What is it? Affinities and systematic position of Dipnoi DBS 402B.1 Presidency University, 2015
What is the question? I ve decided to discuss the controversy, therefore the argument and philosophy of classification/systematics Why is there even an argument? Systematics is the study of biological diversity. At the present time we do not even know, to the nearest order of magnitude, how many species there are in the world. About 10,000 new species across all groups of organisms are described every year, and there s no end in sight. Approximately 1.7 million have been described, but this number is far below the actual biodiversity. Recent studies in rain forests and other major habitats indicate the presence of as many as 30 million kinds of insects alone.
Time to revive systematics Because of the largely unknown nature of biodiversity, systematics remains a tremendous source of discoveries and new ideas in biology. Much of the research in taxonomy and systematics has economic and medical importance. The world supply of trained taxonomists is no where near the number required to research even a small part of unknown or poorly known aspects of biodiversity. Government support for systematic research in this country (read USA) is so low that less than one percent of the known species of organisms are under active investigation. Wilson, E. O. (1985). "Time to revive systematics." Science 230(4731): 1227.
Taxonomy &/or Systematics Taxonomy: The discovery, recognition, definition, and naming of groups of organisms. Systematics: The study of biological diversity; or, more specifically, the ordering of the diversity of nature through construction of a classification that can serve as a general reference system. The task of systematics and the task of classification is to find the best possible general reference system that reflects the evolutionary relationships of the taxa being studied. But finding the best possible reference system is not so easy, and it turns out that there are several approaches or ways to go about performing this task. But whatever the methodology, the final products of systematic research are two-fold: Branching diagrams (i.e., evolutionary trees) Classifications (i.e., species or groups of species placed within named groups)
Cladogram Branching diagrams (sometimes called evolutionary or phylogenetic trees, or cladograms, from the Greek clad or klados, meaning to branch) are simply graphic views of the sequence of evolutionary divergence of groups of organisms through time.
Classification Subphylum Myxinomorpha (hagfishes) Subphylum Vertebrata (vertebrates) Superclass Cephalaspidomorphi (lampreys) Superclass Gnathostomata (jawed vertebrates) Class Chondrichthyes (cartilaginous fishes) Class Teleostomi (true-jawed vertebrates) Subclass Actinopterygii (ray-finned fishes) Subclass Sarcopterygii (lobe-finned fishes) Infraclass Crossopterygii (coelacanths) Infraclass Dipneusti Series Dipnoi (lungfishes) Series Tetrapoda (terrestrial vertebrates)
But we will follow Subphylum Tunicata (Urochordata) Subphylum Cephalochordata (Acraniata) Subphylum Vertebrata (Craniata) Class 'Agnatha Class Chondrichthyes Class Osteichthyes Subclass Actinopterygii Subclass Sarcopterygii Order Dipnoi Infraclass Crossopterygii Order Porolepiformes Superorder unnamed Order Onychodontida Order Actinistia Infraclass Tetrapodomorpha Order Rhizodontida Superorder Osteolepidida Order Osteolepiformes Order Panderichthyida Superclass Tetrapoda http://palaeo.gly.bris.ac.uk/benton/vertclass.html
Why is that a controversy? Sarcopterygii: or lobe finned fish (from sarco: flesh; pteryx: wing) Most of them are now fossilized Six living species: 4 Dipnoi Neoceratodus forsteri (Australian) Lepidosiren paradoxa (South American) Protopterus aethiopicus (Africa) Protopterus amphibius (Africa) 2 Coelacanth Latimeria chalumnae (West Indian) Latimeria menadoensis (Indonesian)
Lobe finned fish 5 cm Lower jaw Scaly covering Dorsal spine
Sarcopterygians Sarcopterygians have been divided in three main groups: lungfishes, actinistians (coelacanths), and the rhipidistians (which form a monophyletic group only if you include lungfishes and tetrapods in them). Their fins are supported by a thick bony axis that is moved by muscles within the fin. They have two dorsal fins. The caudal fin is primitively heterocercal. Their dermal scales have a basal layer of laminar bone (isopedin), a vascular layer of bone, a layer of cosmine and a thin layer of enamel.
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Characters Sarcopterygian fishes differ most strikingly from actinopterygians, as well as from other fishes, in the structure of their scales, a unique kind of scale called a cosmoid scale.
Words Cosmoid scales consist of a layer of cosmine (a dentine-like substance), overlain by a thin, hard layer of enamel; an inner layer of spongy or vascular bone; and a deep layer of lamellar bone. Dentine is present as well, closely associated with the cosmine layer. These cosmoid scales were present in nearly all of the earliest (i.e., extinct) sarcopterygians, but they have been considerably modified through time; for example, cosmine is completely lost in modern lungfishes and is absent as well in the scales of Latimeria. Primitively, all sarcopterygians had two dorsal fins, a feature that clearly distinguishes them from early actinopterygian fishes which had only a single dorsal fin. But, probably the most obvious and most significant distinguishing feature of sarcopterygians are the lobed fins. Instead of fin rays supported by a series of elongate, more-or-less parallel bones, the fins are connected to the girdles (pectoral and pelvic) by a continuous chain of bones that are homologous to tetrapod limbs.
In the right diagram, A and B represent the pectoral fin of extinct sharks; C represents that of a living cod; and D that of a living lungfish. Images
On top of it Not only were the pectoral and pelvic fins of these primitive sarcopterygians mounted on lobes, the same condition characterized the support for the dorsal and anal fins, as shown here in this diagram of the extinct sarcopterygian genus Eusthenopteron.
More on lobe This figure shows the lobed pectoral and pelvic fins of the living Australian lungfish, genus Neoceratodus
Lungfishes Lungfishes are poorly represented by modern forms but they have a rich fossil history that also goes back to the Devonian, about 380 MYBP. Evolutionary trends within lungfishes are characterized by gradual reduction: the earliest members of the group had heavy bony skeletons, a large heterocercal or diphycercal tail, and a heavy armor of thick cosmoidscales.
More As they evolved there occurred a dramatic reduction in the amount of bone, loss of the outer layers of the scales, reduction in the number of skull bones, and the loss of distinct fins, including the tail.
Modern Lungfishes Modern lungfishes are characterized by having: Well-developed air-breathing lungs; A largely cartilaginous skeleton; Internal nostrils; Large plate-like teeth modified for crushing; A spiral valve in the intestine. From a once large and diverse group remaining six species, placed in three families and three genera, all considered to be highly divergent survivors of an ancient Devonian stock: Family Protopteridae: African lungfishes; a single genus, Protopterus, with four species. Family Lepidosirenidae: South American lungfish; a single species, Lepidosirenparadoxa. Family Ceratodontidae: Australian lungfish; a single species, Neoceratodusforsteri.
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So All we have left are the eight living species of sarcopterygian fishes extremely interesting, not so much for their success in terms of number of species produced through time, but for the fact that somewhere among their rich fossil history was an animal that gave rise to the first amphibians, thus leading the way to the origin of tetrapods as a whole. That s the crucial point In 1839, in a paper published by the famous British paleontologist Sir Richard Owen (1810 1890), lungfishes were declared to be fishes up until that time they had always been thought of as amphibians and, at the same time, proclaimed them to be most closely related to tetrapods.
But Owen s hypothesis was well accepted at the time and became the prevailing belief until the turn of the century when Edward Drinker Cope, (1840 1897) the well-known American ichthyologist and paleontologist, proposed instead that crossopterygians are most closely related to tetrapods. Nearly everyone, especially American scientists, swung over to this view.
The savior In 1981, Donn E. Rosen of the American Museum of Natural History, revived Owen s 19th-century idea that lungfishes gave rise to tetrapods. What is the evidence to support this notion? Internal nostrils: the internal nostrils of lungfishes are nearly identical to those of Devonian amphibians as well as to those of modern salamanders; Tetrapod-like limbs: primitive lungfishes have two primary joints in each paired appendage just like those of tetrapods; Tetrapod-like locomotion: the locomotion of living lungfishes is like that of tetrapods. In addition to these three characters, Rosen listed another 17 features shared between lungfishes and primitive tetrapods, declaring that this body of evidence supports a sister-group relationship that is irrefutable.
Internal Nasal Youngolepis. Kenichthys. Eusthenopteron
The comparison A & C: Eusthenopteron B & D: Icthyostega
Constructed images Eusthenopteron Primitive amphibian
Eusthenopteron
Ichthyostega Skeleton 3 Fossil Reconstruction
Evolution of the head
Evolution of the limb
The evolution (A) Osteolepiform fish Eusthenopteron; (B) panderichthyid fish Panderichthys; and (C) labyrinthodont amphibian Ichthyostega
But But, as you might expect, not everyone is convinced. Many scientists want to stick to the Cope hypothesis that argues that a Latimeria-like ancestor was responsible for the rise of tetrapods. They argue that, in many ways, crossopterygians provide a link between fishes and the earliest amphibians, a group called the Labyrinthodontia.
More Supporters of the crossopterygian hypothesis cite about a dozen characters that support a coelacanthlabyrinthodont linkage; Here are just three: 1: The paired fins of certain extinct crossopterygians are more like tetrapod limbs than those of lungfishes.
And more Most of the bones of crossopterygians, particularly those of the cranium, can be homologized, element for element, with those of Labyrinthodont amphibians. Crossopterygians have a specialized tooth structure in which the outer layer of enamel is highly folded; this complex folding, which forms deep grooves on the outside of each tooth, is also a primary character of Labyrinthodont amphibians. It is, in fact, the feature for which they are named: labyrinth = complex folding; dont= tooth.
Read on So Colbert s Evolution of the Vertebrates Vertebrates by Kardong Vertebrate zoology and evolution by yadav & kumar (google books)