The oldest gnathostome teeth.

Mandibular teeth and dentitions are features of jawed vertebrates that were first acquired by the Palaeozoic ancestors1-3 of living chondrichthyans and osteichthyans. The fossil record currently points to the latter part of the Silurian period4-7 (around 425 million years ago) as a minimum date for the appearance of gnathostome teeth and to the evolution of growth and replacement mechanisms of mandibular dentitions in the subsequent Devonian period2,8-10. Here we provide, to our knowledge, the earliest direct evidence for jawed vertebrates by describing Qianodus duplicis, a new genus and species of an early Silurian gnathostome based on isolated tooth whorls from Guizhou province, China. The whorls possess non-shedding teeth arranged in a pair of rows that demonstrate a number of features found in modern gnathostome groups. These include lingual addition of teeth in offset rows and maintenance of this patterning throughout whorl development. Our data extend the record of toothed gnathostomes by 14 million years from the late Silurian into the early Silurian (around 439 million years ago) and are important for documenting the initial diversification of vertebrates. Our analyses add to mounting fossil evidence that supports an earlier emergence of jawed vertebrates as part of the Great Ordovician Biodiversification Event (approximately 485-445 million years ago).

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses.

This review explores the long-standing question: 'Why do cardiovascular shunts occur?' An historical perspective is provided on previous research into cardiac shunts in vertebrates that continues to shape current views. Cardiac shunts and when they occur is then described for vertebrates. Nearly 20 different functional reasons have been proposed as specific causes of shunts, ranging from energy conservation to improved gas exchange, and including a plethora of functions related to thermoregulation, digestion and haemodynamics. It has even been suggested that shunts are merely an evolutionary or developmental relic. Having considered the various hypotheses involving cardiovascular shunting in vertebrates, this review then takes a non-traditional approach. Rather than attempting to identify the single 'correct' reason for the occurrence of shunts, we advance a more holistic, integrative approach that embraces multiple, non-exclusive suites of proposed causes for shunts, and indicates how these varied functions might at least co-exist, if not actually support each other as shunts serve multiple, concurrent physiological functions. It is argued that deposing the 'monolithic' view of shunting leads to a more nuanced view of vertebrate cardiovascular systems. This review concludes by suggesting new paradigms for testing the function(s) of shunts, including experimentally placing organ systems into conflict in terms of their perfusion needs, reducing sources of variation in physiological experiments, measuring possible compensatory responses to shunt ablation, moving experiments from the laboratory to the field, and using cladistics-related approaches in the choice of experimental animals.

The vertebrate diencephalic MCH system: a versatile neuronal population in an evolving brain.

Neurons synthesizing melanin-concentrating hormone (MCH) are described in the posterior hypothalamus of all vertebrates investigated so far. However, their anatomy is very different according to species: they are small and periventricular in lampreys, cartilaginous fishes or anurans, large and neuroendocrine in bony fishes, or distributed over large regions of the lateral hypothalamus in many mammals. An analysis of their comparative anatomy alongside recent data about the development of the forebrain, suggests that although very different, MCH neurons of the caudal hypothalamus are homologous. We further hypothesize that their divergent anatomy is linked to divergence in the forebrain - in particular telencephalic evolution.

The origin of the internal nostril of tetrapods.

The choana, a unique 'internal nostril' opening from the nasal sac into the roof of the mouth, is a key part of the tetrapod (land vertebrate) respiratory system. It was the first component of the tetrapod body plan to evolve, well before the origin of limbs, and is therefore crucial to our understanding of the beginning of the fish-tetrapod transition. However, there is no consensus on the origin of the choana despite decades of heated debate; some have claimed that it represents a palatally displaced external nostril, but others have argued that this is implausible because it implies breaking and rejoining the maxillary-premaxillary dental arcade and the maxillary branch of nerve V. The fossil record has not resolved the dispute, because the choana is fully developed in known tetrapod stem-group members. Here we present new material of Kenichthys, a 395-million-year-old fossil fish from China, that provides direct evidence for the origin of the choana and establishes its homology: it is indeed a displaced posterior external nostril that, during a brief transitional stage illustrated by Kenichthys, separated the maxilla from the premaxilla.

Distribution of muscimol, QNB, and 5HT binding in the vertebrate diencephalon: a comparative study of eight mammals and three non-mammals.

The distribution of muscimol, quinuclidinyl benzilate (QNB), and serotonin (5HT)-bound receptors in the diencephalon was examined by conventional receptor-binding methods in 11 species of amniotes including 2 reptiles, 1 bird, and 8 mammals, selected mostly on the basis of their differing last common ancestor with Anthropoids. We found that receptor binding can help define major subdivisions of the forebrain. The results show that in each of these species, the distribution of muscimol and QNB binding across the four major subdivisions of the diencephalon was consistent; densest in the dorsal thalamus, with hypothalamus and then either ventral thalamus or epithalamus with successively lesser amounts. However, the binding of serotonin (5HT) was most prevalent in the hypothalamus with equivalent amounts in the other diencephalic subdivisions. Myelin- and cell-stained materials showed that the pattern of high-density binding probably is not the secondary result of non-neurochemical factors such as differences in cell or neuropil density or in total available membrane. Perhaps more importantly, the receptor distributions suggest functional roles for major subdivisions across taxa. Results show that GABA-A and muscaranic Ach receptors are common in the dorsal diencephalon across vertebrate species and, therefore, are probably responsible for the gating of information to the cortex. Results show that serotonin is predominant in the hypothalamus. The lack of it in the dorsal thalamus indicates that it is probably not responsible for gating of information to the cortex. Results also show that in nonmammals the amount of GABA-A and muscaranic Ach differs from that found in mammals. For muscaranic Ach, the labeling in marsupials differs from that in placentals. Primates differ from other species (nonmammals and mammals combined) in the amount of 5HT found in the ventral diencephalon and the hypothalamus.

Origin and early evolution of vertebrate skeletonization.

Data from living and extinct faunas of primitive vertebrates imply very different scenarios for the origin and evolution of the dermal and oral skeletal developmental system. A direct reading of the evolutionary relationships of living primitive vertebrates implies that the dermal scales, teeth, and jaws arose synchronously with a cohort of other characters that could be considered unique to jawed vertebrates: the dermoskeleton is primitively composed of numerous scales, each derived from an individual dental papilla; teeth are primitively patterned such that they are replaced in a classical conveyor-belt system. The paleontological record provides a unique but complementary perspective in that: 1) the organisms in which the skeletal system evolved are extinct and we have no recourse but to fossils if we aim to address this problem; 2) extinct organisms can be classified among, and in the same way as, living relatives; 3) a holistic approach to the incorporation of all data provides a more complete perspective on early vertebrate evolution. This combined approach is of no greater significance than in dealing with the origin of the skeleton and, combined with recent discoveries and new phylogenetic analyses, we have been able to test and reject existing hypotheses for the origin of the skeleton and erect a new model in their place.

Two systems of giant axon terminals in the cat medial geniculate body: convergence of cortical and GABAergic inputs.

The thalamus plays a critical role in processing sensory information that involves interactions between extrinsic connections and intrinsic circuitry. Little is known regarding how these different systems might interact. We found an unexpected nuclear convergence of two types of giant axon terminals, each of which must have independent origins, in the dorsal division of the cat medial geniculate body. The first class of giant terminal was labeled after injections of biotinylated dextran amines (BDA) in seven auditory cortical areas. A second type was found in sections immunostained for gamma-aminobutyric acid (GABA); these endings had the same nuclear distribution, and they were numerous. The origin of this GABAergic terminal is unknown. The giant corticothalamic terminals were presumably those described in prior accounts using different tracers (Rouiller and de Ribaupierre [1990] Neurosci. Lett. 208:29-35; Ojima [1994] Cerebral Cortex 6:646-663), but with BDA they are labeled more fully. Clusters of such endings were often linked, and hundreds may occur in a single section. Their boutons formed a substantial proportion of the corticothalamic population. Other types of corticogeniculate axon terminals were also labeled, including two kinds that are much smaller and that match closely the classical descriptions of corticothalamic axons. The giant GABAergic endings were found in all dorsal division nuclei and in thalamic visual nuclei such as the lateral posterior nucleus. Like the giant cortical endings, the giant GABAergic terminals often encircled large, pale, immunonegative profiles that may be dendritic. This implies a close spatial, and perhaps a close functional, relationship between the populations of giant axon terminals. Insofar as physiological studies found that pharmacological inactivation of rat somatic sensory cortex suppresses peripheral information transmission through the posterior thalamus, corticofugal input may be essential for normal processing (Diamond et al. [1992] J. Comp. Neurol. 319:66-84). Our findings suggest that the giant corticothalamic endings could play an important role in descending control. Perhaps they are counterbalanced by a GABAergic system and affect thalamic oscillations implicated in shifts in vigilance and attention.

The dorsal thalamus of jawed vertebrates: a comparative viewpoint.

In anamniotes, the dorsal thalamus comprises: (1) a caudal division, the collothalamus, which receives its predominant input from the midbrain roof and projects ipsilaterally to the telencephalon, predominantly to the striatum, and (2) a rostral division, the lemnothalamus, which predominantly receives a direct retinal (lemniscal) input and projects bilaterally to the telencephalon, predominantly to the pallium. In amniotes, collothalamic nuclei relay visual, auditory, and somatosensory-multisensory inputs from the midbrain roof to the ipsilateral telencephalon, terminating in both striatum and pallium. For example, the collothalamic visual nuclei consist of the LP-pulvinar complex in mammals and nucleus rotundus in diapsid reptiles, birds, and turtles. Among amniotes, the latter nuclei are homologous to each other as discrete nuclei, as are the collothalamic auditory and collothalamic somatosensory-multisensory nuclei. Lemnothalamic nuclei (and nuclear groups) in amniotes predominantly (and/or plesiomorphically) receive lemniscal inputs; some project to the telencephalon bilaterally, and most, in contrast to collothalamic nuclei, do not project to the striatum. In mammals, the lemnothalamic nuclei include most of those in the anterior, medial, intralaminar, and ventral nuclear groups and the dorsal lateral geniculate nucleus. In diapsid reptiles, they include the dorsomedial and dorsolateral anterior nuclei and the dorsal lateral optic nucleus; comparable nuclei are present in birds and turtles, with birds additionally having a discrete somatosensory lemniscal relay nucleus. These lemnothalamic nuclei in each amniote radiation are homologous as a field to the lemnothalamus (i.e., nucleus anterior) in anamniotes. Both divisions of the dorsal thalamus were elaborated to some degree in the common ancestral amniote stock. A further major elaboration of the lemnothalamus characterized the ancestral stock of mammals and may have been one of the key events in early mammalian evolution. Birds have independently, to a lesser degree, elaborated the lemnothalamus.

[Retino-hypothalamic pathways in vertebrates]. / Retino-hypothalamische Verbindungen bei Wirbeltieren.

Forty years ago Hollwich (1948) introduced the conception of an "energetic portion of the visual pathway". Contributions to this conception of a direct connection of the retina with the hypothalamus accumulated since then and summarized here in tabular form give rise to the following conclusions: In fish the main hypothalamic termination of retinofugal axons is the nucleus hypothalamicus opticus. It may pass for the suprachiasmatic nucleus of fishes. In amphibians retino-hypothalamic fibres project to the area praeoptica. In reptiles retinofugal fibres innervate hypothalamic neuronal populations called either Nucleus suprachiasmaticus or Nucleus praeopticus. In birds retinohypothalamic axons project to a circumscribed anterior hypothalamic area termed "suprachiasmatic nucleus" by some authors. In mammals at last the main part of the retinohypothalamic tract terminates in the suprachiasmatic nuclei, especially favouring their caudal and ventrolateral parts. The interneuronal connections are axo-dendritic synapses of the Gray Types-I and II. Connections of retinal neurons, especially with suprachiasmatic hypothalamic nuclei or their homologues, are by now well established facts. They represent relatively constant and phylogenically stable components of the centripetal retinal projection. These projections are probably in all, certainly in most of the vertebrates bilateral. Some former but also newer methods of research (Stumpf and Sar, 1975) also depicted optic fibers which terminate in hypothalamic sites apart from the nucleus suprachiasmaticus and the area hypothalamica anterior (Conrad and Stumpf, 1975). A review of the literature on the existence of nerve fibers directly connecting the retina with the hypothalamus is tabulated.

Comparison of the pineal complex, retina and cerebrospinal fluid contacting neurons by immunocytochemical antirhodopsin reaction.

The presence of rhodopsin was investigated by an indirect immunocytochemical method in the pineal complex of various vertebrates (Carassius auratus, Cyprinus carpio, Hypophthalamichthys molitrix, Lucioperca lucioperca, Triturus vulgaris, Bombina bombina, Rana esculenta, Pseudemys scripta elegans, Lacerta agilis et viridis, white leghorn chickens, rat), in the retina of Lebistes reticulatus, Lucioperca lucioperca, Rana esculenta, Lacerta agilis, Pseudemys scripta elegans, the chicken and the rat, and the cerebrospinal (CSF) contacting neurons of Triturus vulgaris. The outer segments of the photoreceptor terminals of the pineal organ, frontal organ, parapineal organ of lower vertebrates and of the retina of the species investigated, were intensely stained with the antirhodopsin reaction. There was no significant positivity in the pineal organ of the reptiles, the chicken and the rat, and the parietal eye of the lizards. We failed to demonstrate any immunoreactive staining in the CSF contacting neurons of various hypothalamic areas and of the spinal cord. The light microscopic immunocytochemical results seem to contradict a photoreceptive role of the CSF contacting neurons and strengthen the view that the receptory cells of the pineal complex of lower vertebrates are involved in light perception by means of the visual pigment rhodopsin.

Cobalt iontophoresis techniques for tracing afferent and efferent connections in the vertebrate CNS.

In recent years several dye and cobalt iontophoresis techniques have been successfully used by invertebrate neurophysiologists for the localization of neuron somata and their processes. The cobalt iontophoresis technique has now been extended for use in the tracing of nerve fiber pathways and the localization of neuron somata in vertebrates. The brain and spinal cord of an animal are removed following perfusion with saline, and placed in a dish of cold saline. A suction electrode, filled with 300 mM cobalt chloride, is then placed over the cut end of the nerve trunk. Cobalt ions are then iontophoresed (by means of a voltage divider) within the nerve fibers, along their course. Following iontophoresis, the brain is bathed in an ammonium sulfide solution to precipitate the cobalt as black cobalt sulfide. The brain is then processed for histological procedures. A wide variety of vertebrates has been used, including amphibians, reptiles, aves and mammals, with uniform success. The cobalt iontophoresis technique presently in use has a wide range of applicability for neuroanatomical studies.

Comparative ultrastructure of cerebrospinal fluid-contacting neurons and pinealocytes.

The pinealocytes of fishes, amphibians, reptiles, birds and mammals have been compared with cerebrospinal fluid (CSF) contacting neurons. We found that the intraventricular dendrite terminal of the latter resembles the pinealocytic inner segment and that the atypical cilium (9x2+0 tubules) of the CSF contacting neurons is analogous with the outer segment of the pinealocytes, even though the outer segment bears photoreceptor lamellae in lower vertebrates. Regular, but small-sized photoreceptor outer segments were also found on pinealocytes of the chicken. In mammals, too, primitive outer segments are present in the form of 9x2 to cilia similar to those of CSF contacting dendritic terminals. In the Golgi areas of the perikarya of both cell types there are granulated vesicles which may contain transmitter substances and/or neurohormones. The synaptic junctions of the pinealocytes differ from those in the CSF contacting neurons. Many synapses occur on the latter, but they appear only rarely on pinealocytes. The axons of the CSF contacting neurons form synaptic connections with other cells, or terminate as neurohormonal synaptic hemidesmosomes on the basal lamina of the brain surface. The pinealocyte axons give rise to terminals containing synaptic ribbons. Such ribbons do not occur in CSF contacting neurons. In Lacertilians, we found pinealocytic terminals without ribbons on dendrite-like profiles. On the basis of the ultrastructural comparisons, we consider the CSF contacting neurons and pinealocytes to be very similar, but not to represent precisely the same cell type.