Saccorhytus is an early ecdysozoan and not the earliest deuterostome.

The early history of deuterostomes, the group composed of the chordates, echinoderms and hemichordates1, is still controversial, not least because of a paucity of stem representatives of these clades2-5. The early Cambrian microscopic animal Saccorhytus coronarius was interpreted as an early deuterostome on the basis of purported pharyngeal openings, providing evidence for a meiofaunal ancestry6 and an explanation for the temporal mismatch between palaeontological and molecular clock timescales of animal evolution6-8. Here we report new material of S. coronarius, which is reconstructed as a millimetric and ellipsoidal meiobenthic animal with spinose armour and a terminal mouth but no anus. Purported pharyngeal openings in support of the deuterostome hypothesis6 are shown to be taphonomic artefacts. Phylogenetic analyses indicate that S. coronarius belongs to total-group Ecdysozoa, expanding the morphological disparity and ecological diversity of early Cambrian ecdysozoans.

A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group.

BACKGROUND: Vetulicolians are one of the most problematic and controversial Cambrian fossil groups, having been considered as arthropods, chordates, kinorhynchs, or their own phylum. Mounting evidence suggests that vetulicolians are deuterostomes, but affinities to crown-group phyla are unresolved. RESULTS: A new vetulicolian from the Emu Bay Shale Konservat-Lagerstätte, South Australia, Nesonektris aldridgei gen. et sp. nov., preserves an axial, rod-like structure in the posterior body region that resembles a notochord in its morphology and taphonomy, with notable similarity to early decay stages of the notochord of extant cephalochordates and vertebrates. Some of its features are also consistent with other structures, such as a gut or a coelomic cavity. CONCLUSIONS: Phylogenetic analyses resolve a monophyletic Vetulicolia as sister-group to tunicates (Urochordata) within crown Chordata, and this holds even if they are scored as unknown for all notochord characters. The hypothesis that the free-swimming vetulicolians are the nearest relatives of tunicates suggests that a perpetual free-living life cycle was primitive for tunicates. Characters of the common ancestor of Vetulicolia + Tunicata include distinct anterior and posterior body regions - the former being non-fusiform and used for filter feeding and the latter originally segmented - plus a terminal mouth, absence of pharyngeal bars, the notochord restricted to the posterior body region, and the gut extending to the end of the tail.

Incremental evolution of the neural crest, neural crest cells and neural crest-derived skeletal tissues.

Urochordates (ascidians) have recently supplanted cephalochordates (amphioxus) as the extant sister taxon of vertebrates. Given that urochordates possess migratory cells that have been classified as 'neural crest-like'- and that cephalochordates lack such cells--this phylogenetic hypothesis may have significant implications with respect to the origin of the neural crest and neural crest-derived skeletal tissues in vertebrates. We present an overview of the genes and gene regulatory network associated with specification of the neural crest in vertebrates. We then use these molecular data--alongside cell behaviour, cell fate and embryonic context--to assess putative antecedents (latent homologues) of the neural crest or neural crest cells in ascidians and cephalochordates. Ascidian migratory mesenchymal cells--non-pigment-forming trunk lateral line cells and pigment-forming 'neural crest-like cells' (NCLC)--are unlikely latent neural crest cell homologues. Rather, Snail-expressing cells at the neural plate of border of urochordates and cephalochordates likely represent the extent of neural crest elaboration in non-vertebrate chordates. We also review evidence for the evolutionary origin of two neural crest-derived skeletal tissues--cartilage and dentine. Dentine is a bona fide vertebrate novelty, and dentine-secreting odontoblasts represent a cell type that is exclusively derived from the neural crest. Cartilage, on the other hand, likely has a much deeper origin within the Metazoa. The mesodermally derived cellular cartilages of some protostome invertebrates are much more similar to vertebrate cartilage than is the acellular 'cartilage-like' tissue in cephalochordate pharyngeal arches. Cartilage, therefore, is not a vertebrate novelty, and a well-developed chondrogenic program was most likely co-opted from mesoderm to the neural crest along the vertebrate stem. We conclude that the neural crest is a vertebrate novelty, but that neural crest cells and their derivatives evolved and diversified in a step-wise fashion--first by elaboration of neural plate border cells, then by the innovation or co-option of new or ancient metazoan cell fates.

Taxonomic reappraisal of the sphagesaurid crocodyliform Sphagesaurus montealtensis from the Late Cretaceous Adamantina Formation of São Paulo State, Brazil.

Sphagesaurus montealtensis is a sphagesaurid whose original description was based on a comparison with Sphagesaurus huenei, the only species of the clade described to that date. Better preparation of the holotype and the discovery of a new specimen have allowed the review of some characteristics and the identification of several synapomorphies of S. montealtensis with the genus Caipirasuchus: presence of antorbital fenestra; external nares bordered only by the premaxillae; premaxilla with four teeth and one diastema (between the 3rd-4th teeth); one diastema between the 4th premaxillary tooth and the 1" maxillary tooth; dentary with ten teeth and two diastemata (between the 4th-5th and 5th-6th teeth); nasal with a groove parallel to the suture with the frontal bone; nasal long, with an acute anterior margin touching anterolaterally the premaxilla, jugal is a straight bar in the lateral view; frontal is longer than wide; a dorsoventrally expanded and vertically oriented quadrate with a groove separating the medial and lateral condyles; the frontal has a discrete sagittal crest; dentary with six posterior sphagesauriform teeth and four anterior conical teeth, the first three are the smallest of the series and the fourth is slightly laterally compressed. The referral of S. montealtensis to the genus Caipirasuchus, as Caipirasuchus montealtensis comb. nov. is proposed here, based on the new taxonomic observations and the results of a phylogenetic analysis.

Basic features of the ancestral chordate brain: a protochordate perspective.

Basic features of the anterior nerve cord in amphioxus larvae are summarized to highlight its essential similarity with the vertebrate brain. Except for a pineal homolog, the amphioxus brain consists of a much simplified version of the ventral brainstem, including a region probably homologous with the hypothalamus, and a locomotory control center roughly comparable to the vertebrate tegmentum and reticulospinal system. Amphioxus has direct pathways for activating its locomotory circuits in response to mechanical stimuli via epithelial sensory cells, but this response is evidently modulated by inputs from diverse sensory-type cells located in the putative hypothalamic homolog, and from the lamellar body, the pineal homolog. This implies that a basic function of the amphioxus brain is to switch between locomotory activities, of which there are several, and the principal non-locomotory one, namely feeding. A similar involvement in switching between behavioral modes may thus have been a core brain function in ancestral chordates. Currently, however, incomplete knowledge of the physiology and behavior of amphioxus limits how effectively it can be used as an evolutionary model. Eye evolution is briefly discussed to illustrate how a better understanding of living forms can inform the evolutionary debate. An account of recent data on dorsoventral inversion is also included, as this bears directly on the question of where the chordate brain originated in relation to other structures. It now appears likely that key components of the ancestral brain were originally located around the mouth. A secondary repositioning of the latter would therefore have been required before a unitary brain could be assembled and internalized. This association between the mouth and the evolving brain reinforces the idea of a fundamental early connection between core brain structures and the control of feeding activity.

Two New Cynodonts (Therapsida) from the Middle-Early Late Triassic of Brazil and Comments on South American Probainognathians.

We describe two new cynodonts from the early Late Triassic of southern Brazil. One taxon, Bonacynodon schultzi gen. et sp. nov., comes from the lower Carnian Dinodontosaurus AZ, being correlated with the faunal association at the upper half of the lower member of the Chañares Formation (Ischigualasto-Villa Unión Basin, Argentina). Phylogenetically, Bonacynodon is a closer relative to Probainognathus jenseni than to any other probainognathian, bearing conspicuous canines with a denticulate distal margin. The other new taxon is Santacruzgnathus abdalai gen. et sp. nov. from the Carnian Santacruzodon AZ. Although based exclusively on a partial lower jaw, it represents a probainognathian close to Prozostrodon from the Hyperodapedon AZ and to Brasilodon, Brasilitherium and Botucaraitherium from the Riograndia AZ. The two new cynodonts and the phylogenetic hypothesis presented herein indicate the degree to which our knowledge on probainognathian cynodonts is incomplete and also the relevance of the South American fossil record for understanding their evolutionary significance. The taxonomic diversity and abundance of probainognathians from Brazil and Argentina will form the basis of deep and complex studies to address the evolutionary transformations of cynodonts leading to mammals.

The oral sensory structures of Thaliacea (Tunicata) and consideration of the evolution of hair cells in Chordata.

We analyzed the mouth of three species, representative of the three orders of the class Thaliacea (Tunicata)--Pyrosoma atlanticum (Pyrosomatida), Doliolum nationalis (Doliolida), and Thalia democratica (Salpida)--to verify the presence of mechanoreceptors, particularly hair cells. In vertebrates, hair cells are well-known mechanoreceptors of the inner ear and lateral line, typically exhibiting an apical hair bundle composed of a cilium and stereovilli but lacking an axon. For a long time, hair cells were thought to be exclusive to vertebrates. However, evidence of a mechanosensory organ (the coronal organ) employing hair cells in the mouth of tunicates, considered the sister group of vertebrates, suggests that tunicate and vertebrate hair cells may share a common origin. This study on thaliaceans, a tunicate group not yet investigated, shows that both P. atlanticum and D. nationalis possess a coronal organ, in addition to sensory structures containing peripheral neurons (i.e., cupular organs and triads of sensory cells). In contrast, in T. democratica, we did not recognize any oral multicellular sensory organ. We hypothesize that in T. democratica, hair cells were secondarily lost, concomitantly with the loss of branchial fissures, the acquisition of a feeding mechanism based on muscle activity, and a mechanosensory apparatus based on excitable epithelia. Our data are consistent with the hypothesis that hair cells were present in the common ancestor of tunicates and vertebrates, from which hair cells progressively evolved.

Dynamic modification of oral innervation during metamorphosis in Branchiostoma belcheri, the oriental lancelet.

The oral apparatus in lancelets undergoes a remarkable modification during larval development, especially during metamorphosis, when the oral innervation is radically altered. The larval mouth opens on the left side at the early larval stage, and a peripheral nerve network, the oral nerve ring (ONR), develops around it. The ONR enlarges as the mouth expands caudally, eventually receiving fibers from nerves as far back as the tenth on the left side. The mouth shrinks during metamorphosis, and with this change the ONR regresses; the posterior sixth to tenth nerves become freed from the connection with the ONR, whereas the fourth and fifth nerves retain their connections. This modification is the basis for the asymmetric innervation to the velum. There is no mesodermal or mesenchymal restriction for guiding nerve patterning as typically found in vertebrate cranial nerves. Rather, it seems to be the ONR, which has no counterpart in vertebrates, that plays pivotal roles for patterning the nervous system in the oral region. The oral innervation pattern in lancelets represents a derived character state that may be related to the asymmetry of the ancestral body and head.

Prospective protochordate homologs of vertebrate midbrain and MHB, with some thoughts on MHB origins.

The MHB (midbrain-hindbrain boundary) is a key organizing center in the vertebrate brain characterized by highly conserved patterns of gene expression. The evidence for an MHB homolog in protochordates is equivocal, the "neck" region immediately caudal to the sensory vesicle in ascidian larvae being the best accepted candidate. It is argued here that similarities in expression patterns between the MHB and the ascidian neck region are more likely due to the latter being the principal source of neurons in the adult brain, and hence where all the genes involved in patterning the latter will necessarily be expressed. The contrast with amphioxus is exemplified by pax2/5/8, expressed in the neck region in ascidian larvae, but more caudally, along much of the nerve cord in amphioxus. The zone of expression in each case corresponds with that part of the nerve cord ultimately responsible for innervating the adult body, which suggests the spatially restricted MHB-like expression pattern in ascidians is secondarily reduced from a condition more like that in amphioxus. Patterns resembling those of the vertebrate MHB are nevertheless found elsewhere among metazoans. This suggests that, irrespective of its modern function, the MHB marks the site of an organizing center of considerable antiquity. Any explanation for how such a center became incorporated into the chordate brain must take account of the dorsoventral inversion chordates have experienced relative to other metazoans. Especially relevant here is a concept developed by Claus Nielsen, in which the brain is derived from a neural center located behind the ancestral mouth. While this is somewhat counterintuitive, it accords well with emerging molecular data.

The deuterostome ancestor.

Hemichordates, the phylum of bilateral animals closest to chordates, can illuminate the evolutionary origins of various chordate traits to determine whether these were already present in a shared ancestor (the deuterostome ancestor) or were evolved within the chordate line. We find that an anteroposterior map of gene expression domains, representing 42 genes of neural patterning, is closely similar in hemichordates and chordates, though it is restricted to the neural ectoderm in chordates whereas in hemichordates, which have a diffuse nervous system, it encircles the whole body. This map allows an accurate alignment of the anterioposterior axes of members of the two groups. We propose that this map dates back at least to the deuterostome ancestor. The map of dorsoventral expression domains, organized along a Bmp-Chordin developmental axis, is also similar in the two groups in terms of many gene expression domains and for the placement of the gill slits, heart, and post-anal tail. The two groups, however, differ in two major respects along this axis. The nervous system and epidermis are not segregated into distinct territories in hemichordates, as they are in chordates, and furthermore, the mouth is on the Chordin side in hemichordates but the Bmp side in chordates. The dorsoventral dimension has undergone extensive modification in the chordate line, including centralization of the nervous system, segregation of epidermis, derivation of the notochord, perhaps from the gut midline, and relocation of the mouth. Based on the shared domain maps, speculations can be made for the remodeling of the body axis in the chordate line.