Abstract Palaeontological data are key elements for inferring ancestral character states and the assembly of character complexes, but cephalopod fossils preserving soft tissues are very rare. The exceptionally well‐preserved, unique specimen of Jurassic Proteroctopus ribeti Fischer & Riou from the Lagerstätte of La‐Voulte‐sur‐Rhône ( c . 165 Ma, France) is one of the few fossil octopod related taxa, but is rarely considered in evolutionary studies. In this paper, we used synchrotron microtomography to reappraise its external characters and for the first time, to reveal its internal structures. A unique character association is found with two fins, head fused to the body, eight well‐developed arms with cirri and two rows of oblique suckers, a gladius and absence of an ink sac. The phylogenetic analysis indicates that Proteroctopus is a basal member of the Vampyropoda. However, this result should be interpreted with caution due to the number of unknown character states in the matrix. Contrary to previous assumptions, the phylogenetic position of Proteroctopus , as well as its stratigraphic occurrence, suggest that the arrangement of biserial suckers may be the ancestral condition in Vampyropoda.
Iterative segments such as teeth or limbs are a widespread characteristic of living organisms. While their proportions may be governed by similar developmental rules in vertebrates, there is no emerging pattern as regards their relation to size. Placental mammals span eight orders of magnitude in body size and show a wide spectrum of dietary habits associated with size and reflected in their dentitions, especially molars. Although variation in size constitutes an important determinant for variation in biological traits, few major allometric trends have been documented on placental molars so far. Molar proportions have been intensively explored in placentals in relation to developmental models, but often at a small phylogenetic scale. Here, we analyzed the diversity of upper molar proportions in relation to absolute size in a large sample of placental species ($n = 299$) encompassing most of the group's dental diversity. Our phylogenetically informed analyses revealed a 2-fold pattern of evolutionary integration among upper molars: while molars covary in size with each other, their proportions covary with the absolute size of the entire molar field. With increasing absolute size, posterior molars increase in size relative to anterior ones, meaning that large-sized species have relatively large rear molars while the opposite is true for small-sized species. The directionality of proportional increase in the molar row exhibits a previously unsuspected allometric patterning among placentals, showing how large-scale variations in size may have influenced variation in dental morphology. This finding provides new evidence that processes regulating the size of individual molars are integrated with overall patterns of growth and calls for further testing of allometric variation in the dentition and in other segmental series of the vertebrate body. [Dentition; evolution; model; phylogeny; segmentation; size.].
Abstract Since the introduction of the cladistic method in systematics, continuous characters have been integrated into analyses but no methods for their treatment have received unanimous support. Some methods require a large number of character states to discretise continuous characters in order to keep the maximum level of information about taxa differences within the coding scheme. Our objective was to assess the impact of increasing the character state number on the outcomes of phylogenetic analyses. Analysis of a variety of simulated datasets shows that these methods for coding continuous characters can lead to the generation of well‐resolved trees that do not reflect a phylogenetic signal. We call this phenomenon the flattening of the tree‐length distribution; it is influenced by both the relative quantity of continuous characters in relation to discrete characters, and the number of characters in relation to the number of taxa. Bootstrap tests provide a method to avoid this potential bias.
Abstract Reptiles represent one of the most diverse groups of tetrapod vertebrates. Extant representatives of reptiles include lepidosaurs (lizards), testudines (turtles) and archosaurs (crocodiles and birds). In particular, they show an important locomotor diversity with bipedal, quadrupedal and facultatively bipedal taxa. This diversity is accompanied by substantial microanatomical disparity in the limb bones. Although many studies have highlighted the link between locomotion and bone microstructure, the latter has never been quantitatively studied from an angular perspective. Indeed, some taxa show microanatomical heterogeneity in cross-section. Here we show, using elliptic Fourier transforms and statistical analyses integrating phylogeny, how angular microanatomical parameters measured on reptilian femoral cross-sections, such as angular bone compactness, can be related to locomotion in this clade. Although phylogeny appears to have a significant impact on our results, we show that a functional signal exists. In particular, we show that bipeds and quadrupeds present a craniolateral-caudomedial and dorsoventral deficit in bone compactness, respectively. This reflects cross-sectional eccentricity in these directions that we relate to the forces acting upon the femur in different postural contexts. This work contributes to deciphering the complex interplay between phylogeny, femoral cross-sectional microanatomy and locomotion in reptiles.
Ammonite phylogeny has mainly been established based on a stratigraphic approach, with cladistics underconsidered. The main arguments against the use of cladistics are the supposed large amount of homoplasy and the small number of characters. Resolving the phylogeny of the Hildoceratidae (Early Jurassic) is especially challenging because of its large diversity and disparity. Many forms that have not been determined as closely related in previous studies exhibit very similar shapes. Moreover, some groups are morphologically very different, adding difficulties to building a unique coding scheme at a low taxonomic resolution (i.e. species). Here we propose an integrated coding scheme of the peristome shape and the ornamentation, allowing an increased level of comparison. The shape of the peristome is used as a new reference to locate ornamental features and propose new homology hypotheses. In total, 105 taxa have been analysed for 47 characters. We code continuous characters by their means and ranges ± one standard deviation. We test two weighting schemes: equal weights standardized by unit range and implied weighting with several concavity constants. This work has led to redefinition of the phylogenetic inclusivenesses of all the hildoceratid subfamilies. The new coding scheme based on peristome shapes provides the fewest homoplastic characters. The schemes appear promising to improve phylogenetic analyses in ammonoids as well as molluscs as a whole by creating a general coding framework.
Abstract Ammonite soft body remains are rarely preserved. One of the biggest enigmas is the morphology of the ammonite brachial crown that has, up till now, never been recovered. Recently, mysterious hook-like structures have been reported in multiple specimens of Scaphitidae, a large family of heteromorph Late Cretaceous ammonites. A previous examination of these structures revealed that they belong to the ammonites. Their nature, however, remained elusive. Here, we exploit tomographic data to study their arrangement in space in order to clarify this matter. After using topological data analyses and comparing their morphology, number, and distribution to other known cephalopod structures, in both extant and extinct taxa, we conclude that these hook-like structures represent part of the brachial crown armature. Therefore, it appears that there are at least three independent evolutionary origins of hooks: in belemnoids, oegospids, and now in ammonites. Finally, we propose for the first time a hypothetical reconstruction of an ammonite brachial crown.
Abstract The practice of species delimitation using molecular data commonly leads to the revealing of species complexes and an increase in the number of delimited species. In a few instances, however, DNA-based taxonomy has led to lumping together of previously described species. Here, we delimit species in the genus Cryptogemma (Gastropoda: Conoidea: Turridae), a group of deep-sea snails with a wide geographical distribution, primarily by using the mitochondrial COI gene. Three approaches of species delimitation (ABGD, mPTP and GMYC) were applied to define species partitions. All approaches resulted in eight species. According to previous taxonomic studies and shell morphology, 23 available names potentially apply to the eight Cryptogemma species that were recognized herein. Shell morphometrics, radular characters and geographical and bathymetric distributions were used to link type specimens to these delimited species. In all, 23 of these available names are here attributed to seven species, resulting in 16 synonymizations, and one species is described as new: Cryptogemma powelli sp. nov. We discuss the possible reasons underlying the apparent overdescription of species within Cryptogemma, which is shown here to constitute a rare case of DNA-based species lumping in the hyper-diversified superfamily Conoidea.
A rich ammonite fauna from the Lower Toarcian (Lower Jurassic) of the South Riffian ridges (northern Morocco), collected bed-by-bed from 14 new outcrops, is described. The fauna is nearly exclusively composed of already-known taxa, allowing correlation from the chronozone down to the zonule level. Fourteen species are described, among them the new species Dactylioceras (Dactylioceras) laticostatum sp. nov. The stratigraphical interval studied spans the first chronozone of the Toarcian, the Polymorphum Chronozone, but it remains unclear whether the Toarcian Oceanic Anoxic Event is represented in the studied succession. The Early Toarcian is mainly characterized by representatives of the family Dactylioceratidae, which are abundant but commonly crushed or deformed. Two new descriptors are proposed to describe the shell shape of crushed or incomplete specimens and have been used together with the classical ones for the taxonomic treatment of this group. A hypothesis about dimorphism in Dactylioceras (Orthodactylites) semicelatum (Simpson, 1843) and Dactylioceras (Dactylioceras) laticostatum sp. nov. is proposed.http://zoobank.org/urn:lsid:zoobank.org:pub:urn:lsid:zoobank.org:pub:E7934E4A-942C-4C73-9831-22A8C0B0B89A
During the Paleogene, Earth's climate experienced drastic changes, drifting from extreme warmth to cold with the development of the Antarctic polar ice cap. Geochemical records of foraminifera are commonly used as proxies to reconstruct past ocean temperatures. Inconsistencies remain however, warranting the search for new tools to corroborate existing reconstructions. Here we present, three continuous sea subsurface temperature reconstructions from different latitudes and ranging from -58 to -32 Myr, using a multi-proxy approach (δ18O, combined Mg/Ca and Sr/Ca data, and inner shell porosity) applied to the planktonic foraminifera species Subbotina linaperta and its ancestor Subbotina patagonica. Two proxies used in this study (combined Mg/Ca and Sr/Ca data, and inner shell porosity) are based on recent developments and are, for the first time, applied to the fossil record. Elemental derived temperatures (Elemental-T) are estimated using a recent published calibration incorporating the Sr/Ca within the Mg/Ca-T equation. In parallel, the inner test porosity (Porosity-T) appears to be a good alternative when foraminifera shells are too obliterated for geochemical proxies. Correlations between porosity and temperature were established, using the temperature reconstructions based on geochemical proxies (δ18O, Elemental-T). In return, porosity allowed the estimation of sea subsurface temperatures of the equatorial site where the geochemical record could not be trusted due to bad preservation of the shells. Contrarily to the global deep-sea cooling recorded by the oxygen isotope composition of benthic foraminifera, our multi-proxy approach highlights different climatic stories depending on the latitudinal context. At high latitudes of the South Atlantic Ocean, sea subsurface temperatures evidence different warming episodes from Late Paleocene to Middle Eocene before a main cooling and stabilization during the Middle Eocene. On the other hemisphere, sea subsurface temperatures of the North Atlantic remain relatively stable from the PETM to the Late Eocene. At the equatorial Pacific, sea subsurface temperatures stay warm over the whole interval. For the first time, our study highlights a latitudinal sea subsurface temperature gradient for both hemispheres, weaker than what observed today. From the Late Paleocene towards the Middle Eocene the gradients are quite unstable. From -47 Myr, the sea subsurface temperature gradient becomes progressively stronger in the Southern hemisphere compared to the Northern hemisphere, describing the establishment of a constant thermal difference between both hemispheres.
Abstract The lifestyle of extinct tetrapods is often difficult to assess when clear morphological adaptations such as swimming paddles are absent. According to the hypothesis of bone functional adaptation, the architecture of trabecular bone adapts sensitively to physiological loadings. Previous studies have already shown a clear relation between trabecular architecture and locomotor behavior, mainly in mammals and birds. However, a link between trabecular architecture and lifestyle has rarely been examined. Here, we analyzed trabecular architecture of different clades of reptiles characterized by a wide range of lifestyles (aquatic, amphibious, generalist terrestrial, fossorial, and climbing). Humeri of squamates, turtles, and crocodylians have been scanned with microcomputed tomography. We selected spherical volumes of interest centered in the proximal metaphyses and measured trabecular spacing, thickness and number, degree of anisotropy, average branch length, bone volume fraction, bone surface density, and connectivity density. Only bone volume fraction showed a significant phylogenetic signal and its significant difference between squamates and other reptiles could be linked to their physiologies. We found negative allometric relationships for trabecular thickness and spacing, positive allometries for connectivity density and trabecular number and no dependence with size for degree of anisotropy and bone volume fraction. The different lifestyles are well separated in the morphological space using linear discriminant analyses, but a cross‐validation procedure indicated a limited predictive ability of the model. The trabecular bone anisotropy has shown a gradient in turtles and in squamates: higher values in amphibious than terrestrial taxa. These allometric scalings, previously emphasized in mammals and birds, seem to be valid for all amniotes. Discriminant analysis has offered, to some extent, a distinction of lifestyles, which however remains difficult to strictly discriminate. Trabecular architecture seems to be a promising tool to infer lifestyle of extinct tetrapods, especially those involved in the terrestrialization.
