Biology subjects

Ortega-Hernandez, J.

Publications and source records attributed to Ortega-Hernandez, J..

10 recordsLinked to original sources

bioRxiv2026

The trilobite nervous system and the early evolution of the mandibulate central complex

Fossil central nervous systems (CNS) have illuminated several facets of total-group euarthropod evolution, including their phylogenetic affinities, the origin of the head, and their ancestral neuroanatomical organization. However, the CNS remains unknown in trilobites, the most diverse and earliest group of Palaeozoic euarthropods, but can potentially clarify fundamental questions that remain disputed despite thousands of described species and countless studies spanning over two centuries, such as their affinities relative to modern representatives. Here, we describe the CNS of the cheirurid Anacheirurus adserai from the Lower Ordovician Fezouata Shale. Anacheirurus features a brain with well-developed proto, deuto- and tritocerebral neuromeres, an anteriomedian protocerebral protrusion, a central complex, an esophageal foramen, and a ganglionated ventral nerve cord. The CNS is replicated in framboidal iron that differs in texture and composition from the exoskeleton and rock matrix. The overall organization of Anacheirurus's CNS most closely resembles that of Cambrian stem-group mandibulates, and critically shares the presence of a central complex with a protocerebral bridge and lateral accessory lobes with extant pancrustaceans. Our findings provide new morphological support for the hypothesis that Trilobita belongs to the mandibulate stem lineage and bring new insights into the ancestral morphology and evolutionary history of the euarthropod central complex. More broadly, our findings offer direct anatomical evidence of the key sensorimotor system innovations that originated during the Cambrian Information Revolution.

paleontology↗

bioRxiv2026

Surface area calculations of lamellar support respiratory function of trilobite exopodites

Trilobites had biramous appendages with an inner endopodite (walking leg) and outer exopodite (gill) connected to the body through the protopodite (limb base). Whereas both endopodite and protopodite were involved in both locomotion and feeding, the exopodite has been subject to various functional interpretations including respiration, ventilation and swimming. Evidence from sites with exceptional fossil preservation indicate that trilobite exopodites show substantial variability in terms of the number and size of their articles, lamellae and setae, but the implications of this morphological diversity have never been investigated. Here, we created anatomically correct 3D models of exopodites in O. serratus and T. eatoni to calculate the SA of the lamellae and explore its relationship with body size. Our results indicate a large SA for O. serratus at 16,589 mm2 compared to the 2,159 mm2 for the much smaller T. eatoni. We also calculated lamellar SA for nine additional trilobite species with exceptionally well-preserved appendages based on lamellar measurements. The results indicate that lamellae SA of trilobites increased exponentially with overall body size. Trilobite data follows the same trendline of gill SA/biomass observed in extant species and thus supports the interpretation of their exopodites as respiratory structures despite substantial variation in morphology.

paleontology↗

bioRxiv2025

Hallucigenia's diet illuminates the feeding ecology of Cambrian lobopodians

The armoured lobopodian Hallucigenia sparsa embodies the seemingly uncanny nature of the animals that evolved during the Cambrian Explosion over 500 million years ago. Initially regarded as an evolutionary oddball, the exceptional preserved anatomy of H. sparsa has been substantially revised, leading to a better understanding of its relationships with other lobopodians and phylogenetic affinities with extant panarthropod phyla. However, the ecology and behaviour of H. sparsa remain largely enigmatic owing to the difficulties of interpreting its functional morphology and the perceived lack of modern analogues. Restudy of a composite fossil assemblage from the middle Cambrian Burgess Shale demonstrates swarm-like behaviour of several small H. sparsa individuals scavenging on a dead ctenophore. The lack of grasping, masticatory, or piercing mouthparts in H. sparsa points to suction feeding as a viable strategy to consume the gelatinous carcass. Reassessment of the morphology of H. sparsa reveals functional analogues shared with extant pycnogonids, including an elongate anterior end with a terminal mouth opening and enlarged foregut chambers lined up with sclerotized denticles. These observations suggest that suction feeding is ancestral for stem-group Onychophora and highlights the critical trophic role of small-bodied armoured lobopodians as degraders of soft-bodied carcasses in Cambrian benthic ecosystems.

paleontology↗

bioRxiv2025

Predator-prey scaling laws support a suspension-feeding lifestyle in Cambrian lobopodians

The early Paleozoic saw a dramatic diversification of shelly epibenthic metazoans adapted to suspension and filter-feeding, but the extent to which these radiations affected the evolution of non-biomineralized suspension-feeding taxa is uncertain because these organisms are not typically well represented in the fossil record. Luolishaniids are a highly derived and disparate clade of (typically) armoured lobopodians widely interpreted as having a suspension-feeding ecology based on the presence of five or six anterior pairs of setulose appendages. Luolishaniids are globally widespread and represent the only Cambrian non-biomineralized free living epibenthic bilaterians regarded as having a suspension feeding mode of life, but their proposed ecology only relies on a qualitative interpretation of their functional morphology. Here we test the hypothesis that the setulose appendages of luolishaniids were adapted for a suspension feeding function. Quantitative morphological comparisons reveal a positive and statistically significant relationship between body length and the mesh spacing of the setulose anterior limbs of luolishaniids. This pattern correlates with suspension-feeding scaling in extant aquatic organisms, and suggests that luolishaniids primarily fed on mesoplanktonic seston. We provide quantitative evidence for suspension-feeding in luolishaniids, which represents the first statistically supported example of modern-like predator-prey scaling patterns observed in Cambrian soft-bodied metazoans.

paleontology↗

bioRxiv2025

A Quantitative Review of Biodiversity Trends during the Great Ordovician Biodiversification Event

The Great Ordovician Biodiversification Event (GOBE) embodies the most dramatic increase of marine biodiversity and escalation of macroecological complexity during the early Phanerozoic. Despite its critical role, the precise timing and duration of the GOBE remain controversial. Numerous palaeobiological studies have attempted to quantify the GOBE based on estimates of species richness through time for various groups of marine organisms. However, these studies use fossil data restricted to specific geographic regions or employ disparate methodologies that preclude direct analytical comparisons. We present a meta-analysis of Ordovician biodiversity that integrates information from multiple temporal, geographic, and ecological scales. We collate 98 datasets from 54 publications to analyze temporally standardized rates of marine species biodiversity accumulation between the latest Cambrian and throughout the entire Ordovician using an effect-size approach. Our results indicate statistically significant high rates of sustained species accumulation that can be traced from the late Cambrian and until the Middle Ordovician, stabilization during the Late Ordovician and then a precipitous decline caused by the Late Ordovician Mass Extinction. Geographic scale (global vs regional) has no significant bearing on rates of biodiversification, with the only exception observed during the Dapingian-Darriwilian transition, supporting the hypothesis of mass dispersal of generalists during the Early Ordovician. Benthic and suspension-feeding organisms show high rates of biodiversity accumulation throughout most of the Ordovician (Tremadocian-Sandbian), whereas the diversification of nektonic, pelagic and predatory/scavenger organisms was mostly restricted to the Early Ordovician.

paleontology↗

bioRxiv2023

Convergent evolution of ventral adaptations for enrollment in trilobites and extant euarthropods

The ability to enroll for protection is an effective defensive strategy that has convergently evolved multiple times in disparate animal groups ranging from euarthropods to mammals. Enrollment is an evolutionary staple of trilobites, and their biomineralized dorsal exoskeleton offers a versatile substrate for the evolution of interlocking devices. However, it is unknown whether trilobites also featured ventral adaptations for enrolment. Here, we report ventral exoskeletal adaptations that facilitate enrollment in exceptionally preserved trilobites from the Upper Ordovician Walcott-Rust Quarry in New York State, USA. Walcott-Rust trilobites reveal the intricate three-dimensional organization of the non-biomineralized ventral anatomy preserved as calcite casts, including the spatial relationship between the articulated sternites (i.e., ventral exoskeletal plates) and the wedge-shaped protopodites. Enrollment in trilobites is achieved by ventrally dipping the anterior margin of the sternites during trunk flexure, facilitated by the presence of flexible membranes, and the close coupling of the wedge-shaped protopodites. Comparisons with the ventral morphology of extant glomerid millipedes and terrestrial isopods reveal similar mechanisms used for enrollment. The wedge-shaped protopodites of trilobites closely resemble the gnathobasic coxa/protopodite of extant horseshoe crabs. We propose that the trilobites wedge-shaped protopodite simultaneously facilitates tight enrollment and gnathobasic feeding with the trunk appendages.

paleontology↗

bioRxiv2023

Fossil calibrations for molecular analyses and divergence time estimation for true crabs (Decapoda: Brachyura)

True crabs, or Brachyura, comprise over 7,600 known species and are among the most ecologically dominant, economically significant, and popularly recognized group of extant crustaceans. There are over 3,000 fossil brachyuran species known from mid and upper Jurassic, Cretaceous, and Cenozoic deposits across the globe, many of them preserved in exquisite detail, but the origins and early evolution of true crabs remain unresolved. This uncertainty hinders the identification of the stratigraphically earliest occurrence of major brachyuran groups in the fossil record, obscuring our understanding of their phylogenetic relationships and thus the ability to estimate divergence times to answer large scale macroevolutionary questions. Here, we present 36 vetted fossil node calibration points for molecular phylogenetic analysis of crabs (one Anomura and 35 Brachyura) and reassess the earliest occurrences of several key clades based on recent fossil discoveries or re-examination of previous studies. For each calibrated node, we provide the minimum and tip maximum ages for the stratigraphically oldest fossil that can be reliably assigned to the group. Disentangling the anatomical disparity of fossil forms and their phylogenetic relationships is crucial to recognize the earliest branching members among brachyuran groups. This represents a critical first step understanding the evolution of carcinization and decarcinization in true crabs, the appearance of key adaptations, and the transition from sea to land and freshwater. The identification of reliable fossils for deep time calibrations, both as tips and nodes, is pivotal to ensure not only precise but more accurate divergence time estimations when reconstructing the crab tree of life. PLAIN LANGUAGE SUMMARYWe present 36 vetted fossil calibration points for molecular phylogenetic analysis of crabs (one Anomura and 35 Brachyura) and reassess the earliest occurrences of several key groups based on recent fossil discoveries or re-examination of previous studies, together with discussions for each taxon. We also provide some general observations and recommendations on fossil age selection and stratigraphic considerations. The identification of reliable fossils for deep time calibrations, both as tips and nodes, is pivotal to ensure not only precise but more accurate divergence time estimations when reconstructing phylogenetic trees.

paleontology↗

bioRxiv2022

Convergent adaptation of true crabs (Decapoda: Brachyura) to a gradient of terrestrial environments

For much of terrestrial biodiversity, the evolutionary pathways of adaptation from marine ancestors are poorly understood, and have usually been viewed as a binary trait. True crabs, the decapod crustacean infraorder Brachyura, comprise over 7,600 species representing a striking diversity of morphology and ecology, including repeated adaptation to non-marine habitats. Here, we reconstruct the evolutionary history of Brachyura using new and published sequences of 10 genes for 344 tips spanning 88 of 109 brachyuran families. Using 36 newly vetted fossil calibrations, we infer that brachyurans most likely diverged in the Triassic, with family-level splits in the late Cretaceous and early Paleogene. By contrast, the root age is underestimated with automated sampling of 328 fossil occurrences explicitly incorporated into the tree prior, suggesting such models are a poor fit under heterogeneous fossil preservation. We apply recently defined trait-by-environment associations to classify a gradient of transitions from marine to terrestrial lifestyles. We estimate that crabs left the marine environment at least seven and up to 17 times convergently, and returned to the sea from non-marine environments at least twice. Although the most highly terrestrial- and many freshwater-adapted crabs are concentrated in Thoracotremata, Bayesian threshold models of ancestral state reconstruction fail to identify shifts to higher terrestrial grades due to the degree of underlying change required. Lineages throughout our tree inhabit intertidal and marginal marine environments, corroborating the inference that the early stages of terrestrial adaptation have a lower threshold to evolve. Our framework and extensive new fossil and natural history datasets will enable future comparisons of non-marine adaptation at the morphological and molecular level. Crabs provide an important window into the early processes of adaptation to novel environments, and different degrees of evolutionary constraint that might help predict these pathways.

evolutionary biology↗

bioRxiv2021

Before trilobite legs: Pygmaclypeatus daziensis reconsidered and the ancestral appendicular organization of Cambrian artiopods

The Cambrian Stage 3 Chengjiang biota in South China is one of the most influential Konservat-Lagerstatten worldwide thanks to the fossilization of diverse non-biomineralizing organisms through pyritization. Despite their contributions to understanding the evolution of early animals, several Chengjiang species remain poorly known due to their scarcity and/or incomplete preservation. Here, we use micro-computed tomography to reveal in detail the ventral appendage organization of the enigmatic non-trilobite artiopod Pygmaclypeatus daziensis - one of the rarest euarthropods in Chengjiang - and explore its functional ecology and broader evolutionary significance. Pygmaclypeatus daziensis possesses a set of uniramous antennae and 14 pairs of post-antennal biramous appendages, the latter of which show an unexpectedly high degree of heteronomy based on the localized differentiation of the protopodite, endopodite and exopodite along the antero-posterior body axis. The small body size (less than 2 cm), presence of delicate spinose endites, and well-developed exopodites with multiple paddle-shaped lamellae on the appendages of P. daziensis indicate a nekto-benthic mode of life, and a scavenging/detritus feeding strategy. Pygmaclypeatus daziensis shows that appendage heteronomy is phylogenetically widespread within Artiopoda - the megadiverse clade that includes trilobites and their relatives with non-biomineralizing exoskeletons - and suggests that a single exopodite lobe with paddle-like lamellae is ancestral for this clade. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=155 SRC="FIGDIR/small/456779v1_ufig1.gif" ALT="Figure 1"> View larger version (111K): org.highwire.dtl.DTLVardef@b4ddaaorg.highwire.dtl.DTLVardef@44f686org.highwire.dtl.DTLVardef@1950095org.highwire.dtl.DTLVardef@1596739_HPS_FORMAT_FIGEXP M_FIG C_FIG Cover image: Morphological reconstruction of the non-trilobite artiopod Pygmaclypeatus daziensis from the early Cambrian (Stage 3) Chengjiang biota in south China. Artwork by Holly Sullivan (https://www.sulscientific.com/).

paleontology↗

bioRxiv2017

A primitive starfish ancestor from the Early Ordovician of Morocco reveals the origin of crown group Echinodermata

The somasteroids are Ordovician star-shaped animals widely regarded as ancestors of Asterozoa, the group of extant echinoderms that includes brittle stars and starfish. The phylogenetic position of somasteroids makes them critical for understanding the origin and early evolution of crown group Echinodermata. However, the early evolution of asterozoans, the origin of their distinctive body organization and their relationships with other Cambrian and Ordovician echinoderms, such as edrioasteroids, blastozoans, crinoids, and other asterozoans, remain problematic due to the difficulties of comparing the calcitic endoskeleton of these disparate groups. Here we describe the new somasteroid Cantabrigiaster fezouataensis from the Early Ordovician (Tremadocian) Fezouata Lagerstatte in Morocco. Cantabrigiaster shares with other somasteroids the presence of rod-like virgal ossicles that articulate with the ambulacrals, but differs from all other known asterozoans in the absence of adambulacral ossicles defining the arm margins. The unique arm construction evokes parallels with non-asterozoan echinoderms. Developmentally informed Bayesian and parsimony based phylogenetic analyses, which reflect the homology of the biserial ambulacral ossicles in Paleozoic echinoderms according to the Extraxial-Axial Theory, recover Cantabrigiaster as basal within stem group Asterozoa. Our results indicate that Cantabrigiaster is the earliest diverging stem group asterozoan, revealing the ancestral morphology of this major clade and clarifying the affinities of problematic Ordovician taxa. Somasteroids are resolved as a paraphyletic grade within stem and crown group Asterozoa (starfishes), whereas stenuroids are paraphyletic within stem group Ophiuroidea (brittle stars). Cantabrigiaster also illuminates the relationship between Ordovician crown group Echinodermata and its Cambrian stem lineage, which includes sessile forms with incipient radial symmetry such as edrioasteroids and blastozoans. The contentious Pelmatozoa hypothesis (i.e. monophyly of blastozoans and crinoids) is not supported; instead, blastozoans represent the most likely sister-taxon of crown group Echinodermata.\n\nAuthor summaryStarfish and brittle stars, collectively known as asterozoans, constitute a diverse and ecologically successful group of echinoderms that first appear in the fossil record some 480Ma. However, the early evolution of asterozoans, the origin of their distinctive body organization, and their phylogenetic relationships with Cambrian echinoderms remain largely unresolved. We describe Cantabrigiaster fezouataensis gen. et sp. nov., a primitive asterozoan from the Fezouata Lagerstatte, Morocco, with a unique endoskeletal arm organization that reveals the ancestral morphology of this major clade. Bayesian and parsimony based phylogenetic analyses indicate that Cantabrigiaster is the earliest diverging stem group asterozoan, and resolve the phylogenetic position of Ordovician asterozoans such as somasteroids. Our analyses clarify the origin of crown group echinoderms relative to their problematic Cambrian stem group representatives.

paleontology↗