Early fossils illuminate character evolution and interrelationships of Lampridiformes (Teleostei, Acanthomorpha)

Exceptional preservation of a Cretaceous intestine provides a glimpse of the early ecological diversity of spiny-rayed fishes (Acanthomorpha, Teleostei)

Acanthomorph teleosts (spiny-rayed fishes) account for approximately a third of extant vertebrate species. They appeared during the Late Cretaceous and have been a major component of aquatic biodiversity since the early Cenozoic. They occupy today most trophic levels and ecological niches in aquatic environments, however very little is known about those that were adopted by the earliest representatives of the group. Here, we report on an exceptional glimpse into the ecological diversity of early spiny-rayed fishes provided by the unusual preservation of a newly discovered specimen of the freshwater acanthomorph Spinocaudichthys from the Upper Cretaceous of Morocco. A combination of major-to-trace elemental mapping methods reveals that the gross morphology of the specimen's intestine has been remarkably preserved owing to the rapid mineralization of iron hydroxides around it. Differing with the typically short and straight intestinal tract of carnivorous teleosts, the intestine in Spinocaudichthys is long and highly convoluted, indicating a probable herbivorous diet. Acanthomorphs would therefore have conquered various ecological niches in their early evolutionary history, prior to their subsequent phylogenetic diversification in both marine and freshwater environments that followed the K-Pg extinction event.

Phylogenetic classification of bony fishes

BACKGROUND

Fish classifications, as those of most other taxonomic groups, are being transformed drastically as new molecular phylogenies provide support for natural groups that were unanticipated by previous studies. A brief review of the main criteria used by ichthyologists to define their classifications during the last 50 years, however, reveals slow progress towards using an explicit phylogenetic framework. Instead, the trend has been to rely, in varying degrees, on deep-rooted anatomical concepts and authority, often mixing taxa with explicit phylogenetic support with arbitrary groupings. Two leading sources in ichthyology frequently used for fish classifications (JS Nelson's volumes of Fishes of the World and W. Eschmeyer's Catalog of Fishes) fail to adopt a global phylogenetic framework despite much recent progress made towards the resolution of the fish Tree of Life. The first explicit phylogenetic classification of bony fishes was published in 2013, based on a comprehensive molecular phylogeny ( www.deepfin.org ). We here update the first version of that classification by incorporating the most recent phylogenetic results.

RESULTS

The updated classification presented here is based on phylogenies inferred using molecular and genomic data for nearly 2000 fishes. A total of 72 orders (and 79 suborders) are recognized in this version, compared with 66 orders in version 1. The phylogeny resolves placement of 410 families, or ~80% of the total of 514 families of bony fishes currently recognized. The ordinal status of 30 percomorph families included in this study, however, remains uncertain (incertae sedis in the series Carangaria, Ovalentaria, or Eupercaria). Comments to support taxonomic decisions and comparisons with conflicting taxonomic groups proposed by others are presented. We also highlight cases were morphological support exist for the groups being classified.

CONCLUSIONS

This version of the phylogenetic classification of bony fishes is substantially improved, providing resolution for more taxa than previous versions, based on more densely sampled phylogenetic trees. The classification presented in this study represents, unlike any other, the most up-to-date hypothesis of the Tree of Life of fishes.

A fossil unicorn crestfish (Teleostei, Lampridiformes, Lophotidae) from the Eocene of Iran

Lophotidae, or crestfishes, is a family of rare deep-sea teleosts characterised by an enlarged horn-like crest on the forehead. They are poorly represented in the fossil record, by only three described taxa. One specimen attributed to Lophotidae has been described from the pelagic fauna of the middle-late Eocene Zagros Basin, Iran. Originally considered as a specimen of the fossil lophotid †Protolophotus, it is proposed hereby as a new genus and species †Babelichthys olneyi, gen. et sp. nov., differs from the other fossil lophotids by its relatively long and strongly projecting crest, suggesting a close relationship with the modern unicorn crestfish, Eumecichthys. This new taxon increases the diversity of the deep-sea teleost fauna to which it belongs, improving our understanding of the taxonomic composition of the early Cenozoic mesopelagic ecosystems.

Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates

BACKGROUND

Fishes are extremely speciose and also highly disparate in their fin configurations, more specifically in the number of fins present as well as their structure, shape, and size. How they achieved this remarkable disparity is difficult to explain in the absence of any comprehensive overview of the evolutionary history of fish appendages. Fin modularity could provide an explanation for both the observed disparity in fin configurations and the sequential appearance of new fins. Modularity is considered as an important prerequisite for the evolvability of living systems, enabling individual modules to be optimized without interfering with others. Similarities in developmental patterns between some of the fins already suggest that they form developmental modules during ontogeny. At a macroevolutionary scale, these developmental modules could act as evolutionary units of change and contribute to the disparity in fin configurations. This study addresses fin disparity in a phylogenetic perspective, while focusing on the presence/absence and number of each of the median and paired fins.

RESULTS

Patterns of fin morphological disparity were assessed by mapping fin characters on a new phylogenetic supertree of fish orders. Among agnathans, disparity in fin configurations results from the sequential appearance of novel fins forming various combinations. Both median and paired fins would have appeared first as elongated ribbon-like structures, which were the precursors for more constricted appendages. Among chondrichthyans, disparity in fin configurations relates mostly to median fin losses. Among actinopterygians, fin disparity involves fin losses, the addition of novel fins (e.g., the adipose fin), and coordinated duplications of the dorsal and anal fins. Furthermore, some pairs of fins, notably the dorsal/anal and pectoral/pelvic fins, show non-independence in their character distribution, supporting expectations based on developmental and morphological evidence that these fin pairs form evolutionary modules.

CONCLUSIONS

Our results suggest that the pectoral/pelvic fins and the dorsal/anal fins form two distinct evolutionary modules, and that the latter is nested within a more inclusive median fins module. Because the modularity hypotheses that we are testing are also supported by developmental and variational data, this constitutes a striking example linking developmental, variational, and evolutionary modules.

'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.