Rise and fall of †Pycnodontiformes: Diversity, competition and extinction of a successful fish clade

Alternating regimes of shallow and deep-sea diversification explain a species-richness paradox in marine fishes

The deep sea contains a surprising diversity of life, including iconic fish groups such as anglerfishes and lanternfishes. Still, >65% of marine teleost fish species are restricted to the photic zone <200 m, which comprises less than 10% of the ocean's total volume. From a macroevolutionary perspective, this paradox may be explained by three hypotheses: 1) shallow water lineages have had more time to diversify than deep-sea lineages, 2) shallow water lineages have faster rates of speciation than deep-sea lineages, or 3) shallow-to-deep sea transition rates limit deep-sea richness. Here we use phylogenetic comparative methods to test among these three non-mutually exclusive hypotheses. While we found support for all hypotheses, the disparity in species richness is better described as the uneven outcome of alternating phases that favored shallow or deep diversification over the past 200 million y. Shallow marine teleosts became incredibly diverse 100 million y ago during a period of warm temperatures and high sea level, suggesting the importance of reefs and epicontinental settings. Conversely, deep-sea colonization and speciation was favored during brief episodes when cooling temperatures increased the efficiency of the ocean's carbon pump. Finally, time-variable ecological filters limited shallow-to-deep colonization for much of teleost history, which helped maintain higher shallow richness. A pelagic lifestyle and large jaws were associated with early deep-sea colonists, while a demersal lifestyle and a tapered body plan were typical of later colonists. Therefore, we also suggest that some hallmark characteristics of deep-sea fishes evolved prior to colonizing the deep sea.

Rising from the Ashes: The Biogeographic Origins of Modern Coral Reef Fishes

During the excavation of Mayan tombs, little did the archaeologists know that the fossils they discovered in the tomb stones would fundamentally alter our understanding of the earliest origins of coral reef fishes. Located just 500 kilometers from the point where an asteroid impact reconfigured the world's biological systems 66 million years ago, we find the earliest origins of three typical reef fish groups. Their presence in Mexico just 3 million years after this impact finally reconciles the conflict between the fossil and phylogenetic evidence for the earliest origins of reef fishes. The incorporation of these fossils into a global reconstruction of fish evolutionary history reveals a new picture of the early biogeography of reef fishes, with strong Atlantic links. From locations associated with biological destruction and societal collapse, we see evidence of the origins of one of the world's most diverse and spectacular marine ecosystems: coral reefs.

A new hybodontiform shark (Strophodus Agassiz 1838) from the Lower Cretaceous (Valanginian-Hauterivian) of Colombia

The vertebrate marine faunas that inhabited northern South America during the Cretaceous are still poorly known. This study is a contribution to a growing wave of new studies on Lower Cretaceous vertebrates from Colombia. Here we report and describe a new species of a hybodontiform shark of the genus Strophodus, which we named Strophodus rebecae sp. nov., based on isolated teeth, that were collected in Valanginian-Hauterivian rocks of the Rosa Blanca Formation (Carrizal and El Sapo Members) near the town of Zapatoca, Santander Department, Andes of Colombia. In addition, we describe two other fragmented teeth assigned to Strophodus sp. from the Rosa Blanca Fm. The new species from Colombia represents the only Cretaceous record of Strophodus from Gondwana, offering new insights into the paleogeographic distribution of the genus, as well as increasing the knowledge about the scarce hybodontiform paleodiversity known from South America. The presence of Strophodus in the Rosa Blanca Formation suggests that these durophagous (shell-crushing) fishes played an important role as predators of the abundant and diverse invertebrate fauna present in these ancient tropical coastal ecosystems of Gondwana.

Marine biology: Ageing a 'living fossil'

A novel approach to determine the lifespan of an iconic fish, the coelacanth Latimeria chalumnae, reveals that they can live up to a hundred years. This re-focuses attention on the need to measure life-history traits that determine the demographics of fish populations.

Fossilized cell structures identify an ancient origin for the teleost whole-genome duplication

Teleost fishes comprise one-half of all vertebrate species and possess a duplicated genome. This whole-genome duplication (WGD) occurred on the teleost stem lineage in an ancient common ancestor of all living teleosts and is hypothesized as a trigger of their exceptional evolutionary radiation. Genomic and phylogenetic data indicate that WGD occurred in the Mesozoic after the divergence of teleosts from their closest living relatives but before the origin of the extant teleost groups. However, these approaches cannot pinpoint WGD among the many extinct groups that populate this 50- to 100-million-y lineage, preventing tests of the evolutionary effects of WGD. We infer patterns of genome size evolution in fossil stem-group teleosts using high-resolution synchrotron X-ray tomography to measure the bone cell volumes, which correlate with genome size in living species. Our findings indicate that WGD occurred very early on the teleost stem lineage and that all extinct stem-group teleosts known so far possessed duplicated genomes. WGD therefore predates both the origin of proposed key innovations of the teleost skeleton and the onset of substantial morphological diversification in the clade. Moreover, the early occurrence of WGD allowed considerable time for postduplication reorganization prior to the origin of the teleost crown group. This suggests at most an indirect link between WGD and evolutionary success, with broad implications for the relationship between genomic architecture and large-scale evolutionary patterns in the vertebrate Tree of Life.

Rise and fall of †Pycnodontiformes: Diversity, competition and extinction of a successful fish clade

†Pycnodontiformes was a successful lineage of primarily marine fishes that broadly diversified during the Mesozoic. They possessed a wide variety of body shapes and were adapted to a broad range of food sources. Two other neopterygian clades possessing similar ecological adaptations in both body morphology (†Dapediiformes) and dentition (Ginglymodi) also occurred in Mesozoic seas. Although these groups occupied the same marine ecosystems, the role that competitive exclusion and niche partitioning played in their ability to survive alongside each other remains unknown. Using geometric morphometrics on both the lower jaw (as constraint for feeding adaptation) and body shape (as constraint for habitat adaptation), we show that while dapediiforms and ginglymodians occupy similar lower jaw morphospace, pycnodontiforms are completely separate. Separation also occurs between the clades in body shape so that competition reduction between pycnodontiforms and the other two clades would have resulted in niche partitioning. Competition within pycnodontiforms seemingly was reduced further by evolving different feeding strategies as shown by disparate jaw shapes that also indicate high levels of plasticity. Acanthomorpha was a teleostean clade that evolved later in the Mesozoic and which has been regarded as implicated in driving the pycnodontiforms to extinction. Although they share similar body shapes, no coeval acanthomorphs had similar jaw shapes or dentitions for dealing with hard prey like pycnodontiforms do and so their success being a factor in pycnodontiform extinction is unlikely. Sea surface temperature and eustatic variations also had no impact on pycnodontiform diversity patterns according to our results. Conversely, the occurrence and number of available reefs and hardgrounds as habitats through time seems to be the main factor in pycnodontiform success. Decline in such habitats during the Late Cretaceous and Palaeogene might have had deleterious consequences for pycnodontiform diversity. Acanthomorphs occupied the niches of pycnodontiforms during the terminal phase of their existence.