Fossil Lineages Research Articles

Evolution of left–right asymmetry in side-resting (pleurothetic) bivalves: a test of genes as leaders or followers

ABSTRACT Side-resting (pleurothetic) lifestyles evolved independently many times across the Bivalvia in free-lying, byssally attached and cementing lineages. Within some species individuals lie indifferently on either their right or left side (randomly pleurothetic), whereas in others they lie consistently on one side (left-pleurothetic or right-pleurothetic). Morphological differences between valves (asymmetries) typically accompany this behaviour, so it can be studied widely across living and fossil bivalves. Because direction of asymmetry is rarely inherited in cases of random asymmetry, any evolutionary transition from randomly pleurothetic to fixed (left or right) qualifies as a case of ‘genes as followers’. Origins of left-pleurothetic or right-pleurothetic taxa directly from upright (orthothetic) ancestors would represent examples of ‘genes as leaders’. I therefore surveyed pleurothetic lifestyles across the Bivalvia. By mapping pleurothetic states onto a comprehensive phylogenetic tree that included both living and fossil families, I assessed the prevalence of these alternate modes of evolution. Various lines of evidence were also used to hypothesize phylogenetic relations within the Chamidae—the only living bivalve family exhibiting all three pleurothetic states. Randomly pleurothetic forms (direction not inherited) arose from orthothetic ancestors about half as often as fixed pleurothetic forms (direction is inherited). Unlike many other animal groups, fixed pleurothetic forms rarely arose from randomly pleurothetic ancestors, suggesting a ‘genes as leaders’ mode of evolution was most prevalent. However, many fascinating examples of randomly pleurothetic forms among Upper Ordovician and Silurian bivalves suggest that evolutionary transitions from random to fixed may have been more common. Also, randomly pleurothetic forms were short-lived in some fossil lineages, which would reduce the likelihood of them being detected. Hopefully, this preliminary analysis will motivate further study of (1) phylogenetic relations of key groups like the Chamidae, and basal Limida, Ostreida and Pterioidea, and (2) morphological variation among early members of other pleurothetic bivalve groups.

A new Peirosauridae (Crocodyliformes, Notosuchia) from the Adamantina Formation (Bauru Group, Late Cretaceous), with a revised phylogenetic analysis of Sebecia.

Peirosauridae (Crocodyliformes, Notosuchia) is one of the fossil lineages of crocodyliforms ubiquitous in the Cretaceous deposits of the Bauru Basin. Here, we describe a new species of a longirostrine Peirosauridae from the Adamantina Formation (Bauru Basin, Late Cretaceous). The specimen consists of a partially preserved skull with a cranial roof, interorbital region, and fragments of the posterior portion of the rostrum, including the prefrontal and lacrimal; left hemimandible, with 14 alveoli and 12 teeth; and a single cervical rib fragment. The specimen is associated with Peirosauridae by three cranial synapomorphies, and it can be assigned to a new genus and species by presenting seven cranial and one tooth apomorphies. To clarify the position of the new taxon, an updated phylogenetic analysis was performed with increased sampling of taxa of Notosuchia, especially Peirosauridae, and phylogenetically relevant characters. Our results indicated the monophyly of Peirosauridae, formed by two main lineages, the oreinirostral and presumably terrestrial Peirosaurinae and the longirostrine and presumably semi-aquatic Pepesuchinae. The recovering of both lineages as distinct entities was also reinforced through a morphospace analysis. Pepesuchinae were notable by exploring a position of the morphospace not explored by any other Notosuchia. Their longer rostra and the assumption of them being gradually specialized to aquatic habits reflects the unique diversity of these crocodyliforms through the Cretaceous deposits of South America and Africa.

The genomic signatures of evolutionary stasis.

Evolutionary stasis characterizes lineages that seldom speciate and show little phenotypic change over long stretches of geological time. Although lineages that appear to exhibit evolutionary stasis are often called living fossils, no single mechanism is thought to be responsible for their slow rates of morphological evolution and low species diversity. Some analyses of molecular evolutionary rates in a handful of living fossil lineages have indicated that these clades exhibit slow rates of genomic change. Here, we investigate mechanisms of evolutionary stasis using a dataset of 1,105 exons for 481 vertebrate species. We demonstrate that two ancient clades of ray-finned fishes classically called living fossils, gars and sturgeons, exhibit the lowest rates of molecular substitution in protein-coding genes among all jawed vertebrates. Comparably low rates of evolution are observed at fourfold degenerate sites in gars and sturgeons, implying a mechanism of stasis decoupled from selection that we speculate is linked to a highly effective DNA repair apparatus. We show that two gar species last sharing common ancestry over 100 million years ago produce morphologically intermediate and fertile hybrids in the wild. This makes gars the oldest naturally hybridizing divergence among eukaryotes and supports a theoretical prediction that slow rates of nucleotide substitution across the genome slow the accumulation of genetic incompatibilities, enabling hybridization across deeply divergent lineages and slowing the rate of speciation over geological timescales. Our results help establish molecular stasis as a barrier to speciation and phenotypic innovation and provide a mechanism to explain the low species diversity in living fossil lineages.

Plant periderm as a continuum in structural organisation: a tracheophyte-wide survey and hypotheses on evolution.

Periderm is a well-known structural feature with vital roles in protection of inner plant tissues and wound healing. Despite its importance to plant survival, knowledge of periderm occurrences outside the seed plants is limited and the evolutionary origins of periderm remain poorly explored. Here, we review the current knowledge of the taxonomic distribution of periderm in its two main forms - canonical periderm (periderm formed as a typical ontogenetic stage) and wound periderm (periderm produced as a self-repair mechanism) - with a focus on major plant lineages, living and extinct. We supplement the published occurrences with data based on our own observations and experiments. This updated body of data reveals that the distribution of wound periderm is more widespread taxonomically than previously recognized and some living and extinct groups are capable of producing wound periderm, despite canonical periderm being absent from their normal developmental program. A critical review of canonical and wound periderms in extant and fossil lineages indicates that not all periderms are created equal. Their organisation is widely variable and the differences can be characterised in terms of variations in three structural features: (i) the consistency in orientation of periclinal walls within individual files of periderm cells; (ii) the lateral coordination of periclinal walls between adjacent cell files; and (iii) whether a cambial layer and conspicuous layering of inward and outward derivatives can be distinguished. Using a new system of scoring periderm structure based on these criteria, we characterise the level of organisation of canonical and wound periderms in different lineages. Looking at periderms through the lens provided by their level of organisation reveals that the traditional image of periderm as a single generalised feature, is best viewed as a continuum of structural configurations that are all predicated by the same basic process (periclinal divisions), but can fall anywhere between very loosely organized (diffuse periclinal growth) to very tightly coordinated (organized periclinal growth). Overall, wound periderms in both seed plants and seed-free plants have lower degrees of organisation than canonical periderms, which may be due to their initiation in response to inherently disruptive traumatic events. Wound and canonical periderms of seed plants have higher degrees of organisation than those of seed-free plants, possibly due to co-option of the programs responsible for organizing their vascular cambial growth. Given the importance of wound periderm to plant survival, its widespread taxonomic distribution, and its early occurrence in the fossil record, we hypothesise that wound periderm may have had a single origin in euphyllophytes and canonical periderm may have originated separately in different lineages by co-option of the basic regulatory toolkit of wound periderm formation. In one evolutionary scenario, wound periderm regulators activated initially by tissue tearing due to tensional stresses elicited by woody growth underwent heterochronic change that switched their activation trigger from tissue tearing to the tensional stresses that precede it, with corresponding changes in the signalling that triggered the regulatory cascade of periderm development from tearing-induced signals to signalling induced by tension in cells.

Sorting of persistent morphological polymorphisms links paleobiological pattern to population process

AbstractBiological variation fuels evolutionary change. Across longer timescales, however, polymorphisms at both the genomic and phenotypic levels often persist longer than would be expected under standard population genetic models such as positive selection or genetic drift. Explaining the maintenance of this variation within populations across long time spans via balancing selection has been a major triumph of theoretical population genetics and ecology. Although persistent polymorphisms can often be traced in fossil lineages over long periods through the rock record, paleobiology has had little to say about either the long-term maintenance of phenotypic variation or its macroevolutionary consequences. I explore the dynamics that occur when persistent polymorphisms maintained over long lineage durations are filtered into descendant lineages during periods of demographic upheaval that occur at speciation. I evaluate these patterns in two lineages: Ectocion, a genus of Eocene mammals, and botryocrinids, a Mississippian cladid crinoid family. Following origination, descendants are less variable than their ancestors. The patterns by which ancestral variation is sorted cannot be distinguished from drift. Maintained and accumulated polymorphisms in highly variable ancestral lineages such as Barycrinus rhombiferus Owen and Shumard, 1852 may fuel radiations as character states are sorted into multiple descendant lineages. Interrogating the conditions under which trans-specific polymorphism is either maintained or lost during periods of demographic and ecological upheaval can explain how population-level processes contribute to the emergent macroevolutionary dynamics that shape the history of life as preserved in the fossil record.

An Inconvenient Apocalpse, Environmental Collapse, Climate Crisis, and the Fate of Humanity

The evolutionary history of humans, known by the scientific name Homo sapiens (wise man), is the last living representative of a long fossil lineage of human-like populations. This book explores the wisdom of the current human populations, whose eight billion plus population growth and spread dominates present-day earth, and the possibility that these sheer numbers if left unchecked without the right choices, may sooner or later, destroy planet earth and perhaps go extinct. This book explores the questions posing human survival. Therefore, if you care about the survival of the most recent human species, you must read this book and consider the options that confront the future of our existence. Many other life forms that make up living components of ecosystems have gone extinct, and/or have merged with other extant species, so will human evolution follow the same time course?

A new fossil family of aculeate wasp sheds light on early evolution of Apoidea (Hymenoptera)

AbstractIn the present work, we describe a new fossil family of Apoidea supported by phylogenetic analyses involving both fossil and extant groups. †Burmasphecidae fam.n. is based on †Burmasphex Melo & Rosa, a genus described from Burmese amber. We include in this family the monotypic genus †Decasphex Zheng, Zhang & Rasnitsyn also from Burmese amber. Additionally, we describe two new genera and four new species in †Burmasphecidae fam.n.: †Burmasphex mirabilis sp.n.; †Simplisphex gen.n., containing S. scutellatus sp.n. and †S. burmensis sp.n.; and †Callisphex robustus gen. et sp.n. In our phylogenetic study, we extended a pre‐existing matrix of morphological data and analysed it under parsimony and Bayesian inference. In the Bayesian inference analyses, the morphological dataset was partitioned under a homoplasy criterion. We postulate the first phylogenetic hypotheses for the placement of †Angarosphecidae based on the type species, †Angarosphex myrmicopterus Rasnitsyn, plus a new Burmese amber taxon, †A. alethes sp.n. We demonstrate that †Burmasphecidae fam.n. clearly belongs to Apoidea and has a sister relationship with the other representatives of the superfamily. Our results indicate that †Burmasphecidae fam.n. and †Angarosphecidae are distinct lineages, with the second clearly more derived than the first. We discuss the phylogenetic relationships of these fossil lineages with extant groups of both Apoidea and other Aculeata, and present morphological evidence for the first time supporting the Formicidae + Apoidea clade. Finally, we indicate some considerations about the paleoenvironment and the nature of the Burmese amber biota, suggesting an alternative hypothesis to the island endemism described in previous works.

Phoronida-A small clade with a big role in understanding the evolution of lophophorates.

Phoronids, together with brachiopods and bryozoans, form the animal clade Lophophorata. Modern lophophorates are quite diverse-some can biomineralize while others are soft-bodied, they could be either solitary or colonial, and they develop through various eccentric larval stages that undergo different types of metamorphoses. The diversity of this clade is further enriched by numerous extinct fossil lineages with their own distinct body plans and life histories. In this review, I discuss how data on phoronid development, genetics, and morphology can inform our understanding of lophophorate evolution. The actinotrocha larvae of phoronids is a well documented example of intercalation of the new larval body plan, which can be used to study how new life stages emerge in animals with biphasic life cycle. The genomic and embryonic data from phoronids, in concert with studies of the fossil lophophorates, allow the more precise reconstruction of the evolution of lophophorate biomineralization. Finally, the regenerative and asexual abilities of phoronids can shed new light on the evolution of coloniality in lophophorates. As evident from those examples, Phoronida occupies a central role in the discussion of the evolution of lophophorate body plans and life histories.

The biogeography of extant lungfishes traces the breakup of Gondwana

AbstractAimLungfishes are one of the two surviving clades from the once diverse grade of lobe‐finned fishes leading to tetrapods. This classic living fossil lineage, which is the living sister to four‐limbed terrestrial vertebrates, appeared approximately 425 million years ago and rapidly diversified. However, the evolution of lungfishes after their initial radiation is poorly understood, and whether their present distribution tracks ancient geographical change is a classic problem in biogeography.LocationGlobal.TaxonLungfishes (Dipnoi).MethodsHere, we combine mitogenomic, nuclear gene and fossil data to reconstruct the timing of lungfish diversification a Bayesian tip‐dating approach to quantitatively test hypotheses of lungfish historical biogeography and divergence times. We sample all major living and extinct lungfish lineages, including three of the four species of African lungfishes (Protopterusspp.), the Australian lungfishNeoceratodus forsteri, the South American lungfishLepidosiren paradoxaand 13 fossils representing extinct lineages from across the globe.ResultsOur results demonstrate that the divergences of the three major living lungfish clades closely recapitulate the stepwise fragmentation of the Gondwana during the Mesozoic. All of our model‐based biogeographical reconstructions support a Gondwanan vicariance model for the origins of the present distribution of lungfish lineages.ConclusionsIn turn, lungfishes provide an excellent example of how the integration of fossil data may drastically change support for historical biogeographical hypotheses previously discounted by molecular data and are one of the few living animal lineages that record incredibly ancient geographical changes in their phylogeny.

The oldest case of paedomorphosis in rove beetles and description of a new genus of Paederinae from Cretaceous amber (Coleoptera: Staphylinidae)

The ecology of extinct species from the Cretaceous is largely unknown. Morphological features of specimens preserved in amber can help to reveal habitats and evolutionary strategies that occurred in fossil lineages. An unusually small rove beetle (Staphylinidae) from the subfamily Paederinae with a Y-shaped suture on the head and modified tarsi and antennae is newly described here as Midinudon juvenis Tokareva & Żyła gen. et sp. nov. We hypothesise that such a combination of characters represents the earliest example of paedomorphosis in Staphylinidae and discuss other possible reasons that could explain the small size and morphological modifications of the new species. We provide the results of total-evidence phylogenetic analysis and discuss the relationships of Midinudon juvenis Tokareva & Żyła gen. et sp. nov. within Paederinae.

Scorpions trapped in amber: a remarkable window on their evolution over time from the Mesozoic period to present days.

This synoptic review aims to bring some general information on fossil scorpions, namely those trapped in amber - fossilized resin - ranging from Lower Cretaceous through the Palaeocene and up to the Miocene. The question to be addressed is how the study of these fossils can be connected with possible present scorpionism problems. A precise knowledge of these ancient lineages provides information about the evolution of extant lineages, including the buthoids, which contain most known noxious species. Among the Arthropods found trapped in amber, scorpions are considered rare. A limited number of elements have been described from the Late Tertiary Dominican and Mexican amber, while the most ancient Tertiary amber from the Baltic region produced more consistent results in the last 30 years, primarily focusing on a single limited lineage. Contrarily, the Cretaceous amber from Myanmar, also called Burmite, has yielded and continues to yield a significant number of results represented by several distinct lineages, which attest to the considerable degree of diversity that existed in the Burmese amber-producing forests. As in my previous similar contributions to this journal, the content of this note is primarily addressed to non-specialists whose research embraces scorpions in various fields such as venom toxins and public health. An overview knowledge of at least some fossil lineages can eventually help to clarify why some extant elements associated with the buthoids represent dangerous species while others are not noxious.

Origin and evolution of Heterocephalus from East Africa (Rodentia: Heterocephalidae)

Known today by a single species only in the Horn of Africa, rodents of the Heterocephalus genus displayed through Neogene times a larger geographical distribution and a higher diversity. Here we present the state of art of the knowledge of the described fossil Heterocephalus species and follow the different described lineages through time and space. We also present some morphological and classical morphometric analyses upon the skulls and teeth of the modern representative of the genus compared with the fossil ones. This allowed to find some cranio-dental criteria allowing to validate the probable presence of two to three modern species in the horn of Africa and define and discuss some dental trends of evolution of the fossil lineages through time and space. Finally, by gathering the paleoclimatic conditions and collecting environmental parameters of each site were Heterocephalus remains were collected we propose a scenario of evolution of the genus.

Phylogenomic analysis of the bowfin (Amia calva) reveals unrecognized species diversity in a living fossil lineage

The Bowfin (Amia calva), as currently recognized, represents the sole living member of the family Amiidae, which dates back to approximately 150 Ma. Prior to 1896, 13 species of extant Bowfins had been described, but these were all placed into a single species with no rationale or analysis given. This situation has persisted until the present day, with little attention given to re-evaluation of those previously described nominal forms. Here, we present a phylogenomic analysis based on over 21,000 single nucleotide polymorphisms (SNPs) from 94 individuals that unambiguously demonstrates the presence of at least two independent evolutionary lineages within extant Amia populations that merit species-level standing, as well as the possibility of two more. These findings not only expand the recognizable species diversity in an iconic, ancient lineage, but also demonstrate the utility of such methods in addressing previously intractable questions of molecular systematics and phylogeography in slowly evolving groups of ancient fishes.

Gondwanan survivor lineages and the high‐risk biogeography of Anthropocene Southeast Asia

AbstractThe Southeast Asian rainforest region is extremely complex and biodiverse. Fossils have shown that paleo‐Antarctic rainforest lineages (PARLs) now extant in Asia tracked the ever‐wet conditions needed to survive and diversify through deep time. However, the threat of future climate change to the remaining rainforest and PARLs in Southeast Asia has yet to be evaluated to set conservation priorities. We first quantified the woody‐genus floristic relationships of Southeast Asian Island Groups by vetting and analyzing recent compilations of bioregional species data. We then evaluated the contributions to community assembly of woody fossil lineages and Island Group relationships to environmental gradients. To better understand climatic constraints of fossil lineage distributions and forecast distributions under projected future climate, we used exemplar living woody PARLs, including two angiosperms and two gymnosperms. Generalized linear models were used to project potential distributions under future climate pathways that assume no reduction in carbon dioxide emissions. The floristic analyses highlighted strong similarities among Island Groups in the ever‐wet forest areas of Malesia, where PARLs are often concentrated. Ordination outliers represented more seasonal locations. Species distribution models showed that potential future distributions of ancient lineages are constrained by increasing rainfall seasonality and higher seasonal temperatures, with significant differences among exemplar genera. Notably, potential distributions often mapped onto de facto inaccessible areas, where forest clearing and the ubiquitous marine dispersal barriers that characterize the region will drastically inhibit potential relocation. These realities gravely threaten paleo‐conservation values and contemporary rainforest community assembly processes in Southeast Asia.

The pre-eminent role of directional selection in generating extreme morphological change in glyptodonts (Cingulata; Xenarthra).

The prevalence of stasis on macroevolution has been classically taken as evidence of the strong role of stabilizing selection in constraining morphological change. Rates of evolution calculated over longer timescales tend to fall below the expected under genetic drift, suggesting that directional selection signals are erased at longer timescales. Here, we investigated the rates of morphological evolution of the skull in a fossil lineage that underwent extreme morphological modification, the glyptodonts. Contrary to what was expected, we show here that directional selection was the primary process during the evolution of glyptodonts. Furthermore, the reconstruction of selection patterns shows that traits selected to generate a glyptodont morphology are markedly different from those operating on extant armadillos. Changes in both direction and magnitude of selection are probably tied to glyptodonts' invasion of a specialist-herbivore adaptive zone. These results suggest that directional selection might have played a more critical role in the evolution of extreme morphologies than previously imagined.

The Asian lotus (<i>Nelumbo nucifera</i>) pan-plastome: diversity and divergence in a living fossil grown for seed, rhizome, and aesthetics

The Asian lotus (<i>Nelumbo nucifera</i>) has a history of cultivation in Asia dating back over 3,000 years where it has been an important food crop producing edible rhizomes and seeds as well as flowers of great aesthetic and cultural value. Here, we de novo assembled the plastomes of 316 lotus accessions including five North American lotus (<i>N. lutea</i>) and 311 Asian lotus (<i>N. nucifera</i>) to construct a pan-plastome genome map, and investigate the phylogeography and genetic diversity among the only two extant species within this living fossil lineage. A total of 113 unique genes were annotated and plastome sizes varied between 163,457 and 163,672 bp with only minor differences in each of the four major genomic units. The most abundant nucleotide differences among plastomes were single nucleotide variants followed by insertions/deletions and block substitutions mainly found in intergenic spacer regions of the large single copy portion of the plastome. Seven well-supported genetic clusters were resolved using multiple different population structure analyses. The different lotus types (flower, seed, rhizome, or wild) were disproportionally assigned to multiple different genetic clusters. This pattern indicates that the domestication of Asian lotus involved multiple genetic origins and possible matrilineal introgression. Geographic mapping of accessions also revealed that genetic diversity is unevenly distributed with eastern China possessing the highest genetic diversity and regions such as Yunnan, Indonesian, and Thailand possessing unique haplotypes. These results provide an important maternal history of <i>Nelumbo</i> and necessary groundwork for future studies on intergenomic gene transfer, cytonuclear incompatibility, and conservation genetics.

Tooth and long bone scaling in Sardinian ochotonids (Early Pleistocene-Holocene): Evidence for megalodontia and its palaeoecological implications

Body size is a useful character to unravel the biology of fossil taxa and, indirectly, the palaeoenvironment in which they lived. However, the reliability of size proxies is debated, particularly among insular endemics in which large teeth relative to body size have been observed. To shed light on this issue, here we compute statistical models to assess: (i) teeth and long bone scaling and (ii) dentition size evolutionary dynamics in the anagenetic lineage Prolagus figaro-P. sardus from the late Early Pleistocene-Holocene of Sardinia, Italy. Postcranial bones, particularly tibial epiphyses, are shown to be the most suitable parameters for prediction of body mass in Prolagus and other lagomorph taxa. By contrast, p3 should not be used for size reconstructions, whereas m1 width provided the highest correlation with long bones. On the other hand, the postcanine occlusal surface of Sardinian Prolagus (m1 and toothrow area) appeared significantly larger than those of extant lagomorphs, pointing to a higher resistance to tooth wear and a more durable permanent dentition. It seems likely that megalodontia might have evolved in Sardinian Prolagus to cope with the abrasive diet found on islands (food habit hypothesis). Nonetheless, we cannot exclude that other biological features, such as long longevity, could be co-driving forces in this fossil lineage (life history hypothesis). The results achieved throw light on size and scale evolution in lagomorphs, as well as refine the eco-evolutionary responses of small mammals to insular regimes.

The Fossil Record of Elateridae (Coleoptera: Elateroidea): Described Species, Current Problems and Future Prospects.

Simple SummaryBeetle fossils play an important role in our understanding of the origin and evolutionary history of this insect order. Despite the recently increasing rate of fossil research focused on the click-beetles (Coleoptera: Elateridae), the major group in the superfamily Elateroidea, their palaeodiversity has still remained largely understudied. In this study, we summarized current knowledge on the click-beetle fossil record with a main emphasis on the described diversity. We compiled an annotated catalogue of all described fossil species in Elateridae, assessed the reliability of their systematic placement, and discuss the current state of knowledge and prospects of research of the fossil record in the group. This study should serve as a comprehensive basis for all subsequent research dealing with the origin, early evolution and diversity of Elateridae.The Elateridae (click-beetles) are the largest family in Elateroidea; however, their relationships, systematics and classification remain unclear. Our understanding of the origin, evolution, palaeodiversity and palaeobiogeography of Elateridae, as well as reconstruction of a reliable time-calibrated phylogeny for the group, are hampered by the lack of detailed knowledge of their fossil record. In this study, we summarize the current knowledge on all described fossil species in Elateridae, including their type material, geographic origin, age, bibliography and remarks on their systematic placement. Altogether, 261 fossil species classified in 99 genera and nine subfamilies are currently listed in this family. The Mesozoic click-beetle diversity includes 143 species, with most of them described from the Jurassic Karatau, and 118 described species are known from the Cenozoic deposits, mainly from the Eocene North American Florissant Formation and European Baltic amber. Available data on the described past diversity of Elateridae suggest that almost all fossil lineages in this group are in urgent need of revision and numerous Mesozoic species might belong to different families. Our study is intended to serve as a comprehensive basis for all subsequent research focused on the click-beetle fossil record.

Ontogeny of a sexually selected structure in an extant archosaur Gavialis gangeticus (Pseudosuchia: Crocodylia) with implications for sexual dimorphism in dinosaurs.

Despite strong evidence for sexual selection in various display traits and other exaggerated structures in large extinct reptiles, such as dinosaurs, detecting sexual dimorphism in them remains difficult. Their relatively small sample sizes, long growth periods, and difficulties distinguishing the sexes of fossil specimens mean that there are little compelling data on dimorphism in these animals. The extant gharial (Gavialis gangeticus) is a large and endangered crocodylian that is sexually dimorphic in size, but males also possesses a sexually selected structure, the ghara, which has an osteological correlate in the presence of a fossa associated with the nares. This makes the species a unique model for potentially assessing dimorphism in fossil lineages, such as dinosaurs and pterosaurs, because it is a large, slow-growing, egg-laying archosaur. Here we assess the dimorphism of G. gangeticus across 106 specimens and show that the presence of a narial fossa diagnoses adult male gharials. Males are larger than females, but the level of size dimorphism, and that of other cranial features, is low and difficult to detect without a priori knowledge of the sexes, even with this large dataset. By extension, dimorphism in extinct reptiles is very difficult to detect in the absence of sex specific characters, such as the narial fossa.

Phylogenetic constraint and phenotypic plasticity in the shell microstructure of vent and seep pectinodontid limpets

Pectinodontid limpets of the genus Bathyacmaea are endemic to hot vents and cold seeps and exhibit greatly variable shell and radular macro-morphologies, rendering reliable species-level identification challenging. Here, we analyzed shell microstructures of western Pacific vent/seep Bathyacmaea limpets using scanning electron microscopy and Raman spectrophotometry to test its usefulness in providing phylogenetic signals. Bathyacmaea shells comprised of two forms of calcitic microstructure including irregular spherulitic prismatic type-A (ISP type-A) and semi-foliated (SF), as well as the aragonitic crossed lamellar (CL) microstructure. Despite marked differences in macroscopic shell morphologies once leading them to be classified into different species or even genera, six morphotypes of Bathyacmaea nipponica from different chemosynthetic localities and substrates shared an outermost ISP-A layer and alternating layers of SF and CL structures in their outer and inner shell layers. A genetically divergent lineage recovered from the South Chamorro Seamount, however, differed in having a simple three-layered shell composition consisting of ISP-A, SF, and CL structures, in that order, from the outside, and an unusually thin inner shell layer consisting of only CL structure. Moreover, the ratio of aragonite and calcite varied with habitat conditions, with calcite dominating in vents and aragonite dominating in seeps. These results suggest that the shell microstructure of pectinodontids is under phylogenetic constraints and provides useful taxonomic signals, while the mineral polymorphism in aragonite/calcite ratio varies according to environmental factors. Furthermore, microstructures of two ‘species’ from Cretaceous seeps confirmed the same patterns in fossil lineages.