ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012010

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The paper describes the external shape and ornament of four stem-vertebrate mitrates (Mitrocystites mitra, Mitrocystella incipiens, Ateleocystites guttenbergensis, Placocystites forbesianus) and, by using two-dimensional working models, reconstructs their locomotion. A cladogram suggests that the evolution of these species, which occur in the Ordovician and Silurian of Europe and the U.S.A., reflects increasing adaptation to a shallow-burrowing mode of life with differing amounts of yaw. The external ontogeny ofPlacocystites forbesianusis described.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012011

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

A brief historical review of the discussion and evidence for a segmented head is given. The idea that the head is made up of a number of segments is accepted, and the number of segments is identified as five and one-half. The half is due to the fact that the sclerotomie segments alternate with myotomic ones. If the head is segmented, then structures can be assigned to segments. Several figures and a table summarize these assignments. A segmented head is viewed as part of the hypothesis that the vertebrate arose from a cephalochordate-like ancestor lacking a complex pharynx and having a better developed head.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012012

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Evidence for two competing theories of relationships between Recent agnathans (hagfishes and lampreys) and gnathostomes is reviewed. The hypothesis that lampreys and gnathostomes are sister-groups is favored. The more obvious structures (fins, gill chambers, skeleton) seen in fossil agnathans are discussed and conclusions about their distribution are used in an attempt to include the fossil groups within a classification of Recent craniates. It is concluded that heterostracans are the sister-group of lampreys + gnathostomes; that anaspids are the sister-group of lampreys; and that galeaspids are the sister-group of osteostracans + gnathostomes.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012013

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The phylogenetic relationships of two monophyletic groups of fossil jawless craniates, the Osteostraci and Galeaspida, are discussed on the basis of new observations. Maximum congruence of derived character distribution implies that the Galeaspida are the sister-group of the Osteostraci + Gnathostomata. This implies that the dorsal naso-hypophysial opening is not a synapomorphy of the Cephalaspidomorphi (Osteostraci + Anaspida + Petromyzontida), but is likely to have appeared independently, at least in lampreys and osteostracans. An outline of the phylogeny of the Galeaspida is proposed on the basis of the published material.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012014

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

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 JurassicPalaeospinaxby several synapomorphies.Palaeospinaxand 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.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012015

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The microscopic anatomy of the peculiar spine-“brush” complex of the Paleozoic sharkStethacanthus altonensis(Symmoriida: Stethacanthidae) reveals that the spine consists of trabecular dentine throughout—no shaft and mantle components are discernible. Parts of the trabecular dentine contain abundant fibers that suggest attachment surfaces for tendons and other connective tissue membranes to the epaxial musculature. The “brush” consists of parallel, membranous tubes and connecting membranes that form an intricate, internal cavity system. In section, this structure resembles sections of erectile tissues in man. The “brush” may thus have been inflatable. The “brush” is covered with a patch of large, massive, dermal denticles pointing forward. Associated with this spine-“brush” is a second patch of enlarged dermal denticles on top of the head whose crowns face backward. The entire complex consists of a variety of gross anatomical, histological, and microscopic anatomical elements. Sexual display or even injurious courtship activities, such as the lacerating of the flanks of the female by the male, do not require such a complicated mechanism. It is suggested that the spine-“brush” and head denticle patch may have acted in concert to effect a threat posture simulating a tooth-studded, wide-open mouth of a much larger fish.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012016

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

A review of the evidence suggests that placoderms are more closely related to osteichthyans than chondrichthyans. A new phylogeny of placoderms is proposed in which the ptyctodonts are considered the collateral descendants of all other placoderms. The short pectoral girdle with a single ring of bones is judged to be primitive and the sliding neck joint is presumed to be derived.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012017

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Evaluation of the characters used to determine the phylogeny and classification of arthrodires leads to the following conclusions: Actinolepidae are defined only by characters primitive for the order. Holonematidae are not brachythoracans but are related to Phlyctaeniidae. Brachythoraci are difficult to define, but are probably monophyletic and are useful taxonomically. Buchanosteoids, heterosteoids, homosteoids and brachydeiroids all retain primitive characters that indicate an early origin among Brachythoraci, in spite of the specializations attained by each superfamily. Coccosteidae are the central stock of the major brachythoracan radiation, the Coccosteoidea, but can be defined only by the absence of the specializations of the families to which they gave rise; these include the Rhachiosteidae, Pholidosteidae, Camuropiscidae and Incisoscutidae fam. nov., as well as the Dinichthyidae which are grouped with the Leiosteidae and Trematosteidae in a separate superfamily, Dinichthyoidea.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012018

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

A preliminary diagnosis and description is given of a new palaeonisciform,Lawrenciella schaefferin.g., n.sp., from the Pennsylvanian (Late Carboniferous) of Kansas, U.S.A. This species is known only by the braincase and associated parasphenoid. Endocranial characters of the Palaeonisciformes are reviewed and discussed in connection with their use in palaeoniscid systematics. Some remarks are made on features which appear to be significant in the evolution of the palaeonisciform braincase.

ISSN:0272-4634    

DOI:10.1080/02724634.1984.10012019

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor