A Late Devonian coelacanth reconfigures actinistian phylogeny, disparity, and evolutionary dynamics.

The living coelacanth Latimeria (Sarcopterygii: Actinistia) is an iconic, so-called 'living fossil' within one of the most apparently morphologically conservative vertebrate groups. We describe a new, 3-D preserved coelacanth from the Late Devonian Gogo Formation in Western Australia. We assemble a comprehensive analysis of the group to assess the phylogeny, evolutionary rates, and morphological disparity of all coelacanths. We reveal a major shift in morphological disparity between Devonian and post-Devonian coelacanths. The newly described fossil fish fills a critical transitional stage in coelacanth disparity and evolution. Since the mid-Cretaceous, discrete character changes (representing major morphological innovations) have essentially ceased, while meristic and continuous characters have continued to evolve within coelacanths. Considering a range of putative environmental drivers, tectonic activity best explains variation in the rates of coelacanth evolution.

Histological observations and transcriptome analyses reveal the dynamic changes in the gonads of the blotched snakehead (Channa maculata) during sex differentiation and gametogenesis.

BACKGROUND: Blotched snakehead (Channa maculata) displays significant sexual dimorphism, with males exhibiting faster growth rates and larger body sizes compared to females. The cultivation of the all-male population of snakeheads holds substantial economic and ecological value. Nonetheless, the intricate processes governing the development of bipotential gonads into either testis or ovary in C. maculata remain inadequately elucidated. Therefore, it is necessary to determine the critical time window of sex differentiation in C. maculata, providing a theoretical basis for sex control in production practices. METHODS: The body length and weight of male and female C. maculata were measured at different developmental stages to reveal when sexual dimorphism in growth initially appears. Histological observations and spatiotemporal comparative transcriptome analyses were performed on ovaries and testes across various developmental stages to determine the crucial time windows for sex differentiation in each sex and the sex-related genes. Additionally, qPCR and MG2C were utilized to validate and locate sex-related genes, and levels of E2 and T were quantified to understand sex steroid synthesis. RESULTS: Sexual dimorphism in growth became evident starting from 90 dpf. Histological observations revealed that morphological sex differentiation in females and males occurred between 20 and 25 dpf or earlier and 30-35 dpf or earlier, respectively, corresponding to the appearance of the ovarian cavity or efferent duct anlage. Transcriptome analyses revealed divergent gene expression patterns in testes and ovaries after 30 dpf. The periods of 40-60 dpf and 60-90 dpf marked the initiation of molecular sex differentiation in females and males, respectively. Male-biased genes (Sox11a, Dmrt1, Amh, Amhr2, Gsdf, Ar, Cyp17a2) likely play crucial roles in male sex differentiation and spermatogenesis, while female-biased genes (Foxl2, Cyp19a1a, Bmp15, Figla, Er) could be pivotal in ovarian differentiation and development. Numerous biological pathways linked to sex differentiation and gametogenesis were also identified. Additionally, E2 and T exhibited sexual dimorphism during sex differentiation and gonadal development. Based on these results, it is hypothesized that in C. maculata, the potential male sex differentiation pathway, Sox11a-Dmrt1-Sox9b, activates downstream sex-related genes (Amh, Amhr2, Gsdf, Ar, Cyp17a2) for testicular development, while the antagonistic pathway, Foxl2/Cyp19a1a, activates downstream sex-related genes (Bmp15, Figla, Er) for ovarian development. CONCLUSIONS: This study provides a comprehensive overview of gonadal dynamic changes during sex differentiation and gametogenesis in C. maculata, establishing a scientific foundation for sex control in this species.

Blotched snakehead (Channa maculata) exhibits significant sexual dimorphism, as males display faster growth rates and larger body sizes compared to females. The cultivation of the all-male population of snakeheads holds substantial economic and ecological value. However, the mechanisms underlying sex determination and differentiation in C. maculata remain insufficiently elucidated. In this study, sexual dimorphism in growth became evident starting from 90 dpf through the measurement of body length and weight of male and female C. maculata at different developmental stages. Histological observations indicated that morphological sex differentiation in females and males occurred at 20­25 dpf or earlier and 30­35 dpf or earlier, respectively, corresponding to the appearance of the ovarian cavity or efferent duct anlage. Transcriptome analyses revealed divergent gene expression patterns in male and female gonads after 30 dpf, suggesting that the period preceding 30 dpf might be the critical time window for sex control in C. maculata. The periods of 40­60 dpf and 60­90 dpf marked the initiation of molecular sex differentiation in females and males, respectively. Male-biased genes (Sox11a, Dmrt1, Amh, Amhr2, Gsdf, Ar, Cyp17a2) likely play crucial roles in testicular differentiation and spermatogenesis, while female-biased genes (Foxl2, Cyp19a1a, Bmp15, Figla, Er) could be pivotal in ovarian differentiation and oogenesis. Additionally, numerous biological pathways linked to sex differentiation and gametogenesis were identified. Moreover, sexual dimorphism was observed in the levels of E₂ and T during gonadal differentiation and development. Based on these findings, it is hypothesized that in C. maculata, the potential male sex differentiation pathway, Sox11a­Dmrt1­Sox9b, activates downstream sex-related genes (Amh, Amhr2, Gsdf, Ar, Cyp17a2) for testicular development, while the antagonistic pathway, Foxl2/Cyp19a1a, activates downstream sex-related genes (Bmp15, Figla, Er) for ovarian development. This study provides a comprehensive overview of gonadal dynamic changes during sex differentiation and gametogenesis in C. maculata, thereby establishing a scientific foundation for sex control in this species.

Delta wing design in earliest nektonic vertebrates.

The colonization of the pelagic realm by the vertebrates represents one of the major transitions in the evolutionary success of the group and in the establishment of modern complex marine ecosystem. It has been traditionally related with the Devonian rise of jawed vertebrates, but new evidences indicate that first active swimmers, invading the water column, occurred within earlier armoured jawless fishes ("ostracoderms"). These "primitive" fishes lacked conventional fish control surfaces and the precise mechanism used to generate lift and stabilizing forces still remains unclear. We show that, because of their shape, the rigid cephalic shield of Pteraspidiformes, a group of Silurian-Devonian "ostracoderms", generate significant forces for hydrodynamic lift. Particle Image Velocimetry and force measurements in a water channel shows that the flow over real-sized Pteraspidiformes models is similar to that over delta wings, dominated by the formation of leading-edge vortices resulting in enhanced vortex lift forces and delayed stall angles of attack. Additionally, experiments simulating ground effect show that Pteraspidiformes present better hydrodynamic performance under fully pelagic conditions than in a benthic scenario. This suggests that, lacking movable appendages other than the caudal fin, leading-edge vortices were exploited by earliest vertebrates to colonize the water column more than 400 Mya.

Bat consumption by holostean fishes in the Eocene Lake Messel: insights into the trophic adaptability of extinct gars and bowfins.

Direct evidence of trophic interactions between extinct species is rarely available in the fossil record. Here, we describe fish-mammal associations from the middle Eocene of Messel (Germany), consisting of three specimens of holosteans (one Atractosteus messelensis (Lepisosteidae) and two Cyclurus kehreri (Amiidae)) each preserved with a bat specimen (Palaeochiropteryx tupaiodon) lying in close contact with its jaws. This suggests that these fishes probably died after failed swallowing attempts, with the bat wing membrane entangled in their jaws resulting in a fatal handicap. Based on data from modern gars and bowfins, A. messelensis and C. kehreri may have opportunistically attacked drowning and dying individuals or scavenged on floating/sinking carcasses. This hypothesis is also supported by the unusually high number of bat specimens preserved in the deposits of the Eocene Lake Messel, suggesting that this group of small mammals may have represented a substantial food source for generalist feeders. This is the earliest case of chiropterophagy and the first known evidence of bat consumption by lepisosteid and amiid fishes, emphasizing the high trophic variability and adaptability of these groups throughout their evolutionary histories. The newly described associations provide important information for reconstructing the Eocene Lake Messel palaeoecosystem and its trophic web.

Paleoneurology: Evolving an everted brain in actinopterygian fishes.

The 30,000 or so ray-finned fishes (actinopterygians) are the most diverse group of vertebrates. Yet, their brain is peculiar, with the hemispheres everted unlike in other vertebrates. New fossils suggest how actinopterygian brain architecture evolved.

Anatomical study of pericardioperitoneal canal in immature beluga (Huso huso) with ultrasonography.

BACKGROUND: One of the special anatomical structures in sturgeons is the pericardioperitoneal canal (PPC), which has not been studied much. Considering that the presence and absence of this channel and its morphometry have not been investigated in this type of fish, this study was conducted. OBJECTIVE: The purpose of present study was to obtain a complete understanding about anatomical features and ultrasonography of PPC in the heart of immature beluga species in order to provide standard approaches for performing sonography and echocardiography on this sturgeon species. METHODS: Ten immature belugas (Huso huso) were used to perform ultrasonography with Sonosite MicroMaxx ultrasonography machine of ventral approach between two pectoral fins. After performing the steps of ultrasonographic study, gross anatomical studies were also performed, and the morphometric measurement of the canal was also performed. RESULTS: A small PPC was observed, which communicated between the pericardial cavity, and the peritoneal (coelomic) cavity. The cranial part of this channel, which was located immediately after the transverse septum, was on the midline of the body and was seen in midsagittal ultrasonograms. On average, the length of this canal was 3.23 ± 0.05 cm, and its diameter (cranial part) was 0.24 ± 0.04 cm. CONCLUSIONS: In the present study on immature beluga, it was observed that the canal is located behind the sinus venosus and is caudoventral to it, though behind the transverse septum. It has been mentioned that this PPC establishes a connection between the pericardial cavity and the peritoneal cavity, so this connection is between the peritoneal cavity and the space between the two layers of the pericardium and has no connection with the pericardial sac (the space where the heart is located). In this study, the anatomical structure and morphometry of the PPC in immature beluga were investigated, and some approaches were presented to observe the canal in ultrasonography.

Vertical structure of Caribbean deep-reef fishes from the altiphotic to deep-sea boundary.

While recent technical breakthroughs have enabled advances in the description of reefs down to 150 m, the structure and depth zonation of deep-reef communities below 150 m remains largely unknown. Here, we present results from over 10 years of deep-reef fish surveys using human-occupied submersibles at four locations across the Caribbean Sea, constituting one of the only continuous reef-fish surveys from 10 to 480 m (1 site) and 40 to 300 m (3 sites). We identify four vertically stratified deep-reef fish communities between 40 and 300 m bordered by an altiphotic (0-10 m) and a deep-sea (300-480 m) community. We found a strong faunal break around 150 m that separates mesophotic and rariphotic zones and secondary breaks at ~ 70 to 90 m and ~ 180 to 200 m subdividing these zones into upper and lower communities. From 300 to 480 m in Roatán, we found a single fish community dominated by deep-sea families, indicating that the lower boundary of the reef-fish realm occurs at 300 m. No differences were found between communities ranging from 20 to 60 m, suggesting that fishes from the lower altiphotic and upper mesophotic form an ecological continuum. While some variability was observed across sites, the overall depth zonation and key species characterizing depth zones were consistent. Most deep-reef species observed were depth specialists restricted to a single depth zone, but many shallow-reef species extended down to mesophotic depths. Depth segregation among species of a genus was found across ten reef-fish genera and likely constitutes one of the mechanisms driving community distinctiveness and thereby fish diversity across depths.

Fossils indicate marine dispersal in osteoglossid fishes, a classic example of continental vicariance.

The separation of closely related terrestrial or freshwater species by vast marine barriers represents a biogeographical riddle. Such cases can provide evidence for vicariance, a process whereby ancient geological events like continental rifting divided ancestral geographical ranges. With an evolutionary history extending tens of millions of years, freshwater ecology, and distribution encompassing widely separated southern landmasses, osteoglossid bonytongue fishes are a textbook case of vicariance attributed to Mesozoic fragmentation of the Gondwanan supercontinent. Largely overlooked fossils complicate the clean narrative invoked for extant species by recording occurrences on additional continents and in marine settings. Here, we present a new total-evidence phylogenetic hypothesis for bonytongue fishes combined with quantitative models of range evolution and show that the last common ancestor of extant osteoglossids was likely marine, and that the group colonized freshwater settings at least four times when both extant and extinct lineages are considered. The correspondence between extant osteoglossid relationships and patterns of continental fragmentation therefore represents a striking example of biogeographical pseudocongruence. Contrary to arguments against vicariance hypotheses that rely only on temporal or phylogenetic evidence, these results provide direct palaeontological support for enhanced dispersal ability early in the history of a group with widely separated distributions in the modern day.

Secondary growth ovarian follicles of the pigmented sterlet sturgeon Acipenser ruthenus L. 1758 (Acipenseriformes, Chondrostei, Actinopterygii, Osteichthyes) - Microscopic study of oocytes, egg envelope and diversification of follicular cells.

The individual ovarian follicle of sturgeons (Acipenseriformes, Acipenseridae) contains an oocyte surrounded by follicular cells (FCs), basal lamina, and thecal cells. The late stages of the secondary growth of follicles (mid- and advanced vitellogenic) are not fully explained in Acipenseriformes. To explore and discuss the ultrastructure of oocytes, FCs, an egg envelope, and explain how micropylar cells differentiate and the canals of a multiple micropyle are formed, the samples of ovaries of the mature sterlet sturgeon Acipenser ruthenus were examined. The oocytes are polarized, the nucleus is located in the animal hemisphere, contains lampbrush chromosomes and multiple nucleoli. In the ooplasm three regions are present: a perinuclear (contains the mitochondria), an endoplasm (contains the lipid droplets and yolk platelets), and a periplasm (contains the cortical granules, melanosomes, endocytotic and exocytotic vesicles). The melanosomes in animal hemisphere form two concentric rings separated by a lighter region between them. The FCs are differentiated into bright and dark cells that are both translationally and secretory active. Diversification of FCs involves repeated and cytoskeleton-dependent change of shape. In the advanced follicles the FCs are diversified into micropylar, the animal and vegetal regions cells, and the cells that delaminated from the epithelium in the animal region. The egg envelope is present in the perioocytic space and consists of three layers: (1) an inner layer or vitelline envelope, (2) a middle layer, and (3) an outer layer. The inner layer consists of four sublayers: (a) a filamentous sublayer composed of filaments released from the oocytes, (b) a trabecular 1 sublayer and (c) a trabecular 2 sublayer named due to the sequence of the deposition, and composed of filaments, fibres and trabecules, (d) a homogeneous sublayer located between the trabecular 1 and trabecular 2 sublayers composed of filaments that adhere to each other closely. The middle layer contains two sublayers: a porous 1 and a porous 2 (composed of granular material) which are released by the oocyte and FCs. The outer layer consists of fibrillar material released by the FCs. The egg envelope is pierced by radial canals formed around the microvilli of the oocyte and the microvilli-like processes of FCs. A micropylar field in the egg envelope that covers the animal pole of the oocyte contains 1 - 4 micropylar canals. Micropylar cells are involved in their formation. The shape of these cells is icicle-like and the cytoplasm is differentiated into two regions (a basal and apical bearing a projection) equipped with different sets of organelles.

Identification of pre-flexion fish larvae from the western South Atlantic using DNA barcoding and morphological characters.

Knowledge on species composition is the first step necessary for the proper conservation and management of biological resources and ecologically relevant species. High species diversity and a lack of diagnostic characters for some groups can impose difficulties for taxonomic identification through traditional methodologies, and ichthyoplankton (fish larvae and eggs) are a good example of such a scenario. With more than 35.000 valid species of fishes worldwide and overall similar anatomies in early developmental stages in closely related groups, fish larvae are often hard to be identified at the species or even more encompassing taxonomic levels. To overcome this situation, molecular techniques have been applied, with different markers tested over the years. Cytochrome c oxidase I (COI) is the most commonly used marker and now has the broadest public reference libraries, providing consistent results for species identification in different metazoan studies. Here we sequenced the mitochondrial COI-5P fragment of 89 fish larvae collected in the Campos Basin, coastal southeastern Brazil, and compared these sequences with references deposited in public databases to obtain taxonomic identifications. Most specimens identified are species of the Blenniiformes, with Parablennius and Labrisomus the most frequently identified genera. Parablennius included two species (P. marmoreus and P. pilicornis), while Labrisomus included three species (L. cricota, L. conditus and L. nuchipinnis). Anatomy of these molecularly identified specimens were then analyzed with the intention of finding anatomical characters that might be diagnostically informative amongst the early development stage (pre-flexion) larvae. Ventral pigmentation patterns are proposed as useful markers to identify Labrisomus species. However, additional specimens are needed to confirm if the character holds stability through the geographic distribution of the species.

A nitrogen isotopic shift in fish otolith-bound organic matter during the Late Cretaceous.

The nitrogen isotopes of the organic matter preserved in fossil fish otoliths (ear stones) are a promising tool for reconstructing past environmental changes. We analyzed the 15N/14N ratio (δ15N) of fossil otolith-bound organic matter in Late Cretaceous fish otoliths (of Eutawichthys maastrichtiensis, Eutawichthys zideki and Pterothrissus sp.) from three deposits along the US east coast, with two of Campanian (83.6 to 77.9 Ma) and one Maastrichtian (72.1 to 66 Ma) age. δ15N and N content were insensitive to cleaning protocol and the preservation state of otolith morphological features, and N content differences among taxa were consistent across deposits, pointing to a fossil-native origin for the organic matter. All three species showed an increase in otolith-bound organic matter δ15N of ~4‰ from Campanian to Maastrichtian. As to its cause, the similar change in distinct genera argues against changing trophic level, and modern field data argue against the different locations of the sedimentary deposits. Rather, the lower δ15N in the Campanian is best interpreted as an environmental signal at the regional scale or greater, and it may be a consequence of the warmer global climate. A similar decrease has been observed in foraminifera-bound δ15N during warm periods of the Cenozoic, reflecting decreased water column denitrification and thus contraction of the ocean's oxygen deficient zones (ODZs) under warm conditions. The same δ15N-climate correlation in Cretaceous otoliths raises the prospect of an ODZ-to-climate relationship that has been consistent over the last ~80 My, applying before and after the end-Cretaceous mass extinction and spanning changes in continental configuration.

Osteological features of some clupeid fishes (Teleostei: Clupeiformes) of Iran.

This paper presents the conclusions of a comparative analysis of six osteological features: the Structure of the vertebral column, the morphology of the predorsal bones, the vertebral column regionalization, the pterygiophore interdigitation with neural spines of dorsal fin, the pterygiophores interdigitation of with the haemal spines of the anal fin, and the intermuscular bones (IMB) and hypomerals (HM) of 12 clupeid species of the families Alosidae, Dorosomatidae, Dussumieridae and Ehiravidae. Conceivable taxonomically beneficial osteological features are nominated and utilized to discrete the clupeid species explored. Formulae for the structure of the vertebral column, the dorsal- and anal-fin pterygiophores' interdigitation with the neural and haemal spines of the vertebrae are established. These morphological descriptive traits disclose a morphotype that may be related to the mode of swimming of the species searched. The morphological study of the vertebral column of the species in question permits the division of this bony structure into six morphologically different regions. This regionalization is more intricate than the classical division in abdominal and caudal parts only.

Novel neurocranial fenestrae and expansions in Monomitopus and Selachophidium (Teleostei: Ophidiidae), with comments on the morphology, taxonomy, and evolution of the genera.

The Ophidiidae is a group of more than 300 species of fishes characterized by elongated, snake-like bodies and continuous dorsal, anal, and caudal fins. While describing a new species in the genus Monomitopus, we discovered a bilaterally paired fenestra on the dorsomedial surface of the neurocranium. We surveyed the distribution of this fenestra across species of Monomitopus and previously hypothesized allies in the genera Dannevigia, Dicrolene, Homostolus, Neobythites, and Selachophidium, finding variation in its presence and size. We also found a prominent bilaterally paired lateral fenestra and a posterior expansion of the exoccipital in the neurocrania of M. americanus and S. guentheri, with soft tissue connecting the back of the neurocranium to the first epineural and pectoral girdle in S. guentheri. In this study, we describe the distribution of and variation in these features. We integrate morphological characters and DNA data to generate a phylogeny of Monomitopus and allies to understand their relationships and trace the evolutionary history of these novel features. Our results call the monophyly of Monomitopus into question. The presence of the lateral neurocranial fenestra and posterior expansion of the exoccipital support the reclassification of M. americanus as a species of Selachophidium.

Nocturnal substrate association of four coral reef fish groups (parrotfishes, surgeonfishes, groupers and butterflyfishes) in relation to substrate architectural characteristics.

Although numerous coral reef fish species utilize substrates with high structural complexities as habitats and refuge spaces, quantitative analysis of nocturnal fish substrate associations has not been sufficiently examined yet. The aims of the present study were to clarify the nocturnal substrate associations of 17 coral reef fish species (nine parrotfish, two surgeonfish, two grouper and four butterflyfish) in relation to substrate architectural characteristics. Substrate architectural characteristics were categorized into seven types: (1) eave-like space, (2) large inter-branch space, (3) overhang by protrusion of fine branching structure, (4) overhang by coarse structure, (5) uneven structure without large space or overhang, (6) flat and (7) macroalgae. Overall, fishes were primarily associated with three architectural characteristics (eave-like space, large inter-branch space and overhang by coarse structure). The main providers of these three architectural characteristics were tabular and corymbose Acropora, staghorn Acropora, and rock. Species-specific significant positive associations with particular architectural characteristics were found as follows. For the nine parrotfish species, Chlorurus microrhinos with large inter-branch space and overhang by coarse structure; Ch. spilurus with eave-like space and large inter-branch space; Hipposcarus longiceps with large inter-branch space; Scarus ghobban with overhang by coarse structure; five species (Scarus forsteni, S. niger, S. oviceps, S. rivulatus and S. schlegeli) with eave-like space. For the two surgeonfish species, Naso unicornis with overhang by coarse structure; N. lituratus with eave-like space. For the two grouper species, Plectropomus leopardus with eave-like space; Epinephelus ongus with overhang by coarse structure. For the four butterflyfish species, Chaetodon trifascialis with eave-like space and large inter-branch space; C. lunulatus and C. ephippium with large inter-branch space; C. auriga showed no significant associations with any architectural characteristics. Four species (Ch. microrhinos, H. longiceps, S. niger and N. unicornis) also showed clear variations in substrate associations among the different fish size classes. Since parrotfishes, surgeonfishes and groupers are main fisheries targets in coral reefs, conservation and restoration of coral species that provide eave-like space (tabular and corymbose Acropora) and large inter-branch space (staghorn Acropora) as well as hard substrates with coarse structure that provide overhang (rock) should be considered for effective fisheries management in coral reefs. For butterflyfishes, coral species that provide eave-like space (tabular Acropora) and large inter-branch space (staghorn Acropora) should also be conserved and restored for provision of sleeping sites.

Sexually dimorphic eye size in dragonfishes, a response to a bioluminescent signalling gap.

Deep-sea fishes must overcome extremely large nearest-neighbour distances and darkness to find mates. Sexual dimorphism in the size of luminescent structures in many deep-sea taxa, including dragonfishes (family Stomiidae), indicates reproductive behaviours may be mediated by visual signalling. This presents a paradox: if male photophores are larger, females may find males at shorter distances than males find females. Solutions to this gap may include females closing this gap or by males gathering more photons with a larger eye. We examine the eye size of two species of dragonfishes (Malacosteus niger and Phostomias guernei) for sexual dimorphism and employ a model of detection distance to evaluate the potential for such dimorphism to bridge the detection gap. This model incorporates the flux of sexually dimorphic postorbital photophores and eye lens size to predict detection distances. In both species, we found a significant visual detection gap in which females find males before males find females and that male lens size is larger, marking the second known case of size dimorphism in the actinopterygian visual system. Our results indicate the larger eye affords males a significant improvement in detection distance. We conclude that this dimorphic phenotype may have evolved to close the detection gap.

Soft-tissue fossilization illuminates the stepwise evolution of the ray-finned fish brain.

A complex brain is central to the success of backboned animals. However, direct evidence bearing on vertebrate brain evolution comes almost exclusively from extant species, leaving substantial knowledge gaps. Although rare, soft-tissue preservation in fossils can yield unique insights on patterns of neuroanatomical evolution. Paleontological evidence from an exceptionally preserved Pennsylvanian (∼318 Ma) actinopterygian, Coccocephalus, calls into question prior interpretations of ancestral actinopterygian brain conditions. However, the ordering and timing of major evolutionary innovations, such as an everted telencephalon, modified meningeal tissues, and hypothalamic inferior lobes, remain unclear. Here, we report two distinct actinopterygian morphotypes from the latest Carboniferous-earliest Permian (∼299 Ma) of Brazil that show extensive soft-tissue preservation of brains, cranial nerves, eyes, and potential cardiovascular tissues. These fossils corroborate inferences drawn from ✝Coccocephalus, while adding new information about neuroanatomical evolution. Skeletal features indicate that one of these Brazilian morphotypes is more closely related to living actinopterygians than the other, which is also reflected in soft-tissue features. Significantly, the more crownward morphotype shows a key neuroanatomical feature of extant actinopterygians-an everted telencephalon-that is absent in the other morphotype and ✝Coccocephalus. All preserved Paleozoic actinopterygian brains show broad similarities, including an invaginated cerebellum, hypothalamus inferior lobes, and a small forebrain. In each case, preserved brains are substantially smaller than the enclosing cranial chamber. The neuroanatomical similarities shared by this grade of Permo-Carboniferous actinopterygians reflect probable primitive conditions for actinopterygians, providing a revised model for interpreting brain evolution in a major branch of the vertebrate tree of life.

Diversity of Intraspecific Patterns of Brain Region Size Covariation in Fish.

Traits often do not evolve in isolation or vary independently of other traits. Instead, they can be affected by covariation, both within and across species. However, the importance of within-species trait covariation and, critically, the degree to which it varies between species has yet to be thoroughly studied. Brain morphology is a trait of great ecological and behavioral importance, with regions that are hypothesized to vary in size based on behavioral and cognitive demands. Sizes of brain regions have also been shown to covary with each other across various taxa. Here, we test the degree to which covariation in brain region sizes within species has been conserved across 10 teleost fish species. These 10 species span five orders, allowing us to examine how phylogenetic proximity influences similarities in intraspecific trait covariation. Our results showed a trend that similar patterns of brain region size covariation occur in more closely related species. Interestingly, there were certain brain region pairs that showed similar levels of covariation across all species regardless of phylogenetic distance, such as the telencephalon and optic tectum, while others, such as the olfactory bulb and the hypothalamus, varied more independently. Ultimately, the patterns of brain region covariation shown here suggest that evolutionary mechanisms or constraints can act on specific brain regions independently, and that these constraints can change over evolutionary time.

A geometric morphometric protocol to correct postmortem body arching in fossil fishes.

Postmortem body curvature introduces error in fish morphometric data. Compared to living fish, the causes of such body curvature in fossils may be due to additive taphonomic processes that have been widely studied. However, a protocol that helps to correct its effect upon morphometric data remains unexplored. Here, we test two different mathematical approaches (multivariate regression and the so-called 'unbending functions') available to tackle fish geometric morphometric data in two exceptionally preserved gonorynchiformes fossil fishes, Rubiesichthys gregalis and Gordichthys conquensis, from the Las Hoyas deposits (Early Cretaceous, Spain). Although both methods successfully correct body curvature (i.e., removing misleading geometric variation), our results show that traditional approaches applied in living fishes might not be appropriate to fossil ones, because of the additional anatomical alterations. Namely, the best result for 2D fossil fishes is achieved by correcting the arching of the specimens (mathematically "unbending" them). Ultimately, the effect of body curvature on morphometric data is largely taxon independent and morphological diversity mitigates its effect, but size is an important factor to take into account (because larger individuals tend to be less curved).

Occipital-synarcual joint mobility in ratfishes (Chimaeridae) and its possible adaptive role.

The neurocranial elevation generated by axial muscles is widespread among aquatic gnathostomes. The mechanism has two functions: first, it contributes to the orientation of the mouth gape, and second, it is involved in suction feeding. To provide such mobility, anatomical specialization of the anterior part of the vertebral column has evolved in many fish species. In modern chimaeras, the anterior part of the vertebral column develops into the synarcual. Possible biological roles of the occipital-synarcual joint have not been discussed before. Dissections of the head of two species of ratfishes (Chimaera monstrosa and Chimaera phantasma) confirmed the heterocoely of the articulation surface between the synarcual and the neurocranium, indicating the possibility of movements in the sagittal and frontal planes. Muscles capable of controlling the movements of the neurocranium were described. The m. epaxialis is capable of elevating the head, the m. coracomandibularis is capable of lowering it if the mandible is anchored by the adductor. Lateral flexion is performed by the m. lateroventralis, for which this function was proposed for the first time. The first description of the m. epaxialis profundus is given, its function is to be elucidated in the future. Manipulations with joint preparations revealed a pronounced amplitude of movement in the sagittal and frontal planes. Since chimaeras generate weak decrease in pressure in the oropharyngeal cavity when sucking in prey, we hypothesised the primary effect of neurocranial elevation, in addition to the evident lateral head mobility, is accurate prey targeting.

Anatomical insights into fish terrestrial locomotion: A study of barred mudskipper (Periophthalmus argentilineatus) fins based on µCT 3D reconstructions.

Mudskippers are a group of extant ray-finned fishes with an amphibious lifestyle and serve as exemplars for understanding the evolution of amphibious capabilities in teleosts. A comprehensive anatomical profile of both the soft and hard tissues within their propulsive fins is essential for advancing our understanding of terrestrial locomotor adaptations in fish. Despite the ecological significance of mudskippers, detailed data on their musculoskeletal anatomy remains limited. In the present research, we utilized contrast-enhanced high-resolution microcomputed tomography (µCT) imaging to investigate the barred mudskipper, Periophthalmus argentilineatus. This technique enabled detailed reconstruction and quantification of the morphological details of the pectoral, pelvic, and caudal fins of this terrestrial mudskipper, facilitating comparison with its aquatic relatives. Our findings reveal that P. argentilineatus has undergone complex musculoskeletal adaptations for terrestrial movement, including an increase in muscle complexity and muscle volume, as well as the development of specialized structures like aponeuroses for pectoral fin extension. Skeletal modifications are also evident, with features such as a reinforced shoulder-pelvic joint and thickened fin rays. These evolutionary modifications suggest biomechanically advanced fins capable of overcoming the gravitational challenges of terrestrial habitats, indicating a strong selective advantage for these features in land-based environments. The unique musculoskeletal modifications in the fins of mudskippers like P. argentilineatus, compared with their aquatic counterparts, mark a critical evolutionary shift toward terrestrial adaptations. This study not only sheds light on the specific anatomical changes facilitating this transition but also offers broader insights into the early evolutionary mechanisms of terrestrial locomotion, potentially mirroring the transformative journey from aquatic to terrestrial life in the lineage leading to tetrapods.