Wolf-Ernst Reif

Evolution of Dermal Skeleton and Dentition in Vertebrates

The starting point of comparative evolutionary studies of the dermal skeleton of vertebrates is Hertwig’s series of papers (1874, 1876/1879/1882), which directly stimulated many dozens of papers, most of them in German and some of them long forgotten. The literature on comparative histology and histogenesis of the dermal skeleton and on regulatory and morphogenetic processes of the vertebrate integument is so voluminous that it can hardly be summarized. Ever since Hertwig, attempts have been made not only to contribute descriptive and experimental data, but also to arrive at a synthesis. Most of the synthetic papers, however, address only a small section of the theoretical problems, and it seems that some important questions have never been asked.

THE SKELETON SPACE: A FINITE SET OF ORGANIC DESIGNS

The structures of animal skeletons converge repeatedly on a limited number of architectural designs that can be constructed by growing organisms and that are functionally viable, although often not optimal. Properties of materials, construction rules that determine patterns of development, and physical constraints exerted by the requirements of function suggest that organic structure must necessarily approach these recurrent elements of design. A set of potential designs for the elements of animal skeletons is derived in terms of geometric and construction rules and the properties of materials. Skeletons of actual living and extinct organisms are matched with the possibilities defined within this theoretical morphospace. This provides a metric of skeletal complexity and of the extent to which various groups of animals have been able to exploit the range of possibilities of organic structure. These analyses show that the most evolutionarily advanced animals within a given phylum do not have the most complex skeletons; that arthropods are less morphologically diverse than vertebrates and molluscs; that the physical constraints of life on land and in the air substantially limit the variety of skeletal structures suitable for life in these environments; and that overall the range of possible skeletal designs has been very fully exploited by living and extinct organisms. These results strongly support the hypothesis that the essential elements of organic design are inherent in the material properties of the universe. The organizational properties of animal skeletons suggest that their design elements are fixed point attractors, structures that we characterize as topological attractors that evolution cannot avoid.

Types of morphogenesis of the dermal skeleton in fossil sharks

Four types of morphogenesis of the dermal skeleton can be distinguished. They differ with regard to scale growth, scale replacement and insertion of new scales during ontogeny. Three of the types occur exclusively in fossil sharks and have been found in only a few articulate specimens. In only one case (Jurassic hybodontids) it is possible to trace the phyletic transition from one type to another. The adaptive significance, both of different types of morphogenesis of the dermal skeleton as well as different types of scale shapes, is discussed.ZusammenfassungVier Morphogenese-Typen des Dermalskelettes können unterschieden werden. Sie unterscheiden sich hinsichtlich Schuppenwachstum, Schuppenaustausch und Einschub neuer Schuppen. Drei dieser Typen sind beschränkt auf fossile Haie; sie wurden bisher nur in wenigen artikulierten Exemplaren gefunden. Nur in einem Falle (Jurassische Hybodonti-den) konnte der phyletische Übergang von einem Typ zu einem anderen Typ beobachtet werden. Die adaptive Bedeutung der verschiedenen Morphogenese-Typen und verschiedenen Schuppenformen wird diskutiert.

The synthetic theory of evolution: general problems and the German contribution to the synthesis

A metatheoretical and historiographical re-analysis of the Evolutionary Synthesis (the process) and the Synthetic Theory (the result) leads to the following conclusion: The Synthetic Theory is not a reductionistic, but rather a structuralistic theory with a limited range of relevant hierarchical levels. Historically the Synthesis was not a sudden event but a rational long-term project carried out between 1930 and 1950 by a large number of biologists in several countries. In the second part of our paper the contributions of several German biologists to the Synthesis are analyzed.

Morphologie und Ultrastruktur des Hai‐ “Schmelzes”

Sharks at the cladodont and hybodont level (Paleozoic and Mesozoic) have blunt fangs and crushing teeth. These are covered by a thin sculptured uniform “enamel” cap, with a high compressive strength. The radiation of the modern sharks in the middle Mesozoic leads primarily to a broad spectrum of different sharks with a fang, or cutting tooth dentition. This radiation is accompanied by modifications of anatomical characters of the sharks body but also by the development of three new “enamel” types, which give the teeth bending and compressive strength.

Morphogenesis, pattern formation and function of the dentition ofHeterodontus (Selachii)

SummaryAs in all skarks, the ectoderm ofHeterodontus folds in behind the jaw cartilage during embryogenesis. The anterior part of this ectodermal fold becomes organized into the Inner Enamel Epithelium which cooperates with the mesenchyme. Already during the infolding, both tissues begin to form teeth. This process begins with a spontaneous division of the fold along its long axis into tooth-forming and non tooth-forming tissue sections. In this way the tooth formula of the “first dentition” is established. The Inner Enamel Epithelium and the mesenchyme only gradually attain competence for tooth formation, so that the first formed tooth germs become incomplete tooth shards (Fig. 8). Shortly before the end of the embryonic phase, as soon as the infolding stops, the tooth transport mechanism begins to work.Heterodontus hatches with a characteristic “first dentition”, which has a typical dental formula of 17to19/13to15.Because of allometric growth, the number of tooth families increases strongly in the course of a lifetime, but to different degrees in the various species. The “first dentition” is composed of teeth having numerous needle-like cusps; it is only slightly heterodont and serves in feeding on soft-bodied benthonic animals. In the course of ontogeny, a highly heterodont dentition becomes differentiated. The anterior part of the adult dentition is composed of teeth with 1–3 cusps, while the distal part bears molariform teeth. On the basis of these distal teeth, two groups of species may be distinguished: a primitive group (Francisci-type) distributed in the Indopacific and having slender, keeled crushing teeth (probably specialized for a diet of echinoderms), and a group of species derived from it (Portusjacksoni-type) found only in the western Pacific and having broad, powerful crushing teeth capable of cracking hardshelled molluscs.It is characteristic that the most distal tooth family of the first dentition becomes the main crushing tooth within the molariform group in the course of ontogeny. This tooth family lies at the point where the most force can be brought to bear by the jaw apparatus, and remains there life-long; that is, it does not change position relative to the jaw.Since the infolded ectoderm lies on the inner side of the jaw, its growth is controlled by the growth of the jaw. The jaw on the Portusjacksoni-type grows forward sharply to form a beak so that the infolded ectoderm must grow forward with it and thus can form extra tooth families. The infolded ectoderm expands outwards in the course of ontogeny toward the distal ends of the rami. As soon as a gap appears between tooth primordia in this fold, it is filled by a new tooth germ whether the gap results from forward migration of an already-formed tooth or from elongation of the folded ectoderm during growth.In theHeterodontus dentition there are two tooth-form gradients: On one hand, tooth-form in each family changes in the course of time; on the other, the dentition is heterodont at each ontogenetic stage. Thus, within each dentition there is a tooth form and tooth size gradient. Tooth form and size, therefore, are determined by two coordinate parameters:1.the age of the animal and2.the position in the dentition. The observations on the dentition ofHeterodontus lead to the interpretation that the insertion of new teeth and tooth families, as well as their form and size, is regulated by a complex system of reference points which transmit positional information to the tooth-forming cells.

Wound healing in Sharks

SummaryPieces of skin together with scales measuring 7×7 mm were removed from a mature Nurse Shark and a mature Leopard Shark. In the following 2 weeks the wound area secreted mucus, later it contracted and the epidermis regenerated, then it expanded again and newly formed scales erupted. Most of the scar area was covered with scales after 4 months. In the regenerated dermal skeleton the scales show a high degree of variability in that they are much larger and much more complex than the scales of control areas. The repair scales are no longer arranged in diagonal rows and are not so perfectly oriented in a caudal direction as the control scales. Wolperts concept of positional information in pattern formation can explain this type of regeneration behaviour. The fact that regeneration of the dermal skeleton is not perfect leads to the assumption that the flow of positional information was temporarily inhibited by the surgery.The present experiment does not indicate whether the anomalous repairscales will eventually be replaced by normal scales if the flow of positional information is readjusted. The author found, however, several skin samples in his own collection which showed a certain kind of anomaly in the dermal skeleton. While the shapes of all the scales were normal, in some small areas the scales did not point in caudal direction but formed an angle with the caudal direction. They were all oriented parallel to each other. It can be assumed that these small areas are old scars. This leads to the finding that size, shape, and variability of the scales that replace the repair scales are normal and that the scar area gains a new polarity which is at an angle to the original polarity.To test these conclusions a new excision would have to be made. It would probably take 2 years until all the repair scales were replaced.ZusammenfassungBei einem adulten Ammenhai (Ginglymostoma cirratum) und einem adulten Leopardenhai (Triakis semifasciata) wurde je ein Hautstück von 7×7 mm Größe herausgeschnitten. Während der zwei Wochen nach der Operation sonderte die Wunde Schleim ab, später kontrahierte sie und regenerierte die Epidermis; danach expandierte sie, und neugebildete Schuppen brachen durch. Nach 4 Monaten war der größte Teil der Narben mit neu gebildeten Schuppen bedeckt. Im regenerierten Hautskelett zeigen die Schuppen ein hohes Maß an Variabilität, sie sind viel größer und komplizierter gebaut als Kontroll-Schuppen.Die Reparatur-Schuppen sind nicht mehr in Diagonalreihen angeordnet und nicht so perfekt in kaudaler Richtung orientiert wie die KontrollSchuppen. Die Diskussion dieser Beobachtungen zeigt, daß Wolperts Konzept der Positions-information in der Musterbildung dieses Regenerationsverhalten erklären kann. Die Tatsache, daß die Regeneration unvollständig ist, führt zu der Annahme, daß der Fluß der Positionsinformation zeitweilig durch die Operation unterbrochen wurde.Dieses Experiment beantwortet jedoch nicht die Frage, ob die anomalen Reparatur-Schuppen allmählich durch normale Schuppen ersetzt werden, auf Grund der Tatsache, daß sich der Fluß der Positionsinformation wieder normalisiert. Der Autor fand jedoch in seiner Sammlung Hautproben, die eine bestimmte Anomalie im Hautskelett aufwiesen. Die Form aller Schuppen war normal; in einigen Bereichen waren die Schuppen jedoch nicht in kaudaler Richtung ausgerichtet, sondern bildeten einen Winkel zur kaudalen Richtung. Untereinander standen sie jedoch parallel. Es ist sehr wahrscheinlich, daß es sich bei diesen Bereichen um alte Narben handelt. Dies führt zu dem Schluß, daß die Schuppen, die die Reparatur-Schuppen ersetzen, normale Form, Größe und Variabilität haben und daß die Narben-Region eine neue Polarität erlangt, die einen Winkel zur ursprünglichen Polarität bildet. Um diesen Schluß zu überprüfen, müßte eine neue Operation durchgeführt werden. Vermutlich dauert es 2 Jahre bis alle Reparaturschuppen ersetzt sind.

The search for a macroevolutionary theory in German paleontology

SummarySix schools of thought can be detected in the development of evolutionary theory in German paleontology between 1859 and World War II. Most paleontologists were hardly affected in their research by Darwins Origin of Species. The traditionalists (School 1) accepted evolution within lower taxa (genera and families) but not for organisms in general. They also rejected Darwins theory of selection. The early Darwinians (School 2) accepted Darwins theory of transmutation and theory of selection as axioms and applied them fruitfully to the fossil record, thereby laying the foundation for the new research areas of phylogeny and paleo-biology. The enthusiasm of the early Darwinians faded when the fossil record and the problems of its interpretion became more widely known. The pluralists of the turn of the century (School 3) invented and adopted a wealth of hypothetical mechanisms in order to explain individual features of the fossil record. They failed, however, to provide one coherent theory. Dissatisfaction with this situation led to adoption of a dogmatic neo-Lamarckism (School 4), which was regarded as a coherent theory providing a fruitful research program. The rejection of the Lamarckian mechanism early in this century left paleontologists with only one kind of evolutionary mechanism: inner causes.Like many neo-Lamarckians several orthogeneticists (School 5) were highly interested in adaptation and did not see any contradiction between the inner causes of evolution and adaptation. The dominance of stratigraphical research programs in paleontology led in the 1930s and 1940s to a decrease in interest in adaptation. Stratigraphical records of taxa were accepted as meaningful in the context of evolutionary theory. Orthogenesis and the new concepts of saltation and cyclicism were amalgamated into one theory: typostrophism (School 6). This theory dominated German paleontology for decades after the war and only recently has the synthetic theory been seriously considered.Evolution was never very intensively discussed in German paleontology in the hundred years after Darwins book. Most information used here comes from textbooks or from papers given on special occasions. It has been impossible to summarize how members of one school defended their views or discussed the ideas of competing schools.

Hilgendorf’s (1863) dissertation on the steinheim planorbids (Gastropoda; Miocene): The development of a phylogenetic research program for paleontology

KurzfassungHilgendorfs (1866) Publikation über die Phylogenie der SchneckePlanorbis multiformis aus Steinheim, Schwäbische Alb, ist einer der wichtigsten Beiträge der Paläontologie zum frühen Darwinismus. Zugleich half sie, die Grundlagen für eine paläontologische Phylogenetik zu legen. Hilgendorfs unpublizierte Dissertation aus dem Jahre 1863, die kürzlich wiederentdeckte Sammlung zu seiner Dissertation und seine Tübinger Promotionsakte machen es nun möglich, die Entwicklung von Hilgendorfs Ideen zu verfolgen. Hilgendorfs Sammlung enthält den ältesten bisher bekanntgewordenen Stammbaum.AbstractHilgendorf’s (1866) publication on the phylogenyof Planorbis multiformis from Steinheim, Swabian Mountains, is one of the most important contributions of paleontology to early Darwinism. At the same time it helped to lay the foundation for paleontological phylogenetics. Hilgendorf’s unpublished dissertation of 1863, the newly rediscovered collection to his dissertation and the document-file ofHilgendorf’s graduation from Tübingen University help to trace back the origin of Hilgendorf’s ideas.Hilgendorf’s collection contains the oldest phylogenetic tree, which is known so far.

Morphogenesis and histology of large scales of batoids (Elasmobranchii)

KurzfassungGroßschuppen kommen nur in drei Familien der Batoiden vor. Obwohl sie zahlreiche verschiedene Formen haben, dienen sie alle dem Schutz bei bodenbezogen lebenden Arten. Die Krone besteht aus Enameloid, Orthodentin und Osteodentin. Drei verschiedene Typen von Basalplatten kommen vor: (a) dünne Basalplatte, die aus azellulärem Knochen besteht; (b) Basalplatte, die sekundär stark verdickt ist; sie besteht aus massivem azellulärem Knochen und dünnen Denteonen, die die Gefäßkanäle umgeben; (c) Basalplatte, die sekundär stark verdickt ist; sie besteht aus einem besonderen Typ von mikrospongiösem Knochen, der sonst bei Elasmobranchiern nicht auftritt.Die Schuppen haben eine oder mehrera Kronen-Elemente. Keine Schuppe gehört einem wachsenden Typ an.Alle Großschuppen wurden wahrscheinlich regelmäßig ausgetauscht.Zum ersten Mal wurde Dentin innerhalb der Basalplatte von Elasmobranchier-Schuppen gefunden.AbstractLarge scales occur only in three families of batoids. Though they have a wide spectrum of different shapes they all serve protective functions in bottom-dwelling species. The crown consists of enameloid, orthodentine and osteodentine. Three different types of basal plates occur: (a) thin basal plate consisting of acellular bone; (b) basal plate which is secondarily thickened; it consists of massive acellular bone and thin denteons which surround the vascular canals; (c) basal plate which is secondarily thickened, consisting of a peculiar type of microspongy bone which has never been found in other elasmobranchs.The scales have either one or several crown elements. None of the scales, however, belongs to a growing type.All large scales were probably replaced regularly.It is the first time that dentine was found within the basal plate of an elasmobranch scale.