The first articulated skeletons of enigmatic Late Cretaceous billfish-like actinopterygians.

Only few candidates of Mesozoic fishes with a similar body plan and ecological niche to the modern billfishes are suggested as their analogues. Several specimens were recovered from Cenomanian deposits in Germany and Lebanon and display a billfish-like fusiform body with elongated premaxillae. They are found close to the plethodids and show a unique combination of characters (rostrum pointed and extremely elongated, double articular head of the quadrate, anteroposteriorly elongated abdominal centra indicating a slender body and different types of scales on the body) allowing their inclusion in a new genus. Two 'Protosphyraena' species are also assigned to this new genus. This fish can be considered as an ecological analogue to the extant xiphioids sharing their feeding habits. This fish was abundant and roamed, as an apex predator, the Central Tethys and the Boreal realms during the Cenomanian.

Revision of the Middle Triassic coelacanth Ticinepomis Rieppel 1980 (Actinistia, Latimeriidae) with paleobiological and paleoecological considerations.

Coelacanths form today an impoverished clade of sarcopterygian fishes, which were somewhat more diverse during their evolutionary history, especially in the Triassic. Since the first description of the coelacanth Ticinepomis peyeri from the Besano Formation of the UNESCO World Heritage Site of Monte San Giorgio (Canton Ticino, Switzerland), the diversity of coelacanths in the Middle Triassic of this area of the western Paleo-Tethys has been enriched with discoveries of other fossil materials. At Monte San Giorgio, two specimens of Heptanema paradoxum and several specimens of the unusual coelacanth Rieppelia heinzfurreri, have been reported from the Meride Limestone and the Besano Formation, respectively. Another unusual coelacanth, Foreyia maxkuhni, and two specimens referred to Ticinepomis cf. T. peyeri have been described from the isochronous and paleogeographical close Prosanto Formation at the Ducanfurgga and Strel sites (near Davos, Canton Graubünden). In the framework of the revision of the coelacanth material from the Besano Formation kept in the collection of the Paläontologisches Institut und Museum der Universität Zürich (Switzerland), we reviewed the genus Ticinepomis on the basis of the holotype and four new referred specimens. Several morphological traits that were little and/or not understood in T. peyeri are here clarified. We re-evaluate the taxonomic attribution of the material of Ticinepomis cf. T. peyeri from the Prosanto Formation. Morphological characters are different enough from the type species, T. peyeri, to erect a new species, Ticinepomis ducanensis sp. nov., which is shown to be also present in the Besano Formation of Monte San Giorgio, where it is represented by fragmentary bone elements. The recognition of a new coelacanth species indicates that the diversity of this slow-evolving lineage was particularly high in this part of the Western Tethys during the Middle Triassic, especially between 242 and 240 million years ago.

Early Mesozoic burst of morphological disparity in the slow-evolving coelacanth fish lineage.

Since the split of the coelacanth lineage from other osteichthyans 420 million years ago, the morphological disparity of this clade has remained remarkably stable. Only few outliers with peculiar body shape stood out over the evolutionary history, but they were phylogenetically and stratigraphically independent of each other. Here, we report the discovery of a new clade of ancient latimeriid coelacanths representing a small flock of species present in the Western Tethys between 242 and 241 million years ago. Among the four species, two show highly derived anatomy. A new genus shows reversal to plesiomorphic conditions in its skull and caudal fin organisation. The new genus and its sister Foreyia have anatomical modules that moved from the general coelacanth Bauplau either in the same direction or in opposite direction that affect proportions of the body, opercle and fins. Comparisons with extant genetic models shows that changes of the regulatory network of the Hedgehog signal gene family may account for most of the altered anatomy. This unexpected, short and confined new clade represents the only known example of a burst of morphological disparity over the long history of coelacanths at a recovery period after the Permian-Triassic Mass Extinction.

The first Jurassic coelacanth from Switzerland.

Coelacanths form a clade of sarcopterygian fish represented today by a single genus, Latimeria. The fossil record of the group, which dates back to the Early Devonian, is sparse. In Switzerland, only Triassic sites in the east and southeast of the country have yielded fossils of coelacanths. Here, we describe and study the very first coelacanth of the Jurassic period (Toarcian stage) from Switzerland. The unique specimen, represented by a sub-complete individual, possesses morphological characteristics allowing assignment to the genus Libys (e.g., sensory canals opening through a large groove crossed by pillars), a marine coelacanth previously known only in the Late Jurassic of Germany. Morphological characters are different enough from the type species, Libys polypterus, to erect a new species of Libys named Libys callolepis sp. nov. The presence of Libys callolepis sp. nov. in Lower Jurassic beds extends the stratigraphic range of the genus Libys by about 34 million years, but without increasing considerably its geographic distribution. Belonging to the modern family Latimeriidae, the occurrence of Libys callolepis sp. nov. heralds a long period, up to the present day, of coelacanth genera with very long stratigraphic range and reduced morphological disparity, which have earned them the nickname of 'living fossils'.

A fossil assemblage from the mid-late Maastrichtian of Gavdos Island, Greece, provides insights into the pre-extinction pelagic ichthyofaunas of the Tethys.

The global body-fossil record of marine 'fishes' from the time interval immediately preceding the Cretaceous-Paleogene Extinction is markedly poor. This deficiency appears to be further exacerbated with regards to offshore and deep-water taxa, obscuring our understanding of the state and composition of corresponding vertebrate faunas at the onset of this major extinction event. Recent fieldwork in the mid-late Maastrichtian exposures of the Pindos Unit in Gavdos Island, Greece, yielded a small but informative sample of fossil 'fishes', which inhabited the Tethys approximately three to four million years before the extinction. In this work we describe this sample, which comprises between eight and nine discrete morphotypes of various size classes, belonging to †Ichthyodectoidei, Aulopiformes (†Dercetidae, †Enchodontidae, †Ichthyotringidae), cf. †Sardinioididae, as well as the hexanchid shark †Gladioserratus sp. The new material expands the faunal list for the Maastrichtian of Gavdos Island, and the Pindos Unit as a whole, and further allows for the description of a new genus and species of †Enchodontidae and a new species of †Ichthyotringidae. The two new taxa are found to be widespread in the Maastrichtian of the Pindos Unit. The overall character of the assemblage agrees with previous interpretations of an offshore and rather deep depositional environment for the fossiliferous horizons. Furthermore, it exhibits a higher diversity than, and little taxonomic overlap with penecontemporaneous teleost assemblages from the Tethys, and informs on the otherwise poorly known Maastrichtian offshore and deep-water marine ichthyofaunas of the region.

The first late cretaceous mawsoniid coelacanth (Sarcopterygii: Actinistia) from North America: Evidence of a lineage of extinct 'living fossils'.

Today, the only living genus of coelacanth, Latimeria is represented by two species along the eastern coast of Africa and in Indonesia. This sarcopterygian fish is nicknamed a "living fossil", in particular because of its slow evolution. The large geographical distribution of Latimeria may be a reason for the great resilience to extinction of this lineage, but the lack of fossil records for this genus prevents us from testing this hypothesis. Here we describe isolated bones (right angular, incomplete basisphenoid, fragments of parasphenoid and pterygoid) found in the Cenomanian Woodbine Formation in northeast Texas that are referred to the mawsoniid coelacanth Mawsonia sp. In order to assess the impact of this discovery on the alleged characteristic of "living fossils" in general and of coelacanths in particular: 1) we compared the average time duration of genera of ray-finned fish and coelacanth in the fossil record; 2) we compared the biogeographic signal from Mawsonia with the signal from the rest of the vertebrate assemblage of the Woodbine formation; and 3) we compared these life traits with those of Latimeria. The stratigraphical range of Mawsonia is at least 50 million years. Since Mawsonia was a fresh, brackish water fish with probably a low ability to cross large sea barriers and because most of the continental components of the Woodbine Fm vertebrate assemblage exhibit Laurasian affinities, it is proposed that the Mawsonia's occurrence in North America is more likely the result of a vicariant event linked to the break-up of Pangea rather than the result of a dispersal from Gondwana. The link between a wide geographic distribution and the resilience to extinction demonstrated here for Mawsonia is a clue that a similar situation existed for Latimeria, which allowed this genus to live for tens of millions of years.

Giant Mesozoic coelacanths (Osteichthyes, Actinistia) reveal high body size disparity decoupled from taxic diversity.

The positive correlation between speciation rates and morphological evolution expressed by body size is a macroevolutionary trait of vertebrates. Although taxic diversification and morphological evolution are slow in coelacanths, their fossil record indicates that large and small species coexisted, which calls into question the link between morphological and body size disparities. Here, we describe and reassess fossils of giant coelacanths. Two genera reached up to 5 m long, placing them among the ten largest bony fish that ever lived. The disparity in body size adjusted to taxic diversity is much greater in coelacanths than in ray-finned fishes. Previous studies have shown that rates of speciation and rates of morphological evolution are overall low in this group, and our results indicate that these parameters are decoupled from the disparity in body size in coelacanths. Genomic and physiological characteristics of the extant Latimeria may reflect how the extinct relatives grew to such a large size. These characteristics highlight new evolutionary traits specific to these "living fossils".

The last known freshwater coelacanths: New Late Cretaceous mawsoniid remains (Osteichthyes: Actinistia) from Southern France.

Coelacanths are iconic fishes represented today by a single marine genus. The group was a little bit more diversified in the Mesozoic, with representatives in marine and continental environments in the Late Cretaceous. Here we describe isolated skull bones of the last know freshwater coelacanths found in several fossil sites from the Early Campanian to the Early Maastrichtian of Southern France (in the Departments of Aude, Bouches-du-Rhône, Hérault, and Var). The sample does not allow distinguishing different species, and all material is referred to Axelrodichthys megadromos Cavin, Valentin, Garcia originally described from the locality of Ventabren in Southern France. A reconstruction of the skull is proposed. Previously unrecognized features are described, including parts of the postparietal portion of the skull, of the suspensorium and of the mandible. The new data confirm the assignation of the species to the mawsoniids, and more specifically to Axelrodichthys. A cladistic analysis scoring new character states provides a similar topology than a previous analysis, i.e. A. megadromos is placed in a polytomy with Axelrodichthys araripensis and Lualabaea lerichei, two species from the Early Cretaceous of Brazil and from the Late Jurassic of the Democratic Republic of the Congo, respectively. A. megadromos appears to have been restricted to freshwater environments, to the contrary of oldest Western Gondwanan representatives of the family that were able to live in brackish and marine waters. A. megadromos is the last representative of the mawsoniids and its occurrence in Europe is probably the result of a dispersal event from Western Gondwana that happened somewhen in the Cretaceous. Based on the available data, the mawsoniids went extinct in the mid-Maastrichthian, i.e. before the end-Cretaceous mass extinction. But it is possible that the fossil record of this family, which has been only recently recognized in Late Cretaceous European deposits, will geographically and stratigraphically widen with further discoveries.

Resolving homology in the face of shifting germ layer origins: Lessons from a major skull vault boundary.

The vertebrate skull varies widely in shape, accommodating diverse strategies of feeding and predation. The braincase is composed of several flat bones that meet at flexible joints called sutures. Nearly all vertebrates have a prominent 'coronal' suture that separates the front and back of the skull. This suture can develop entirely within mesoderm-derived tissue, neural crest-derived tissue, or at the boundary of the two. Recent paleontological findings and genetic insights in non-mammalian model organisms serve to revise fundamental knowledge on the development and evolution of this suture. Growing evidence supports a decoupling of the germ layer origins of the mesenchyme that forms the calvarial bones from inductive signaling that establishes discrete bone centers. Changes in these relationships facilitate skull evolution and may create susceptibility to disease. These concepts provide a general framework for approaching issues of homology in cases where germ layer origins have shifted during evolution.

Heterochronic evolution explains novel body shape in a Triassic coelacanth from Switzerland.

A bizarre latimeriid coelacanth fish from the Middle Triassic of Switzerland shows skeletal features deviating from the uniform anatomy of coelacanths. The new form is closely related to a modern-looking coelacanth found in the same locality and differences between both are attributed to heterochronic evolution. Most of the modified osteological structures in the new coelacanth have their developmental origin in the skull/trunk interface region in the embryo. Change in the expression of developmental patterning genes, specifically the Pax1/9 genes, may explain a rapid evolution at the origin of the new coelacanth. This species broadens the morphological disparity range within the lineage of these 'living fossils' and exemplifies a case of rapid heterochronic evolution likely trigged by minor changes in gene expression.

A unique Cretaceous-Paleogene lineage of piranha-jawed pycnodont fishes.

The extinct group of the Pycnodontiformes is one of the most characteristic components of the Mesozoic and early Cenozoic fish faunas. These ray-finned fishes, which underwent an explosive morphological diversification during the Late Cretaceous, are generally regarded as typical shell-crushers. Here we report unusual cutting-type dentitions from the Paleogene of Morocco which are assigned to a new genus of highly specialized pycnodont fish. This peculiar taxon represents the last member of a new, previously undetected 40-million-year lineage (Serrasalmimidae fam. nov., including two other new genera and Polygyrodus White, 1927) ranging back to the early Late Cretaceous and leading to exclusively carnivorous predatory forms, unique and unexpected among pycnodonts. Our discovery indicates that latest Cretaceous-earliest Paleogene pycnodonts occupied more diverse trophic niches than previously thought, taking advantage of the apparition of new prey types in the changing marine ecosystems of this time interval. The evolutionary sequence of trophic specialization characterizing this new group of pycnodontiforms is strikingly similar to that observed within serrasalmid characiforms, from seed- and fruit-eating pacus to flesh-eating piranhas.

Genome Compositional Organization in Gars Shows More Similarities to Mammals than to Other Ray-Finned Fish.

Genomic GC content can vary locally, and GC-rich regions are usually associated with increased DNA thermostability in thermophilic prokaryotes and warm-blooded eukaryotes. Among vertebrates, fish and amphibians appeared to possess a distinctly less heterogeneous AT/GC organization in their genomes, whereas cytogenetically detectable GC heterogeneity has so far only been documented in mammals and birds. The subject of our study is the gar, an ancient "living fossil" of a basal ray-finned fish lineage, known from the Cretaceous period. We carried out cytogenomic analysis in two gar genera (Atractosteus and Lepisosteus) uncovering a GC chromosomal pattern uncharacteristic for fish. Bioinformatic analysis of the spotted gar (Lepisosteus oculatus) confirmed a GC compartmentalization on GC profiles of linkage groups. This indicates a rather mammalian mode of compositional organization on gar chromosomes. Gars are thus the only analyzed extant ray-finned fishes with a GC compartmentalized genome. Since gars are cold-blooded anamniotes, our results contradict the generally accepted hypothesis that the phylogenomic onset of GC compartmentalization occurred near the origin of amniotes. Ecophysiological findings of other authors indicate a metabolic similarity of gars with mammals. We hypothesize that gars might have undergone convergent evolution with the tetrapod lineages leading to mammals on both metabolic and genomic levels. Their metabolic adaptations might have left footprints in their compositional genome evolution, as proposed by the metabolic rate hypothesis. The genome organization described here in gars sheds new light on the compositional genome evolution in vertebrates generally and contributes to better understanding of the complexities of the mechanisms involved in this process.

'Fish' (Actinopterygii and Elasmobranchii) diversification patterns through deep time.

Actinopterygii (ray-finned fishes) and Elasmobranchii (sharks, skates and rays) represent more than half of today's vertebrate taxic diversity (approximately 33000 species) and form the largest component of vertebrate diversity in extant aquatic ecosystems. Yet, patterns of 'fish' evolutionary history remain insufficiently understood and previous studies generally treated each group independently mainly because of their contrasting fossil record composition and corresponding sampling strategies. Because direct reading of palaeodiversity curves is affected by several biases affecting the fossil record, analytical approaches are needed to correct for these biases. In this review, we propose a comprehensive analysis based on comparison of large data sets related to competing phylogenies (including all Recent and fossil taxa) and the fossil record for both groups during the Mesozoic-Cainozoic interval. This approach provides information on the 'fish' fossil record quality and on the corrected 'fish' deep-time phylogenetic palaeodiversity signals, with special emphasis on diversification events. Because taxonomic information is preserved after analytical treatment, identified palaeodiversity events are considered both quantitatively and qualitatively and put within corresponding palaeoenvironmental and biological settings. Results indicate a better fossil record quality for elasmobranchs due to their microfossil-like fossil distribution and their very low diversity in freshwater systems, whereas freshwater actinopterygians are diverse in this realm with lower preservation potential. Several important diversification events are identified at familial and generic levels for elasmobranchs, and marine and freshwater actinopterygians, namely in the Early-Middle Jurassic (elasmobranchs), Late Jurassic (actinopterygians), Early Cretaceous (elasmobranchs, freshwater actinopterygians), Cenomanian (all groups) and the Paleocene-Eocene interval (all groups), the latter two representing the two most exceptional radiations among vertebrates. For each of these events along with the Cretaceous-Paleogene extinction, we provide an in-depth review of the taxa involved and factors that may have influenced the diversity patterns observed. Among these, palaeotemperatures, sea-levels, ocean circulation and productivity as well as continent fragmentation and environment heterogeneity (reef environments) are parameters that largely impacted on 'fish' evolutionary history, along with other biotic constraints.

Contrasting "Fish" Diversity Dynamics between Marine and Freshwater Environments.

Two theoretical models have been proposed to describe long-term dynamics of diversification: the equilibrium model considers the Earth as a closed system with a fixed maximum biological carrying capacity, whereas the expansion model hypothesizes a continuously increasing diversification of life. Based on the analysis of the fossil record of all organisms, Benton suggested contrasting models of diversity dynamics between marine and continental realms. Diversity in marine environments is characterized by phases of rapid diversification followed by plateaux, i.e., an equilibrium model directly derived from insular biogeography theories, whereas diversity in continental environments is characterized by exponential growth. Previous studies that aimed at testing these models with empirical data were based on datasets extracted directly from the reading of the vagaries of the raw fossil record, without correcting for common fossil record biases (preservation and sampling). Although correction of datasets for the incompleteness of the fossil record is now commonly performed for addressing long-term biodiversity variations, only a few attempts have been made to produce diversity curves corrected by phylogenetic data from extant and extinct taxa. Here we show that phylogenetically corrected diversity curves for "fish" (actinopterygians and elasmobranchs) during the last 200 million years fit an equilibrium model in the marine realm and an expansion model in the freshwater realm. These findings demonstrate that the rate of diversification has decreased for marine fish over the Cenozoic but is in sharp expansion for freshwater fish.

Taxonomic composition and trophic structure of the continental bony fish assemblage from the early late cretaceous of Southeastern Morocco.

The mid-Cretaceous vertebrate assemblage from south-eastern Morocco is one of the most diversified continental vertebrate assemblages of this time worldwide. The bony fish component (coelacanths, lungfishes and ray-finned fishes) is represented by relatively complete specimens and, mostly, by fragmentary elements scattered along 250 kilometres of outcrops. Here we revisit the bony fish assemblage by studying both isolated remains collected during several fieldtrips and more complete material kept in public collections. The assemblage comprises several lungfish taxa, with the first mention of the occurrence of Arganodus tiguidiensis, and possibly two mawsoniid coelacanths. A large bichir cf. Bawitius, is recorded and corresponds to cranial elements initially referred to 'Stromerichthys' from coeval deposits in Egypt. The ginglymodians were diversified with a large 'Lepidotes' plus two obaichthyids and a gar. We confirm here that this gar belongs to a genus distinctive from Recent gars, contrary to what was suggested recently. Teleosteans comprise a poorly known ichthyodectiform, a notopterid, a probable osteoglossomorph and a large tselfatiiform, whose cranial anatomy is detailed. The body size and trophic level for each taxon are estimated on the basis of comparison with extant closely related taxa. We plotted the average body size versus average trophic level for the Kem Kem assemblage, together with extant marine and freshwater assemblages. The Kem Kem assemblage is characterized by taxa of proportionally large body size, and by a higher average trophic level than the trophic level of the extant compared freshwater ecosystems, but lower than for the extant marine ecosystems. These results should be regarded with caution because they rest on a reconstructed assemblage known mostly by fragmentary remains. They reinforce, however, the ecological oddities already noticed for this mid-Cretaceous vertebrate ecosystem in North Africa.

Cretaceous stem chondrichthyans survived the end-Permian mass extinction.

Cladodontomorph sharks are Palaeozoic stem chondrichthyans thought to go extinct at the end-Permian mass extinction. This extinction preceded the diversification of euselachians, including modern sharks. Here we describe an outer-platform cladodontomorph shark tooth assemblage from the Early Cretaceous of southern France, increasing the fossil record of this group by circa 120 million years. Identification of this material rests on new histological observations and morphological evidence. Our finding shows that this lineage survived mass extinctions most likely by habitat contraction, using deep-sea refuge environments during catastrophic events. The recorded gap in the cladodontomorph lineage represents the longest gap in the fossil record for an extinct marine vertebrate group. This discovery demonstrates that the deep-sea marine diversity, poorly known during most of the fish evolutionary history, contains essential data for a complete understanding of the long-term evolution of marine fish paleobiodiversity.

Diversity of Mesozoic semionotiform fishes and the origin of gars (Lepisosteidae).

Gars (Lepisosteidae) are ray-finned fishes with controversial relationships to other actinopterygian lineages. When fossil taxa are considered, gars are grouped with Mesozoic macrosemiids and 'semionotids' in the Semionotiformes, but the intra-relationships within this order are still elusive. Here, the evolutionary history of gars is reinvestigated using a set of well-preserved extinct semionotiform taxa in a phylogenetic analysis. Results indicate that the gar lineage roots in a clade of Late Jurassic-Cretaceous semionotiform fishes. The closest relatives to gars were plant-eating and detritivorous freshwater fishes. The occurrence of semionotiform remains in Early and early Late Cretaceous continental deposits worldwide possibly reflects an important radiation of this group, comparable to the present-day diversification of cypriniforms. Other Late Triassic to Early Cretaceous semionotiform taxa are gathered in a single clade with weakly supported internal nodes, pointing out the necessity to better understand the osteology of these fishes.

Using ghost lineages to identify diversification events in the fossil record.

Observed rises in taxic diversity could reflect bias of the fossil record or a genuine diversification. Here we outline a new method that attempts to differentiate between these two possible explanations. The method is based on the calculations of average ghost lineage duration through successive intervals of time. Biases due to variation in preservational conditions affect taxa independently from their position in the tree of life. A genuine radiation event will affect some parts of the tree of life more than others. During periods of rapid diversification, there will be a high proportion of new taxa showing short ghost lineages and therefore the average ghost lineage duration will drop as diversity rises, allowing us to distinguish such events from preservational bias during which ghost lineage duration remains unchanged. We test the method on Aptian-Maastrichtian (Cretaceous) ray-finned fish diversity. The result shows that a peak of diversity in the Cenomanian is associated with a drop in average ghost lineage duration, indicating that a genuine biological radiation occurred at that time.

Minute theropod eggs and embryo from the Lower Cretaceous of Thailand and the dinosaur-bird transition.

We report on very small fossil eggs from the Lower Cretaceous of Thailand, one of them containing a theropod embryo, which display a remarkable mosaic of characters. While the surficial ornamentation is typical of non-avian saurischian dinosaurs, the three-layered prismatic structure of the eggshell is currently known only in extant and fossil eggs associated with birds. These eggs, about the size of a goldfinch's, mirror at the reproductive level the retention of small body size that was paramount in the transition from non-avian theropods to birds. The egg-layer may have been a small feathered theropod similar to those recently found in China.