Humberto G. Ferrón

The evolution of gigantism in active marine predators

Abstract A novel hypothesis to better understand the evolution of gigantism in active marine predators and the diversity of body sizes, feeding strategies and thermophysiologies of extinct and living aquatic vertebrates is proposed. Recent works suggest that some aspects of animal energetics can act as constraining factors for body size. Given that mass-specific metabolic rate decreases with body mass, the body size of active predators should be limited by the high metabolic demand of this feeding strategy. In this context, we propose that shifts towards higher metabolic levels can enable the same activity and feeding strategy to be maintained at bigger body sizes, offering a satisfactory explanation for the evolution of gigantism in active predators, including a vast quantity of fossil taxa. Therefore, assessing the metabolic ceilings of living aquatic vertebrates and the thermoregulatory strategies of certain key extinct groups is now crucial to define the energetic limits of predation and provide quantitative support for this model.

Squamation and ecology of thelodonts

Thelodonts are an enigmatic group of Paleozoic jawless vertebrates that have been well studied from taxonomical, biostratigraphic and paleogeographic points of view, although our knowledge of their ecology and mode of life is still scant. Their bodies were covered by micrometric scales whose morphology, histology and the developmental process are extremely similar to those of extant sharks. Based on these similarities and on the well-recognized relationship between squamation and ecology in sharks, here we explore the ecological diversity and lifestyles of thelodonts. For this we use classic morphometrics and discriminant analysis to characterize the squamation patterns of a significant number of extant shark species whose ecology is well known. Multivariate analyses have defined a characteristic squamation pattern for each ecological group, thus establishing a comparative framework for inferring lifestyles in thelodonts. We then use this information to study the squamation of the currently described 147 species of thelodonts, known from both articulated and disarticulated remains. Discriminant analysis has allowed recognizing squamation patterns comparable to those of sharks and links them to specific ecological groups. Our results suggest a remarkable ecological diversity in thelodonts. A large number of them were probably demersal species inhabiting hard substrates, within caves and crevices in rocky environments or reefs, taking advantage of the flexibility provided by their micromeric squamations. Contrary to classical interpretations, only few thelodonts were placed among demersal species inhabiting sandy and muddy substrates. Schooling species with defensive scales against ectoparasites could be also abundant suggesting that social interactions and pressure of ectoparasites were present in vertebrates as early the Silurian. The presence of species showing scales suggestive of low to moderate speed and a lifestyle presumably associated with open water environments indicates adaptation of thelodonts to deep water habitats. Scale morphology suggests that some other thelodonts were strong-swimming pelagic species, most of them radiating during the Early Devonian in association with the Nekton Revolution.

Lonchidion derenzii, sp. nov., a New Lonchidiid Shark (Chondrichthyes, Hybodontiforms) from the Upper Triassic of Spain, with Remarks on Lonchidiid Enameloid

http://zoobank.org/urn:lsid:zoobank.org:pub:0A620E00-7B8D-4D7C-8BEE-7063C7BA0204 Citation for this article: Manzanares, E., C. Pla, C. Martínez-Pérez, H. Ferrón, and Héctor Botella. 2016. Lonchidion derenzii, sp. nov., a new lonchidiid shark (Chondrichthyes, Hybodontiforms) from the Upper Triassic of Spain, with remarks on lonchidiid enameloid. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1253585.

A Serravallian (Middle Miocene) shark fauna from Southeastern Spain and its palaeoenvironment significance

Abstract The study of a new Serravallian (Middle Miocene) locality from the Southeastern Spain has yielded a shark assemblage characterized by microremains of at least seven taxa (Deania calcea, †Isistius triangulus, †Squaliolus cf. S. schaubi, †Paraetmopterus sp., Pristiophorus sp., Scyliorhinus sp. and a cf. Squaliformes indet) of three different orders (Squaliformes, Pristiophoriformes and Carcharhiniformes). In addition, associated macroremains have also been found, including teeth of †Cosmopolitodus hastalis, Isurus sp., Hemipristis serra, Odontaspis sp., Carcharhinus spp. and †Otodus (Megaselachus) megalodon. The assemblage contains taxa with disparate environmental preferences including not only neritic and epipelagic sharks but also an important number of meso and bathypelagic representatives. The migration of deep water taxa to shallower waters through submarine canyons/coastal upwelling is proposed as the most plausible cause for explaining the origin of such assemblage. Interestingly, the composition of the deep-water taxa here reported contrast with the chondrichthyans assemblages from the Pliocene and extant Mediterranean communities. This entails a complex biogeographic history, where the Messinian salinity crisis strongly affected the posterior evolution of the Mediterranean ecosystems but some other factors, such us the existence of anoxic events during the Quaternary, could have also played an important role.

Middle-Late Triassic chondrichthyans remains from the Betic Range (Spain)

PurposeIn the present study, we described, for first time, the chondrichthyan fauna from several Middle-Late Triassic sections in the Betic Domain and compare it with other recent described coeval faunas from the Iberian Ranges.MethodsSpecimens were retrieved after the dissolution (with 10% acetic acid) of carbonate rocks.ResultsThe assemblage comprises of seven species belonging to six genera (Hybodus plicatilis, Omanoselache bucheri comb. nov., Omanoselache contrarius comb. nov., Lonchidion derenzii, Lissodus aff. L. lepagei, Pseudodalatias henarejensis and cf. Rhomaleodus budurovi), most of them non-nesoselachian. Chondrichthyans remains occur in levels dating from Ladinian to Carnian according with bivalves, ammonoids and conodonts.ConclusionsThe findings are comparable, in taxonomical terms, to the chondrichthyan fauna from the Ladinian of the Iberian Range that was recently described, although chondrichthyans seems noticeably less abundant in the Betic Domain. Most of the species found occur also in the Iberian Range, with the exception of Lonchidion derenzii and cf Rhomaleodus budurovi, which occur in the Boyar Section, dated as Carnian. The small size of all teeth recovered, belonging probably to young specimens, suggest that the very shallow epicontinental environments recorded in Middle-Upper Triassic rocks from the Betic Domain could be used as nursery areas.ResumenObjetivoEn el presente estudio describimos por primera vez, la fauna de condrictios de diversas secciones del Triásico Medio-Tardío en el Dominio Bético y lo comparamos con otras faunas parecidas descritas recientemente en la Cordillera Ibérica.MetodologíaLos ejemplares fueron recuperados después de la disolución (con ácido acético al 10%) de rocas carbonatadas.ResultadosLa asociación se compone de siete especies pertenecientes a seis géneros (Hybodus plicatilis, Omanoselache bucheri comb. nov., Omanoselache contrarius comb. nov., Lonchidion derenzii, Lissodus aff. L. lepagei, Pseudodalatias henarejensis y cf. Rhomaleodus budurovi), la mayoría de ellos no neoseláceos. Los niveles donde aparecen los restos de condrictios datan del Ladiniense hasta el Carniense según las asociaciones de bivalvos, amonoideos y conodontos.ConclusionesEstos hallazgos son comparables, en términos taxonómicos, a la fauna de condrictios del Ladiniense de la Cordillera Ibérica que fue descrita recientemente, aunque éstos parecen ser menos abundantes en el Dominio Bético. Muchas de las especies encontradas aquí también aparecen en la cordillera Ibérica, con las excepciones de Lonchidion derenzii y cf Rhomaleodus budurovi que aparecen en la sección Boyar, datada como Carniense. El pequeño tamaño de los dientes recuperados, pertenecientes a individuos juveniles, sugieren que los ambientes epicontinentales someros registrados en las rocas del Triásico Medio-Superior del Dominio Bético podrían haber sido usadas como zonas de cría.

Regional endothermy as a trigger for gigantism in some extinct macropredatory sharks

Otodontids include some of the largest macropredatory sharks that ever lived, the most extreme case being Otodus (Megaselachus) megalodon. The reasons underlying their gigantism, distribution patterns and extinction have been classically linked with climatic factors and the evolution, radiation and migrations of cetaceans during the Paleogene. However, most of these previous proposals are based on the idea of otodontids as ectothermic sharks regardless of the ecological, energetic and body size constraints that this implies. Interestingly, a few recent studies have suggested the possible existence of endothermy in these sharks thus opening the door to a series of new interpretations. Accordingly, this work proposes that regional endothermy was present in otodontids and some closely related taxa (cretoxyrhinids), playing an important role in the evolution of gigantism and in allowing an active mode of live. The existence of regional endothermy in these groups is supported here by three different approaches including isotopic-based approximations, swimming speed inferences and the application of a novel methodology for assessing energetic budget and cost of swimming in extinct taxa. In addition, this finding has wider implications. It calls into question some previous paleotemperature estimates based partially on these taxa, suggests that the existing hypothesis about the evolution of regional endothermy in fishes requires modification, and provides key evidence for understanding the evolution of gigantism in active macropredators.

Obruchevacanthus ireneae gen. et sp. nov., a new ischnacanthiform (Acanthodii) from the Lower Devonian of Spain

New disarticulated material of typically ischnacanthid scales, tooth whorls, and ?dentigerous jaw bones that occur recurrently together at many levels of the Lower Devonian of the Iberian Chain (Spain) is described. Based on their stratigraphical occurrence, histological evidence and comparison with similar ischnacanthid assemblages from other localities, we suggest including all of these remains in a unique natural assemblage, Obruchevacanthus ireneae gen. et sp. nov. This new taxon shares some features with Trundlelepis cervicostulata from the Lower Devonian of New South Wales (Australia), as for example the presence of a poorly developed pore-canal system in their scales. This feature suggests close phylogenetic relationship between the two taxa, supporting their assignment to the family Poracanthodidae. However, as this porecanal system is only present in a few percents of the total scales and it is very poorly developed in both Obruchevacanthus ireneae gen. et sp. nov. and Trundlelepis cervicostulata, they could represent a derived group of poracanthodids. These new data provided here increase our knowledge of the taxonomic diversity and evolution of the order Ischnacanthiformes, being so far the only ischnacanthid present at the studied area.

Ecomorphological inferences in early vertebrates: reconstructing Dunkleosteus terrelli (Arthrodira, Placodermi) caudal fin from palaeoecological data

Our knowledge about the body morphology of many extinct early vertebrates is very limited, especially in regard to their post-thoracic region. The prompt disarticulation of the dermo-skeletal elements due to taphonomic processes and the lack of a well-ossified endoskeleton in a large number of groups hinder the preservation of complete specimens. Previous reconstructions of most early vertebrates known from partial remains have been wholly based on phylogenetically closely related taxa. However, body design of fishes is determined, to a large extent, by their swimming mode and feeding niche, making it possible to recognise different morphological traits that have evolved several times in non-closely related groups with similar lifestyles. Based on this well-known ecomorphological correlation, here we propose a useful comparative framework established on extant taxa for predicting some anatomical aspects in extinct aquatic vertebrates from palaeoecological data and vice versa. For this, we have assessed the relationship between the locomotory patterns and the morphological variability of the caudal region in extant sharks by means of geometric morphometrics and allometric regression analysis. Multivariate analyses reveal a strong morphological convergence in non-closely related shark species that share similar modes of life, enabling the characterization of the caudal fin morphology of different ecological subgroups. In addition, interspecific positive allometry, affecting mainly the caudal fin span, has been detected. This phenomenon seems to be stronger in sharks with more pelagic habits, supporting its role as a compensation mechanism for the loss of hydrodynamic lift associated with the increase in body size, as previously suggested for many other living and extinct aquatic vertebrates. The quantification of shape change per unit size in each ecological subgroup has allowed us to establish a basis for inferring not only qualitative aspects of the caudal fin morphology of extinct early vertebrates but also to predict absolute values of other variables such as the fin span or the hypocercal and heterocercal angles. The application of this ecomorphological approach to the specific case of Dunkleosteus terrelli has led to a new reconstruction of this emblematic placoderm. Our proposal suggests a caudal fin with a well-developed ventral lobe, narrow peduncle and wide span, in contrast to classical reconstructions founded on the phylogenetic proximity with much smaller placoderms known from complete specimens. Interestingly, this prediction gains support with the recent discovery of fin distal elements (ceratotrichia) in a well preserved D. terrelli, which suggests a possible greater morphological variability in placoderm caudal fins than previously thought.