Estuarine fish and tetrapod evolution: insights from a Late Devonian (Famennian) Gondwanan estuarine lake and a southern African Holocene equivalent

The early diversification of ray-finned fishes (Actinopterygii): hypotheses, challenges and future prospects

Actinopterygii makes up half of living vertebrate diversity, and study of fossil members during their Palaeozoic rise to dominance has a long history of descriptive work. Although research interest into Palaeozoic actinopterygians has increased in recent years, broader patterns of diversity and diversity dynamics remain critically understudied. Past studies have investigated macroevolutionary trends in Palaeozoic actinopterygians in a piecemeal fashion, variably using existing compendia of vertebrates or literature-based searches. Here, we present a comprehensive occurrence-based dataset of actinopterygians spanning the whole of the Palaeozoic. We use this to produce the first through-Palaeozoic trends in genus and species counts for Actinopterygii. Diversity through time generally tracks metrics for sampling, while major taxonomic problems pervading the Palaeozoic actinopterygian record obscure diversity trends. Many described species are concentrated in several particularly problematic 'waste-basket' genera, hiding considerable morphological and taxonomic diversity. This taxonomic confusion also feeds into a limited understanding of phylogenetic relationships. A heavy sampling bias towards Europe and North America exists in both occurrence databases and available phylogenetic matrices, with other regions underrepresented despite yielding important data. Scrutiny of the extent to which spatial biases influence the actinopterygian record is lacking, as is research on other forms of bias. Low richness in some time periods may be linked to geological biases, while the effects of taphonomic biases on Palaeozoic actinopterygians have not yet been investigated. Efforts are already underway both to redescribe poorly defined taxa and to describe taxa from underrepresented regions, helping to address taxonomic issues and accuracy of occurrence data. New methods of sampling standardisation utilising up-to-date occurrence databases will be critical in teasing apart biological changes in diversity and those resulting from bias. Lastly, continued phylogenetic work will enable the use of phylogenetic comparative methods to elucidate the origins of actinopterygian biogeography and subsequent patterns of radiation throughout their rise to dominate aquatic faunas.

A high latitude Gondwanan species of the Late Devonian tristichopterid Hyneria (Osteichthyes: Sarcopterygii)

We describe the largest bony fish in the Late Devonian (late Famennian) fossil assemblage from Waterloo Farm near Makhanda/Grahamstown, South Africa. It is a giant member of the extinct clade Tristichopteridae (Sarcopterygii: Tetrapodomorpha) and most closely resembles Hyneria lindae from the late Famennian Catskill Formation of Pennsylvania, USA. Notwithstanding the overall similarity, it can be distinguished from H. lindae on a number of morphological points and is accordingly described as a new species, H. udlezinye sp. nov. The preserved material comprises most of the dermal skull, lower jaw, gill cover and shoulder girdle. The cranial endoskeleton appears to have been unossified and is not preserved, apart from a fragment of the hyoid arch adhering to a subopercular, but the postcranial endoskeleton is represented by an ulnare, some semi-articulated neural spines, and the basal plate of a median fin. The discovery of H. udlezinye shows that Hyneria is a cosmopolitan genus extending into the high latitudes of Gondwana, not a Euramerican endemic. It supports the contention that the derived clade of giant tristichopterids, which alongside Hyneria includes such genera as Eusthenodon, Edenopteron and Mandageria, originated in Gondwana.

Fossil bone histology reveals ancient origins for rapid juvenile growth in tetrapods

Patterns of growth throughout the lifetime of an animal reflect critical life history traits such as reproductive timing, physiology, and ecological interactions. The ancestral growth pattern for tetrapods has traditionally been described as slow-to-moderately paced, akin to modern amphibians, with fast growth and high metabolic rates considered a specialized physiological trait of amniotes. Here, we present bone histology from an ontogenetic series of the Early Carboniferous stem tetrapod Whatcheeria deltae, and document evidence of fibrolamellar bone-primary bone tissue associated with fast growth. Our data indicate that Whatcheeria juveniles grew rapidly and reached skeletal maturity quickly, allowing them to occupy a large-bodied predator niche in their paleoenvironment. This life history strategy contrasts with those described for other stem tetrapods and indicates that a diversity of growth patterns existed at the origins of tetrapod diversification. Importantly, Whatcheeria marks an unexpectedly early occurrence of fibrolamellar bone in Tetrapoda, both temporally and phylogenetically. These findings reveal that elevated juvenile growth is not limited to amniotes, but has a deep history in the tetrapod clade and may have played a previously unrecognized role in the tetrapod invasion of land.

Coombs Hill: A Late Devonian fossil locality in the Witpoort Formation (Witteberg Group, South Africa)

Coombs Hill, a new fossil locality in the Witpoort Formation (Witteberg Group) of South Africa, preserves a record of Famennian (Late Devonian) life in Gondwana. Fossil plants collected at Coombs Hill are preliminarily assigned to several classes. Shelly invertebrates include a variety of bivalve mollusc forms, some of which appear to be preserved in life position. Biodiversity at Coombs Hill is comparable to that of the well-known Waterloo Farm lagerstätte in ordinal diversity, but exhibits differences in species composition. Ongoing taxonomic analysis will provide a rare window into the ecology of high-latitude environments during this pivotal stage of Earth history, which immediately preceded the end-Devonian extinction. Sandstone dominated sedimentary facies at Coombs Hill suggest a high-energy coastal marine setting, with brackish back-barrier estuarine/lagoonally derived fossiliferous mudstones. Exact stratigraphic placement within the Witpoort Formation is hampered by structural deformation, and precise age comparisons with Waterloo Farm are currently tenuous.Significance: A new fossil locality at Coombs Hill comprises the second known site with a suite of well preserved continental and marginal marine fossils from the Witpoort Formation, providing an exceptionally rare example of high-latitude life during the critical latest Devonian Famennian age. Several new plant taxa will be diagnosed from this locality, which also gives important insights into the morphology of Archaeopteris notosaria, South Africa’s earliest known tree. Discovery of a second palaeontologically significant site in the Witpoort Formation provides impetus for further structural and sedimentary facies analyses to align the unit with datable global eustatic events, and to clarify its internal chronology.

Non-ammocoete larvae of Palaeozoic stem lampreys

Ammocoetes-the filter-feeding larvae of modern lampreys-have long influenced hypotheses of vertebrate ancestry^1-7. The life history of modern lampreys, which develop from a superficially amphioxus-like ammocoete to a specialized predatory adult, appears to recapitulate widely accepted scenarios of vertebrate origin. However, no direct evidence has validated the evolutionary antiquity of ammocoetes, and their status as models of primitive vertebrate anatomy is uncertain. Here we report larval and juvenile forms of four stem lampreys from the Palaeozoic era (Hardistiella, Mayomyzon, Pipiscius, and Priscomyzon), including a hatchling-to-adult growth series of the genus Priscomyzon from Late Devonian Gondwana. Larvae of all four genera lack the defining traits of ammocoetes. They instead display features that are otherwise unique to adult modern lampreys, including prominent eyes, a cusped feeding apparatus, and posteriorly united branchial baskets. Notably, phylogenetic analyses find that these non-ammocoete larvae occur in at least three independent lineages of stem lamprey. This distribution strongly implies that ammocoetes are specializations of modern-lamprey life history rather than relics of vertebrate ancestry. These phylogenetic insights also suggest that the last common ancestor of hagfishes and lampreys was a macrophagous predator that did not have a filter-feeding larval phase. Thus, the armoured 'ostracoderms' that populate the cyclostome and gnathostome stems might serve as better proxies than living cyclostomes for the last common ancestor of all living vertebrates.

The oldest Devonian circumpolar ray-finned fish?

Actinopterygians (ray-finned fishes) are the most diverse group of living fishes, but have a sparse Devonian fossil record restricted to low palaeolatitudes. Here we report a new actinopterygian from the Paraná Basin of Brazil, which occupied a circumpolar position in the Palaeozoic. Available geological evidence supports a Middle Devonian or older age for this taxon, which shares features of the mandibular symphysis with the latest Devonian Tegeolepis. A phylogenetic analysis resolves these two as sister taxa. This new record expands the palaeogeographic distribution of Devonian ray-fins and suggests that gaps in their fossil record might be filled by exploring poorly sampled high-latitude localities within the Malvinokaffric Realm.