approach, our study provides an empirical case that the multiple phylogenetic analyses of morphological data are mutually complementary rather than redundant

A critical appraisal of appendage disparity and homology in fishes

Fishes are both extremely diverse and morphologically disparate. Part of this disparity can be observed in the numerous possible fin configurations that may differ in terms of the number of fins as well as fin shapes, sizes and relative positions on the body. Here, we thoroughly review the major patterns of disparity in fin configurations for each major group of fishes and discuss how median and paired fin homologies have been interpreted over time. When taking into account the entire span of fish diversity, including both extant and fossil taxa, the disparity in fin morphologies greatly complicates inferring homologies for individual fins. Given the phylogenetic scope of this review, structural and topological criteria appear to be the most useful indicators of fin identity. We further suggest that it may be advantageous to consider some of these fin homologies as nested within the larger framework of homologous fin‐forming morphogenetic fields. We also discuss scenarios of appendage evolution and suggest that modularity may have played a key role in appendage disparification. Fin modules re‐expressed within the boundaries of fin‐forming fields could explain how some fins may have evolved numerous times independently in separate lineages (e.g., adipose fin), or how new fins may have evolved over time (e.g., anterior and posterior dorsal fins, pectoral and pelvic fins). We favour an evolutionary scenario whereby median appendages appeared from a unique field of competence first positioned throughout the dorsal and ventral midlines, which was then redeployed laterally leading to paired appendages.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151971/1/faf12402_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151971/2/faf12402.pd

Placoderm interrelationships: a new interpretation, with a short review of placoderm classifications

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The origin of the myelination program in vertebrates

SummaryThe myelin sheath was a transformative vertebrate acquisition, enabling great increases in impulse propagation velocity along axons. Not all vertebrates possess myelinated axons, however, and when myelin first appeared in the vertebrate lineage is an important open question. It has been suggested that the dual, apparently unrelated acquisitions of myelin and the hinged jaw were actually coupled in evolution [1,2]. If so, it would be expected that myelin was first acquired during the Devonian period by the oldest jawed fish, the placoderms [3]. Although myelin itself is not retained in the fossil record, within the skulls of fossilized Paleozoic vertebrate fish are exquisitely preserved imprints of cranial nerves and the foramina they traversed. Examination of these structures now suggests how the nerves functioned in vivo. In placoderms, the first hinge-jawed fish, oculomotor nerve diameters remained constant, but nerve lengths were ten times longer than in the jawless osteostraci. We infer that to accommodate this ten-fold increase in length, while maintaining a constant diameter, the oculomotor system in placoderms must have been myelinated to function as a rapidly conducting motor pathway. Placoderms were the first fish with hinged jaws and some can grow to formidable lengths, requiring a rapid conduction system, so it is highly likely that they were the first organisms with myelinated axons in the craniate lineage

Reactivity of actinides mono-cations with NH3_3 in gas phase: A study using ICP-MS and quantum chemistry

International audienceOver the last decades, reactions between actinides mono-cations and small molecules in gas phase attracted a great interest. Gas phase reactivity is a simple approach (no matrix effect) to better interpret and understand the role of actinide’s electronic structure and contribution of 5f electrons to their reactivity. Experimental studies using different mass spectrometry techniques revealed differences in actinide reactivity with several gaseous molecules (NH3, O2, CO2, CH4, C2H4 …). An experimental correlation between actinides reactivity and their electronic promotion energies was established [1]. The electronic promotion energy is the energy required for the actinides to have 2 non-f electrons. The lower this energy is, the better the reactivity is. To verify this correlation, an experimental study using reaction cell in Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was achieved with NH3. Th+, Pa+, U+, Np+ and Cm+, whose electronic promotion energies are below 0.5 eV, react completely to form AnNH+ unlike Pu+ and Am+, whose energies are above 1 eV. In addition, from an analytical point of view, these differences in reactivity helps to solve isobaric and polyatomic interferences that complicate the analysis of some isotopes in ICP-MS such as 238U/238Pu, 238U1H/239Pu or 242Pu/242mAm/242Cm.Quantum chemical computations were also performed to characterize reaction mechanisms between actinides mono-cations (Ac+, Th+, Pa+ ,U+, Np+, Pu+, Am+ and Cm+) and NH3. The geometries of the transition states and the thermodynamic properties were determined using Density Functional Theory (DFT) and Coupled Cluster theory (CCSD(T)) . In the case of Ac+, Th+, Pa+, U+, Np+ and Cm+, after formation of the An+--NH3 complex, actinide mono-cation inserts itself in N H bond until H2 is eliminated via two transition states and intermediate species HAnNH2+. Figure 1 shows the reaction scheme for Cm+ with NH3 (a similar mechanism is observed for Ac+, Th+, Pa+,U+ and Np+). For Pu+ and Am+, the limiting step is the formation of the first transition state (Figure 1). This step is only possible if the actinide has two non-f electrons: the electronic promotion energy of Pu+ and Am+ required to obtain this reactive configuration is too high.Comparison of experimental and theoretical studies established a correlation between actinide reactivity and electronic structure. This prove the need for actinide mono cations to have a reactive electronic configuration with two non-f electrons. The higher the electron promotion energy, the more difficult it is to achieve a reactive configuration to cross the first transition state