Vertebrate History Research Articles

Comparative Vertebrate Evolutionary Analyses of Type I Collagen: Potential of COL1a1 Gene Structure and Intron Variation for Common Bone-Related Diseases

Collagen type I alpha 1 (COL1a1), which encodes the primary subunit of type I collagen, the main structural and most abundant protein in vertebrates, harbors hundreds of mutations linked to human diseases like osteoporosis and osteogenesis imperfecta. Previous studies have attempted to predict the phenotypic severity associated with type I collagen mutations, yet an evolutionary analysis that compares historical and recent selective pressures, including across noncoding regions, has never been conducted. Here, we use a comparative genomic and species evolutionary analysis representing ∼450 My of vertebrate history to investigate functional constraints associated with both exons and introns of the >17-kb COL1a1 gene. We find that although the COL1a1 amino acid sequence is highly conserved, there are both spatial and temporal signatures of varying selective constraint across protein domains. Furthermore, sites of high evolutionary constraint significantly correlate with the location of disease-associated mutations, the latter of which also cluster with respect to specific severity classes typically categorized in clinical studies. Finally, we find that COL1a1 introns are significantly short in length with high GC content, patterns that are shared across highly diverged vertebrates, and which may be a signature of strong stabilizing selection for high COL1a1 gene expression. In conclusion, although previous studies focused on COL1a1 coding regions, the current results implicate introns as areas of high selective constraint and targets of bone-related phenotypic variation. From a broader perspective, our comparative evolutionary approach provides further resolution to models predicting mutations associated with bone-related function and disease severity.

Evolution of paired fins and the lateral somitic frontier

Recent research on the evolutionary origins of the paired fins (pectoral and pelvic, evolving into the four tetrapod limbs) has focused on genetic, developmental, and fossil evidence. However, a combined synthesis of this evidence has largely been lacking. In living animals, identification of dorsal and ventrolateral zones of fin-producing competence, the distinction between primaxial and abaxial regions separated by the lateral somitic frontier, and the recognition that fin developmental mechanisms could have been co-opted from unpaired to paired fins can be used to evaluate vertebrate fin evolution, including fossil forms. These fossil taxa dominate early vertebrate history and possess an unusually wide variety of fin or fin-like structures; establishing homologies with the paired fins of jawed vertebrates has been challenging, in part owing to the absence of supporting girdles in most jawless vertebrates. An evolutionary scenario is proposed where these fin-like structures develop from somites, in the same manner as dorsal, unpaired fins. Girdles and the paired fins of osteostracans and jawed vertebrates develop from lateral plate mesoderm when the lateral somitic frontier and the abaxial region of the body evolve.

End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates

The Devonian marks a critical stage in the early evolution of vertebrates: It opens with an unprecedented diversity of fishes and closes with the earliest evidence of limbed tetrapods. However, the latter part of the Devonian has also been characterized as a period of global biotic crisis marked by two large extinction pulses: a "Big Five" mass extinction event at the Frasnian-Famennian stage boundary (374 Ma) and the less well-documented Hangenberg event some 15 million years later at the Devonian-Carboniferous boundary (359 Ma). Here, we report the results of a wide-ranging analysis of the impact of these events on early vertebrate evolution, which was obtained from a database of vertebrate occurrences sampling over 1,250 taxa from 66 localities spanning Givetian to Serpukhovian stages (391 to 318 Ma). We show that major vertebrate clades suffered acute and systematic effects centered on the Hangenberg extinction involving long-term losses of over 50% of diversity and the restructuring of vertebrate ecosystems worldwide. Marine and nonmarine faunas were equally affected, precluding the existence of environmental refugia. The subsequent recovery of previously diverse groups (including placoderms, sarcopterygian fish, and acanthodians) was minimal. Tetrapods, actinopterygians, and chondrichthyans, all scarce within the Devonian, undergo large diversification events in the aftermath of the extinction, dominating all subsequent faunas. The Hangenberg event represents a previously unrecognized bottleneck in the evolutionary history of vertebrates as a whole and a historical contingency that shaped the roots of modern biodiversity.

Vertebrate extinctions and reorganizations during the Late Silurian Lau Event

During the Silurian Period, as shown here by data from Gotland, Sweden, and other regions of the Baltoscandian paleocontinent, fish faunas were struck hard by extinctions caused by the late Ludlow Lau Event, also known for being associated with the largest positive stable carbon isotope excursion of the Phanerozoic. This event had a profound impact on the early evolutionary history of vertebrates, wiping out two-thirds of the fish taxa and causing significant ecological reorganizations over an estimated time span of similar to 200 ka. Our data show that immediately prior to the event, jawed acanthodians dominated the fish faunas, whereas the event led to a diverse fauna and a brief but marked dominance of the jawless thelodonts. The stepwise changes observed mimic those of conodonts, suggesting a similar mode of life and response to atmospheric oceanic perturbations for these clades. (Less)

The tail end of hummingbird evolution: parallel flight system development in living and ancient birds

Evolutionary innovations are central to debates about biological uniformitarianism because their very novelty implies a distinct evolutionary dynamic. Traditional scenarios for innovations in the development of avian powered flight exemplify the kinds of distinctions considered to occur at different times during the history of innovations. Thus, the progressive change of the wing stroke mechanism early in its evolution is considered to have imposed strong functional and historical constraints on tail shape diversity, whereas attainment of the modern flight stroke mechanism is considered to have liberated the tail to radiate into a wide variety of other functions and forms. Detailed analyses of living hummingbirds revealed that these highly aerial birds actually expressed many parallel functional constraints and historically progressive patterns observed earlier in avian history: (1) more basal lineages had relatively weak wing muscles (patagial muscles and tendons, TPB), convex to square tails, and more linear flight employed in nonterritorial foraging; (2) more derived lineages had a stronger TPB, forked tails, accentuated growth of tail fork, and more manoeuvrable and agile flight employed in territorial foraging; and (3) the most derived lineage had the strongest TPB, greatly reduced tails, and mainly bee-like flight. These associations make functional sense because convex tails increase stability and efficiency in linear flight, concave tails augment lift for turning flight in territorial defence, and tails become aerodynamically disadvantageous if the wings provide sufficient lift. Derived hummingbird lineages also demonstrated the same expansion of tail shape and taxonomic diversity associated with perfection of the modern wing stroke mechanism earlier in avian history. Thus, living hummingbirds are a microcosm of overall avian flight evolution. Other living avian (‘aerial courser') and extinct reptilian (Pterosaur) clades with extraordinary flight abilities provide evidence for similar patterns, suggesting a broadly defined uniformitarianism (early constraint followed by later radiation) at the limits of the flight performance envelope throughout vertebrate history. Correlated evolution of TPB and tail form suggests that natural selection on an integrated flight system was the principal mechanism fostering the avian patterns, although strengthening of wing muscles in derived lineages may have facilitated expansion of caudal morphological diversity through a balance between natural and sexual selection on males. These findings suggest that wing muscles, locomotor integration, and phylogenetic patterns are essential for understanding function and adaptation of tails in living as well as ancient birds. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97, 467–493.

An Ancient Gene Network Is Co-opted for Teeth on Old and New Jaws

Vertebrate dentitions originated in the posterior pharynx of jawless fishes more than half a billion years ago. As gnathostomes (jawed vertebrates) evolved, teeth developed on oral jaws and helped to establish the dominance of this lineage on land and in the sea. The advent of oral jaws was facilitated, in part, by absence of hox gene expression in the first, most anterior, pharyngeal arch. Much later in evolutionary time, teleost fishes evolved a novel toothed jaw in the pharynx, the location of the first vertebrate teeth. To examine the evolutionary modularity of dentitions, we asked whether oral and pharyngeal teeth develop using common or independent gene regulatory pathways. First, we showed that tooth number is correlated on oral and pharyngeal jaws across species of cichlid fishes from Lake Malawi (East Africa), suggestive of common regulatory mechanisms for tooth initiation. Surprisingly, we found that cichlid pharyngeal dentitions develop in a region of dense hox gene expression. Thus, regulation of tooth number is conserved, despite distinct developmental environments of oral and pharyngeal jaws; pharyngeal jaws occupy hox-positive, endodermal sites, and oral jaws develop in hox-negative regions with ectodermal cell contributions. Next, we studied the expression of a dental gene network for tooth initiation, most genes of which are similarly deployed across the two disparate jaw sites. This collection of genes includes members of the ectodysplasin pathway, eda and edar, expressed identically during the patterning of oral and pharyngeal teeth. Taken together, these data suggest that pharyngeal teeth of jawless vertebrates utilized an ancient gene network before the origin of oral jaws, oral teeth, and ectodermal appendages. The first vertebrate dentition likely appeared in a hox-positive, endodermal environment and expressed a genetic program including ectodysplasin pathway genes. This ancient regulatory circuit was co-opted and modified for teeth in oral jaws of the first jawed vertebrate, and subsequently deployed as jaws enveloped teeth on novel pharyngeal jaws. Our data highlight an amazing modularity of jaws and teeth as they coevolved during the history of vertebrates. We exploit this diversity to infer a core dental gene network, common to the first tooth and all of its descendants.

Birth of the jawed vertebrates

The discovery of embryos in certain fossil fishes not only shows that internal fertilization and live birth evolved early in vertebrate history, but also raises questions about the origin of jawed vertebrates.

The lateral somitic frontier in ontogeny and phylogeny

The vertebrate musculoskeletal system comprises the axial and appendicular systems. The postcranial axial system consists of the vertebrae, ribs and associated muscles, and the appendicular system comprises the muscles and skeleton of the paired appendages and their respective girdles. The morphology, proportions, and arrangements of these parts have undergone tremendous variation during vertebrate history. Despite this vertebrate diversity, the cells that form all of the key parts of the musculoskeletal system during development arise from two populations of embryonic mesoderm, the somites and somatic lateral plate. Nowicki et al. (2003. Mech Dev 120:227-240) identified two dynamic domains in the developing chick embryo. The primaxial domain is populated exclusively by cells from the somites. The abaxial domain includes muscle and bone that develop within lateral plate-derived connective tissue. The boundary between the two domains is the lateral somitic frontier. We hypothesize that the primaxial and abaxial domains are patterned independently and that morphological evolution of the musculoskeletal system is facilitated by partially independent developmental changes in the abaxial and primaxial domain. Here we present our hypothesis in detail and review recent experimental and comparative studies that use the concept of the lateral somitic frontier in the analysis of the evolution of the highly derived chelonian and limbless squamate body plans.

Nucleotide diversity of the homoeologous adh1 loci in the American allotetraploid Oryza species

Polyploidy is a prominent process in plants and has been significant in the evolutionary history of vertebrates and other eukaryotes. Polyploidy induces genome-wide duplication. Studies on nucleotide diversity at homoeologous loci may provide insight into the evolutionary dynamics of duplicate genes. In the present study, we estimated the levels and patterns of nucleotide diversity for alcohol dehydrogenase 1 (adh1) loci in three American allotetraploid Oryza species: O. alta, O. grandiglumis, and O. latifolia. Higher nucleotide diversity was revealed in the C genome. This result suggested the different evolutionary dynamics of homoeologous adh1 loci in these allotetraploid Oryza species.

First Dinosaur Tracks from the Arabian Peninsula

BackgroundThe evolutionary history of Mesozoic terrestrial vertebrates from the Arabian Peninsula is virtually unknown. Despite vast exposures of rocky outcrops, only a handful of fossils have yet been described from the region. Here we report a multi-taxon dinosaur track assemblage near Madar village, 47 km north of Sana'a, Republic of Yemen. This represents the first dinosaur tracksite from the Arabian Peninsula, and the only multi-taxon dinosaur ichnosite in the Middle East.Methodology/FindingsMeasurements were taken directly from trackway impressions, following standard ichnological conventions. The presence of bipedal trackmakers is evidenced by a long series of pes imprints preserving smoothly rounded posterior margins, no evidence of a hallux, bluntly rounded digit tips and digital divarication angles characteristic of ornithopod dinosaurs. Nearby, eleven parallel quadrupedal trackways document a sauropod herd that included large and small individuals traveling together. Based on the morphology of manus impressions along with a narrow-gauged stance, the quadrupedal trackways were made by non-titanosauriform neosauropods. Additional isolated tracks and trackways of sauropod and ornithopod dinosaurs are preserved nearby.Conclusions/SignificanceTaken together, these discoveries present the most evocative window to date into the evolutionary history of dinosaurs of the Arabian Peninsula. Given the limited Mesozoic terrestrial record from the region, this discovery is of both temporal and geographic significance, and massive exposures of similarly-aged outcrops nearby offer great promise for future discoveries.

Major transitions in vertebrate evolution

Acknowledgments Introduction: Studying Evolutionary Transitions among Vertebrates Hans-Dieter Sues and Jason S. Anderson 1. Plasticity of and Transitions between Skeletal Tissues in Vertebrate Evolution and Development Brian K. Hall and P. Eckard Witten 2. Homologies and Evolutionary Transitions in Early Vertebrate History Philippe Janvier 3. Paired Fins of Jawless Vertebrates and Their Homologies across the Agnathan-Gnathostome Transition Mark V. H. Wilson, Gavin F. Hanke, and Tiiu Marss 4. MODEs of Developmental Evolution: An Example with the Origin and Definition of the Autopodium Hans C. E. Larsson 5. Incorporating Ontogeny into the Matrix: A Phylogenetic Evaluation of Developmental Evidence for the Origin of Modern Amphibians Jason S. Anderson 6. The Cranial Anatomy of Basal Diadectomorphs and the Origin of Amniotes Robert R. Reisz 7. Snake Phylogeny, Origins, and Evolution: The Role, Impact, and Importance of Fossils (18692006) Michael W. Caldwell 8. The Beginnings of Birds: Recent Discoveries, Ongoing Arguments, and New Directions Luis M. Chiappe and Gareth J. Dyke 9. Successive Diversifications in Early Mammalian Evolution Zhe-Xi Luo 10. The Terrestrial to Aquatic Transition in Cetacea Mark D. Uhen Contributors Index

Functional diversification of the toll-like receptor gene family

Phylogenetic analyses supported the hypothesis that the vertebrate toll-like receptors (TLRs) include two very ancient groups that arose by gene duplication prior to the divergence of protostomes and deuterostomes: (1) the TLR1 family (including mammalian TLR1, TLR2, TLR6, and TLR10); and (2) a clade including the remainder of mammalian TLRs. Correlating data on ligand type, subcellular localization, and gene expression in leukocytes and other tissues with the phylogeny provided evidence that certain major functional specializations within the TLRs occurred after ancient gene duplication events and that these traits have been retained through further events of gene duplication. For example, the recognition of bacterial lipoproteins appears to have arisen in the ancestor of the TLR1 family and continues to characterize members of that family whose ligands are known. Likewise, expression on the endosomal membrane and the recognition of nucleic acids appears to have been arisen in the ancestor of the TLR7 family and some related TLRs. On the other hand, gene expression patterns across tissues appear to have been much more volatile over the evolution of the vertebrate TLRs, since genes may show expression profiles similar to those of distantly related genes but dissimilar to those of closely related genes. Thus, the vertebrate TLRs provide an example of a multi-gene family in which gene duplication has been followed by extensive changes in certain aspects of gene function, while others have been conserved throughout vertebrate history.

MicroRNAs and the advent of vertebrate morphological complexity

The causal basis of vertebrate complexity has been sought in genome duplication events (GDEs) that occurred during the emergence of vertebrates, but evidence beyond coincidence is wanting. MicroRNAs (miRNAs) have recently been identified as a viable causal factor in increasing organismal complexity through the action of these approximately 22-nt noncoding RNAs in regulating gene expression. Because miRNAs are continuously being added to animalian genomes, and, once integrated into a gene regulatory network, are strongly conserved in primary sequence and rarely secondarily lost, their evolutionary history can be accurately reconstructed. Here, using a combination of Northern analyses and genomic searches, we show that 41 miRNA families evolved at the base of Vertebrata, as they are found and/or detected in lamprey, but not in either ascidians or amphioxus (or any other nonchordate taxon). When placed into temporal context, the rate of miRNA acquisition and the extent of phenotypic evolution are anomalously high early in vertebrate history, far outstripping any other episode in chordate evolution. The genomic position of miRNA paralogues in humans, together with gene trees incorporating lamprey orthologues, indicates that although GDEs can account for an increase in the diversity of miRNA family members, which occurred before the last common ancestor of all living vertebrates, GDEs cannot account for the origin of these novel families themselves. We hypothesize that lying behind the origin of vertebrate complexity is the dramatic expansion of the noncoding RNA inventory including miRNAs, rather than an increase in the protein-encoding inventory caused by GDEs.

Cyclostome embryology and early evolutionary history of vertebrates

Modern agnathans include only two groups, the lampreys and the hagfish, that collectively comprise the group Cyclostomata. Although accumulating molecular data support the cyclostomes as a monophyletic group, there remain some unsettled questions regarding the evolutionary relationships of these animals in that they differ greatly in anatomical and developmental patterns and in their life histories. In this review, we summarize recent developmental data on the lamprey and discuss some questions related to vertebrate evolutionary development raised by the limited information available on hagfish embryos. Comparison of the lamprey and gnathostome developmental patterns suggests some plesiomorphic traits of vertebrates that would have already been established in the most recent common ancestor of the vertebrates. Understanding hagfish development will further clarify the, as yet, unrecognized ancestral characters that either the lampreys or hagfishes may have lost. We stress the immediate importance of hagfish embryology in the determination of the most plausible scenario for the early history of vertebrate evolution, by addressing questions about the origins of the neural crest, thyroid, and adenohypophysis as examples.

Chitinase family GH18: evolutionary insights from the genomic history of a diverse protein family

BackgroundChitinases (EC.3.2.1.14) hydrolyze the β-1,4-linkages in chitin, an abundant N-acetyl-β-D-glucosamine polysaccharide that is a structural component of protective biological matrices such as insect exoskeletons and fungal cell walls. The glycoside hydrolase 18 (GH18) family of chitinases is an ancient gene family widely expressed in archea, prokaryotes and eukaryotes. Mammals are not known to synthesize chitin or metabolize it as a nutrient, yet the human genome encodes eight GH18 family members. Some GH18 proteins lack an essential catalytic glutamic acid and are likely to act as lectins rather than as enzymes. This study used comparative genomic analysis to address the evolutionary history of the GH18 multiprotein family, from early eukaryotes to mammals, in an effort to understand the forces that shaped the human genome content of chitinase related proteins.ResultsGene duplication and loss according to a birth-and-death model of evolution is a feature of the evolutionary history of the GH18 family. The current human family likely originated from ancient genes present at the time of the bilaterian expansion (approx. 550 mya). The family expanded in the chitinous protostomes C. elegans and D. melanogaster, declined in early deuterostomes as chitin synthesis disappeared, and expanded again in late deuterostomes with a significant increase in gene number after the avian/mammalian split.ConclusionThis comprehensive genomic study of animal GH18 proteins reveals three major phylogenetic groups in the family: chitobiases, chitinases/chitolectins, and stabilin-1 interacting chitolectins. Only the chitinase/chitolectin group is associated with expansion in late deuterostomes. Finding that the human GH18 gene family is closely linked to the human major histocompatibility complex paralogon on chromosome 1, together with the recent association of GH18 chitinase activity with Th2 cell inflammation, suggests that its late expansion could be related to an emerging interface of innate and adaptive immunity during early vertebrate history.

Hagfish embryology with reference to the evolution of the neural crest

Hagfish, which lack both jaws and vertebrae, have long been the subject of intense interest owing to their position at a crucial point in the evolutionary transition to a truly vertebrate body plan. However, unlike the comparatively well characterized vertebrate agnathan lamprey, little is known about hagfish development. The inability to analyse hagfish at early embryonic stages has frustrated attempts to resolve questions with important phylogenetic implications, including fundamental ones relating to the emergence of the neural crest. Here we report the obtainment of multiple pharyngula-stage embryos of the hagfish species Eptatretus burgeri and our preliminary analyses of their early development. We present histological evidence of putative neural crest cells, which appear as delaminated cells that migrate along pathways corresponding to neural crest cells in fish and amphibians. Molecular cloning studies further revealed the expression of several regulatory genes, including cognates of Pax6, Pax3/7, SoxEa and Sox9, suggesting that the hagfish neural crest is specified by molecular mechanisms that are general to vertebrates. We propose that the neural crest emerged as a population of de-epithelialized migratory cells in a common vertebrate ancestor, and suggest that the possibility of classical and molecular embryology in hagfish opens up new approaches to clarifying the evolutionary history of vertebrates.

Vertebrate Vitellogenin Gene Duplication in Relation to the “3R Hypothesis”: Correlation to the Pelagic Egg and the Oceanic Radiation of Teleosts

The spiny ray-finned teleost fishes (Acanthomorpha) are the most successful group of vertebrates in terms of species diversity. Their meteoric radiation and speciation in the oceans during the late Cretaceous and Eocene epoch is unprecedented in vertebrate history, occurring in one third of the time for similar diversity to appear in the birds and mammals. The success of marine teleosts is even more remarkable considering their long freshwater ancestry, since it implies solving major physiological challenges when freely broadcasting their eggs in the hyper-osmotic conditions of seawater. Most extant marine teleosts spawn highly hydrated pelagic eggs, due to differential proteolysis of vitellogenin (Vtg)-derived yolk proteins. The maturational degradation of Vtg involves depolymerization of mainly the lipovitellin heavy chain (LvH) of one form of Vtg to generate a large pool of free amino acids (FAA 150–200 mM). This organic osmolyte pool drives hydration of the ooctye while still protected within the maternal ovary. In the present contribution, we have used Bayesian analysis to examine the evolution of vertebrate Vtg genes in relation to the “3R hypothesis” of whole genome duplication (WGD) and the functional end points of LvH degradation during oocyte maturation. We find that teleost Vtgs have experienced a post-R3 lineage-specific gene duplication to form paralogous clusters that correlate to the pelagic and benthic character of the eggs. Neo-functionalization allowed one paralogue to be proteolyzed to FAA driving hydration of the maturing oocytes, which pre-adapts them to the marine environment and causes them to float. The timing of these events matches the appearance of the Acanthomorpha in the fossil record. We discuss the significance of these adaptations in relation to ancestral physiological features, and propose that the neo-functionalization of duplicated Vtg genes was a key event in the evolution and success of the teleosts in the oceanic environment.

Investigating hearing in sturgeon: Getting closer to the origin of hearing in vertebrates

It has been hypothesized that hearing arose early in vertebrate evolution to use sound to glean information about the environment, including the presence of predators and prey. The importance of sound to sense the acoustic scene is especially true underwater where the visual horizon is limited. To create an acoustic scene from sound, an animal needs to localize and identify sound sources. We hypothesize that the sound localization capabilities of primitive bony fishes such as sturgeon are as capable as those of modern bony fishes such as goldfish and, perhaps, many terrestrial vertebrates. We present results from experiments investigating the neural mechanisms of directional hearing and frequency analysis in the auditory periphery of the sturgeon. Extracellular single units were recorded from the eighth nerve during stimulation with a three‐axis shaker system that subjects the fish ear to linear accelerations in known directions at various frequencies. This system enables the characterization of both spatial and frequency tuning of the afferents innervating the ear. The results show that similar neural mechanisms for encoding sound direction and frequency exist in the periphery of modern and primitive bony fish, thereby supporting the hypothesis that basic auditory functions arose very early in vertebrate history.

Origin of dental occlusion in tetrapods: signal for terrestrial vertebrate evolution?

Evolutionary changes of the dentition in tetrapods can be associated with major events in the history of terrestrial vertebrates. Dental occlusion, the process by which teeth from the upper jaw come in contact with those in the lower jaw, appears first in the fossil record in amniotes and their close relatives near the Permo-Carboniferous boundary approximately 300 million years ago. This evolutionary innovation permitted a dramatic increase in the level of oral processing of food in these early tetrapods, and has been generally associated with herbivory. Whereas herbivory in extinct vertebrates is based on circumstantial evidence, dental occlusion provides direct evidence about feeding strategies because jaw movements can be reconstructed from the wear patterns of the teeth. Examination of the evolution of dental occlusion in Paleozoic tetrapods within a phylogenetic framework reveals that this innovation developed independently in several lineages of amniotes, and is represented by a wide range of dental and mandibular morphologies. Dental occlusion also developed within diadectomorphs, the sister taxon of amniotes. The independent, multiple acquisition of this feeding strategy represents an important signal in the evolution of complex terrestrial vertebrate communities, and the first steps in the profound changes in the pattern of trophic interactions in terrestrial ecosystems.

Central connections of anamniote auditory otolith endorgans

The ascending auditory pathways of anamniotes (fish and amphibians) are organized similarly to those of amniotes (reptiles, birds, and mammals). A common pattern for the flow of acoustic information—inner ear to first-order nuclei to higher-order auditory nuclei in the medulla, midbrain, thalamus, and cerebrum—probably evolved early in vertebrate history and was retained in extant fish and in terrestrial vertebrates (with taxon-specific modifications) despite variability in inner-ear acoustic receptors. Otolith endorgans subserve hearing in jawed fish; auditory fibers segregate from otolith endorgan vestibular fibers and supply auditory divisions of functionally mixed (auditory-vestibular) first-order nuclei. Amphibian auditory otolith endorgans are primarily seismic detectors complementing new sound-pressure receptors of the amphibian and basilar papillae. Otolithic and papillar auditory endorgans supply common areas within functionally mixed (auditory-vestibular) first-order nuclei. Uniquely in anurans, they also supply a discrete first-order auditory nucleus analogous to the amniote cochlear nuclei. Discrete first-order auditory nuclei are correlated with the evolution of a tympanic ear, an independently evolved condition in anurans and among amniotes. Thus, at the first-order level, the amphibian auditory pathway retains primitive organizational features present in fish auditory pathways. Convergent neuroanatomical specializations in anurans and amniotes may reflect common functional requirements.