John G. Maisey

HEADS AND TAILS: A CHORDATE PHYLOGENY

Abstract— A cladistic analysis of chordates is presented, based on some 320 nested characters. All the principal higher taxa are defined by synapomorphies, including extinct acanthodians and placoderms. The data base draws broadly from adult anatomy (including osteological data for Recent and fossil taxa), embryology, physiology, and biochemistry. A conventional sequence of chordate higher taxa is generated (hemichordates, urochordates, cephalochordates, craniates). Among the craniates, cyclostomes are considered paraphyletic. Gnathostomes are monophyletic, but two fossil “agnathan” groups (galeaspids, osteostracans) are regarded as stem gnathostomes. Chondrichthyans and osteichthyans are monophyletic. New arguments for osteichthyan affinity of acanthodians are presented. The phylogenetic position of placoderms is still problematic, but they can no longer be perceived as stem chondrichthyans or even as “elasmobranchiomorphs.” Recent dipnoans and tetrapods are sister groups, but new paleontological discoveries refute many of their supposed osteological synapomorphies, thereby reopening the possibility of a closer relationship between tetrapods and osteolepiform rhipidistians.

Chondrichthyan phylogeny: a look at the evidence

ABSTRACT Chondrichthyans (sharks, rays and holocephalans) are subjected to cladistic analysis in order to identify possible monophyletic groups. Chondrichthyan monophyly is established on the basis of several apomorphic characters, of which the most convincing is the presence of a mineralized layer of prismatic perichondral tissue of a unique type. Modern elasmobranchs are united with the Jurassic Palaeospinax by several synapomorphies. Palaeospinax and some other fossil sharks are sequenced as successive sister-groups to modern elasmobranchs. It is concluded that elasmobranchs and chimaeroids are monophyletic sister-groups, but that sharks are not monophyletic unless holocephalans are included.

Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography

Living cartilaginous fishes, or chondrichthyans, include numerous elasmobranch (sharks and rays) species but only few chimaeroid (ratfish) species. The early history of chimaeroids, or holocephalans, and the modalities of their divergence from elasmobranchs are much debated. During Carboniferous times, 358–300 million years (Myr) ago, they underwent a remarkable evolutionary radiation, with some odd and poorly understood forms, including the enigmatic iniopterygians that were known until now from poorly informative flattened impressions. Here, we report iniopterygian skulls found preserved in 3 dimensions in ≈300-Myr-old concretions from Oklahoma and Kansas. The study was performed by using conventional X-ray microtomography (μCT), as well as absorption-based synchrotron microtomography (SR-μCT) [Tafforeau P, et al. (2006) Applications of X-ray synchrotron microtomography for non-destructive 3D studies of paleontological specimens. Appl Phys A 83:95–202] and a new holotomographic approach [Guigay P, Langer M, Boistel R, Cloetens P (2007) Mixed transfer function and transport of intensity approach for phase retrieval in the Fresnel region. Opt Lett 32:1617–1619], which revealed their peculiar anatomy. Iniopterygians also share unique characters with living chimaeroids, suggesting that the key chimaeroid skull features were already established 300 Myr ago. Moreover, SR-μCT of an articulated skull revealed a strikingly brain-shaped structure inside the endocranial cavity, which seems to be an exceptional case of soft-tissue mineralization of the brain, presumably as a result of microbially induced postmortem phosphatization. This was imaged with exceptional accuracy by using holotomography, which demonstrates its great potential to image preserved soft parts in dense fossils.

A primitive chondrichthyan braincase from the Early Devonian of South Africa

Abstract The earliest-known chondrichthyan braincase, from the Early Devonian (Emsian) Gydo Formation of South Africa, is described along with parts of the visceral skeleton. Only the ventral surface of the braincase is exposed, but CT-scanning permits the investigation of its unprepared regions and internal features. There is a persistent cranial fissure (as in osteichthyans) separating the trabecular and parachordal regions. The parachordal cartilage is identical to isolated endoskeletal elements referred to Pucapampella from the Middle Devonian of Bolivia. The semicircular canals are arranged as in osteichthyans and chimaeroids, with a crus commune connecting the anterior and posterior canals dorsally. The elongate palatoquadrate has an ethmoidal and palatobasal articulation (it is unknown if a postorbital articulation was present). The mandibular joint is positioned lateral to the widest part of the parachordal region, and the hyoid arch probably helped support the jaw. The South African specimen demonstrates unequivocally that some features previously known only in osteichthyans (e.g., ventral otic fissure, posterior dorsal fontanelle, palatobasal articulation) were also present in primitive chondrichthyans and are actually plesiomorphic hold-overs whose distribution was primitively more universal among gnathostomes. Fossils rarely overturn the phylogenetic status of morphological characters at such deep phylogenetic levels, perhaps because their wider original distribution pattern was short-lived and is rarely recovered from the fossil record.

Predator-prey relationships and trophic level reconstruction in a fossil fish community

SynopsisAll living species occupy an ecological niche, and are positioned within a trophic hierarchy. Extinct organisms presumably held similar behavioral and coevolutionary characteristics in the past, and were susceptible to the same kinds of natural ecological pressures operating today. Paleoecological investigations are limited by the incompleteness of the fossil record, and particularly by a lack of behavioral data that are so fundamental to ecological studies of living communities and habitats. Opportunities to examine the coevolutionary structure of ancient communities from empirical data are extremely rare. One such opportunity is provided by the Lower Cretaceous Santana Formation of north-eastern Brazil, a series of richly fossiliferous strata approximately 110 million years old. Many fossil fishes from the Santana Formation contain identifiable prey, including decapod crustaceans and fishes. A trophic hierarchy of these organisms is reconstructed here, and their ecological relationships are discussed. Comparison is made with a similar fish fauna from the Upper Jurassic Solnhofen Limestone of Germany. Low-level, intermediate and high-level predators are identified in each fauna. Predator-prey relationships in the Santana fauna are strongly hierarchical, and are more focussed at the intermediate predator level than in Solnhofen. Comparison with a model of predator-prey relationships between fishes and benthic fauna of the Baltic Sea (which like the Araripe Basin represents a semi-enclosed environment) suggests that heavy predation on teleosts such asRhacolepis, occupying an intermediate trophic level, may have permitted benthic decapods to proliferate and exclude other benthic organisms. Less intense predation on fishes at the intermediate trophic level would allow their numbers to increase, thereby increasing the intensity of predation on the benthos at the base of the trophic hierarchy.

A New Paleozoic Symmoriiformes (Chondrichthyes) from the Late Carboniferous of Kansas (USA) and Cladistic Analysis of Early Chondrichthyans

Background The relationships of cartilaginous fishes are discussed in the light of well preserved three-dimensional Paleozoic specimens. There is no consensus to date on the interrelationship of Paleozoic chondrichthyans, although three main phylogenetic hypotheses exist in the current literature: 1. the Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are grouped along with the modern sharks (neoselachians) into a clade which is sister group of holocephalans; 2. the Symmoriiformes are related to holocephalans, whereas the other Paleozoic shark-like chondrichthyans are related to neoselachians; 3. many Paleozoic shark-like chondrichthyans, such as the Symmoriiformes, are stem chondrichthyans, whereas stem and crown holocephalans are sister group to the stem and crown neoselachians in a crown-chondrichthyan clade. This third hypothesis was proposed recently, based mainly on dental characters. Methodology/Principal Findings On the basis of two well preserved chondrichthyan neurocrania from the Late Carboniferous of Kansas, USA, we describe here a new species of Symmoriiformes, Kawichthys moodiei gen. et sp. nov., which was investigated by means of computerized X-ray synchrotron microtomography. We present a new phylogenetic analysis based on neurocranial characters, which supports the third hypothesis and corroborates the hypothesis that crown-group chondrichthyans (Holocephali+Neoselachii) form a tightly-knit group within the chondrichthyan total group, by providing additional, non dental characters. Conclusions/Significance Our results highlight the importance of new well preserved Paleozoic fossils and new techniques of observation, and suggest that a new look at the synapomorphies of the crown-group chondrichthyans would be worthwhile in terms of understanding the adaptive significance of phylogenetically important characters.

Redescription of †Ellimma branneri and †Diplomystus shengliensis, and Relationships of Some Basal Clupeomorphs

Abstract Two extinct clupeomorphs, †Ellimma branneri from the Cretaceous of Brazil and †Diplomystus shengliensis from the Eocene of China, are redescribed. †Ellimma branneri was formerly classified within the Clupeiformes, but it lacks derived characters of clupeiforms and clupeoids. Dorsal scute “wings” are expanded and subrectangular in †Ellimma and other members of the family †Paraclupeidae Chang and Chou (1977), approximately equal to †Ellimmichthyidae Grande (1982a). Consequently, †Ellimma branneri is classified here within the family †Paraclupeidae. †Paraclupeidae are known from the Lower Cretaceous to the middle Eocene. In the present work, two monophyletic groups are identified within the †Paraclupeidae. One group (subfamily †Paraclupeinae of Chang and Grande, 1997), known only from the Lower Cretaceous (Hauterivian–Albian), includes †Paraclupea, †Ellimmichthys, and †Ellimma. These taxa are united by strongly sculptured, skull-roofing bones with ridges radiating from the growth center, and a dorsal scute ornament of prominent ridges. †Scutatuspinosus may also belong in this group. The other group includes †Diplomystus (Upper Cretaceous–Eocene) and †Armigatus (Upper Cretaceous), which are united by a single homoplaseous character (presence of a posteriorly expanded third hypural, leaving no gap between hypurals 2 and 4): this character also occurs in pristigasteroids, †Erichalcis, osteoglossids, some elopomorphs (†Lebanichthys lewisi, and most Albula spp.), and a number of ostariophysans not included in our analysis. †Paraclupeines are customarily regarded as being more closely related to the Clupeiformes than to other teleosts (i.e., as clupeomorphs), although no derived characters are uniquely shared by †Ellimma branneri and modern Clupeiformes. The relationships of †Ellimma and certain other extinct herring-like teleosts (including other †paraclupeines) with the Clupeiformes are unclear, and they may collectively form a paraphyletic assemblage. No biogeographical hypothesis satisfactorily explains the known distribution of nonmarine †paraclupeine fishes in the Cretaceous. A substantial portion of their nonmarine fossil record is missing (as evidenced by the recent discovery of a possible †paraclupeine, †Ezkutuberezi carmeni Poyato-Ariza et al., 2000, in Spain), and some aspects of their early distribution pattern may have involved marine dispersal. Eocene †Diplomystus occurs on both sides of the Pacific Ocean, but the “Pacifica” hypothesis (which lacks empirical support) is abandoned as an explanation for such Eocene (and younger) trans-Pacific distribution patterns of nonmarine fishes. Instead, a “freshwater Arctic Ocean” hypothesis is favored. According to this hypothesis (for which there are several independent lines of geological evidence), temporary desalination of the Arctic Ocean occurred during the Paleocene and early Eocene, which may have permitted freshwater fishes to move unimpeded by salt-water barriers between Asia and North America; this temporary desalination event may eventually become recognized as a significant factor in the holarctic distribution patterns of various Tertiary-Recent freshwater fishes.

Redescription of Santanichthys diasii (Otophysi, Characiformes) from the Albian of the Santana Formation and Comments on Its Implications for Otophysan Relationships

Abstract A detailed redescription of Santanichthys diasii is presented, based on several new acid-prepared and very well-preserved specimens. S. diasii has a complete Weberian apparatus and is at present the earliest otophysan fish known (early Cretaceous, Albian). In addition, this fish displays at least one synapomorphy of modern characiforms (large and globular lagenar capsules that extend well lateral to the cranium) and we consequently suggest that it is a stem characiform. If this is correct, it represents a significant temporal extension for characiforms. We discuss the phylogenetic implications of its unique combination of features in light of earlier phylogenetic hypotheses.

Phylogeny of Early Vertebrate Skeletal Induction and Ossification Patterns

The past 40 years has witnessed the demise of paleontology as a panacea with the ability to vindicate great phylogenetic theories. Today the role of fossils has been quite sharply defined (e.g., Hennig, 1965; Schaeffer et al., 1972; Nelson, 1978; Gaffney, 1979; Cracraft, 1979; Patterson, 1981a,b). The fossil record nevertheless provides useful data by refuting putative synapomorphies and by revealing nonhomology among living taxa, by suggesting sequential acquisitions of characters, and by providing supplemental biogeographic data (Patterson, 1981a). In these regards, fossils perform like newly discovered Recent taxa, but the paleontological data are potentially more dynamic in adding the element of geological time (hence giving minimum dates for taxic divergence and for biogeographic and other evolutionary events). Furthermore, the chances of discovering phylogenetically intermediate taxa (whether they are called “sister groups,” “ancestors,” “stem taxa,” “missing links,” or whatever) are inherently greater in fossil biotas than in Recent ones (vide the taxic paucity of “living fossils,” such as monotremes, coelacanths, cladistians [polypterids], agnathans, etc., versus the relative abundance of primitive Mesozoic mammals, fossil sarcopterygians, “palaeoniscoids,” and Paleozoic agnathans).

The Giant Cretaceous Coelacanth (Actinistia, Sarcopterygii) Megalocoelacanthus dobiei Schwimmer, Stewart & Williams, 1994, and Its Bearing on Latimerioidei Interrelationships

We present a redescription of Megalocoelacanthus dobiei, a giant fossil coelacanth from Upper Cretaceous strata of North America. Megalocoelacanthus has been previously described on the basis of composite material that consisted of isolated elements. Consequently, many aspects of its anatomy have remained unknown as well as its phylogenetic relationships with other coelacanths. Previous studies have suggested that Megalocoelacanthus is closer to Latimeria and Macropoma than to Mawsonia. However, this assumption was based only on the overall similarity of few anatomical features, rather than on a phylogenetic character analysis. A new, and outstandingly preserved specimen from the Niobrara Formation in Kansas allows the detailed description of the skull of Megalocoelacanthus and elucidation of its phylogenetic relationships with other coelacanths. Although strongly flattened, the skull and jaws are well preserved and show many derived features that are shared with Latimeriidae such as Latimeria, Macropoma and Libys. Notably, the parietonasal shield is narrow and flanked by very large, continuous vacuities forming the supraorbital sensory line canal. Such an unusual morphology is also known in Libys. Some other features of Megalocoelacanthus, such as its large size and the absence of teeth are shared with the mawsoniid genera Mawsonia and Axelrodichthys. Our cladistic analysis supports the sister-group relationship of Megalocoelacanthus and Libys within Latimeriidae. This topology suggests that toothless, large-sized coelacanths evolved independently in both Latimeriidae and Mawsoniidae during the Mesozoic. Based on previous topologies and on ours, we then review the high-level taxonomy of Latimerioidei and propose new systematic phylogenetic definitions.