Dieter Korn

Morphological pathways in the evolution of Early and Middle Devonian ammonoids

Abstract The principal conch parameters—whorl expansion rate, whorl overlap rate, umbilical width, and whorl thickness—of Early and Middle Devonian ammonoids have been extensively investigated. Stratophenetic analyses show long-term trends in the transformations of these characters over long periods of time, but sudden and rapid reversals can also be observed. On the basis of these four quantifiable conch parameters and supplementary qualitative characters, ten ammonoid morphs were distinguished. Reconstruction of the evolutionary history of these morphs reflects the existence of two major phylogenetic lineages, both already visible in Early Devonian faunas. The agoniatitid lineage is characterized by slow character development and leads to the Frasnian gephuroceratids; the anarcestid lineage displays rapid morphological evolution that leads to the late Givetian pharciceratids as well as the Middle and Late Devonian tornoceratids. Morphological evolution is interpreted as partly limited by geometrical and physical constraints.

Early evolutionary trends in ammonoid embryonic development.

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.

Morphospace occupation of ammonoids over the Devonian-Carboniferous boundary

KurzfassungIn einer Fallstudie wird eine stratophänetische Analyse der aufeinanderfolgenden Ammonoideen-Faunen aus den Cephalopodenkalken des höchsten Devon und untersten Karbon im Rheinischen Schiefergebirge durchgeführt. Die Untersuchung konzentrierte sich auf die Entwicklung der Windungsexpansionsrate (WER), einem wichtigen Merkmal bei Ammonoideen; es beeinflußt die Länge der Wohnkammer und somit die Orientierung des Tieres in der Wassersäule sowie seine Mobilität. Die Studie führt zu dem Ergebnis, daß der Hangenberg-Event einen vollständigen Wechsel in der Ammonoideen-Geometrie verursachte. Alle Clymenien und Tornoceraten starben am oder unmittelbar nach dem Hangenberg-Event aus; der von diesen devonischen Gruppen zurückgelassene Morphospace wurde durch die überlebenden Prionoceraten nur unvollständig neu besetzt.AbstractIn a case study, a stratophenetic analysis of the succeeding ammonoid faunas of the latest Devonian and earliest Carboniferous cephalopod limestones of the Rhenish Massif has been made. This investigation concentrated on the development of the whorl expansion rate (WER), a character very important for ammonoids since it indicates the body chamber length and hence the orientation in the water column as well as mobility. The study leads to the conclusion that the Hangenberg Event caused an almost complete change in the morphospace adopted by ammonoids. All clymenüds as well as tornoceratids became extinct at or immediately after the Hangenberg Event, and the morphospace left behind by these Devonian groups was reoccupied only incompletely by the surviving prionoceratid ammonoids.

Tournaisian and Viséan ammonoid stratigraphy in North Africa

Fifteen ammonoid assemblages can be distinguished in the Tournaisian and Visean succession of Morocco and Algeria; these are (in ascending order): Gattendorfia-Eocanites Assemblage, Gattendorfia-Kahlacanites Ass., Goniocyclus-Protocanites Ass., Acrocanites-Kazakhstania Ass., Pericyclus-Progoniatites Ass., Ammonellipsites-Merocanites Ass., lower Bollandites-Bollandoceras Ass., upper Bollandites-Bollandoceras Ass., Entogonites-Maxigoniatites Ass., Goniatites tympanus Ass., Goniatites rodioni Ass., Goniatites gerberi Ass., Dombarites granofalcatus Ass., Platygoniatites rhanemensis Ass., and Ferganoceras torridum Ass. Several of these faunal complexes can be subdivided into species zones, which can be correlated with ammonoid successions in other regions such as Central Europe. The new family Acrocanitidae is erected and placed in the Prionoceratoidea. Additional new taxa are Acrocanites smidensis n. sp., Jdaidites serpentinus n. gen. n. sp., Beyrichoceras merzougense n. sp., and Goniatites tympanus n. sp.

The youngest African clymeniids (Ammonoidea, Late Devonian) – failed survivors of the Hangenberg Event

A co-occurrence of the clymeniid Postclymenia evoluta Schmidt, 1924 and the goniatite Acutimitoceras hilarum Korn, 2002 is reported from the Anti-Atlas of Morocco. Both species occur in the same limestone horizon within the Acutimitoceras prorsum Zone, that has yielded an exclusive conodont fauna of the Upper praesulcata Zone (latest Devonian, above the Hangenberg Black Shale). This record is firm evidence that some clymeniids survived the global Hangenberg Event, but soon later became extinct without descendants.

THE LAZARUS AMMONOID FAMILY GONIATITIDAE, THE TETRANGULARLY COILED ENTOGONITIDAE, AND MISSISSIPPIAN BIOGEOGRAPHY

Abstract A small early Late Viséan (Mississippian) ammonoid assemblage with Entogonites saharensis new species and Goniatites lazarus new species is described from the eastern Anti–Atlas of Morocco, being the first African record of Entogonites. The family Goniatitidae is a typical Lazarus taxon, which, after a gap representing approximately 10 million years, reappears in the fossil record. The genera Entogonites and Goniatites have a wide paleogeographic distribution (northwest Laurentia, northern and southern Variscides, north Gondwana). This indicates ammonoid cosmopolitism at the genus level at the end of the middle Viséan, before late Viséan ammonoid provinces formed. Entogonites with tetrangularly coiled juvenile whorls and with a low aperture probably had a planktonic life- style.

Palaeozoic Ammonoids – Diversity and Development of Conch Morphology

Palaeozoic ammonoids are not only valuable index fossils, but are also valuable for discriminating global crises in biodiversity such as the end-Eifelian Kacak Event, the Frasnian Kellwasser Events, the late Famennian Hangenberg Event, and the Permian-Triassic Event. They are also useful for solving various questions in palaeobiology; their suture lines and conch geometry permit morphometric studies using multivariate and shape analyses. Results from such analyses accord with results derived from taxa-based diversity curves (Hangenberg Event), but may contrast with them in other cases (the Kellwasser and Permian-Triassic Events). The disparity curve of conch shapes in Palaeozoic ammonoids is markedly discontinuous. Character unfolding at the beginning of their evolutionary history in the Early Devonian was rapid; the subsequent evolutionary development exhibits phases of wide morphologic extent (i.e. high disparity: middle to late Famennian, late Visean to Bashkirian, Artinskian to Wordian) alternating with intervals of less morphologic extent (i.e. low disparity: earliest Famennian, earliest Tournaisian). Palaegeographic distribution of morphospace occupation is discussed in the context of the early rediversification and the end-Permian endemism and diversity-decline.

Allometric growth and intraspecific variability in the basal Carboniferous ammonoidGattendorfia crassa Schmidt, 1924

Gattendorfia crassa is an Early Carboniferous (Mississippian) goniatite species with strikingly allometric conch growth. Analysis of 15 high-precision cross-sections of this species demonstrates the small intraspecific variability of some of the conch form characters, but remarkable variability in others. While the whorl expansion rate, umbilical width, and conch thickness vary within narrow limits, the expansion rates of the whorl height and whorl width are remarkably plastic. Variability of most of the characters tends to be smallest in intermediate growth stages, whereas juveniles and adults are more variable. The differences in morphological plasticity are interpreted in terms of the function of the ammonoid conch, especially the orientation of the aperture during life.KurzfassungGattendorfia crassa ist eine unterkarbonische Goniatiten-Art mit auffallend allometrischem Gehäuse-Wachstum. Die Analyse von 15 Hochpräzisions-Querschnitten dieser Art zeigt die geringe intraspezifische Variabilität einiger Gehäuseparameter und die bemerkenswerte Variabilität in anderen Merkmalen. Während die Windungsexpansionsrate, Nabelweite und Gehäusebreite nur in engen Grenzen variieren, sind die Expansionsraten der Windungshöhe und Windungsbreite sehr plastisch. Bei den meisten Merkmalen ist die Variabilität in intermediären Stadien am geringsten, während Jugend- und Adultstadien sehr variabel sind. Die Unterschiede in der morphologischen Plastizität werden durch die Funktion des Gehäuses interpretiert, besonders durch dessen Orientierung der Mündung zu Lebzeiten.

Typostrophism in Palaeozoic Ammonoids

KurzfassungDie antidarwinistische „Typostrophentheorie“ von O.H.Schindewolf wird mit den Ammonoiden-Beipielen getestet, auf welchen sie begründet worden ist. Es kann gezeigt werden dass keines der drei theoretischen Elemente der Theorie (Saltationismus, Internalismus und Zyklismus) durch empirische Befunde gestützt werden kann. Vermeintliche Saltationen („Typogenese“) werden durch das Fehlen von Zwischenformen vorgetäuscht. Internalistische und orthogenetische Entwicklung („Typostase“) kann nur postuliert werden, wenn mögliche Funktionen abgelehnt werden. Vorprogrammiertes Aussterben von „degenerierten“ Entwicklungslinien („Typolyse“) kann ausgeschlossen werden, wenn Ammonoideen-Morphologien frei von anthropozentrischen Ansichten betrachtet werden. Auf Grund der Studie von paläozoischen Ammonoideen gibt es keinen Grund, die „Typostrophenlehre“ oder einige der sie aufbauenden Elemente, wie das „Typus-Konzept“ und „Proterogenese“, dem darwinistischen Evolutionsmodell vorzuziehen.AbstractThe anti-Darwinian “Typostrophe Theory” of O.H.Schindewolf can be put to the test by revisiting the ammonoid examples on which this macroevolutionary model was founded. It is shown that none of the three theoretical elements saltationism, internalism, and cyclism can be supported by empirical data obtained from ammonoid research. Putative saltations (“Typogenesis”) were feigned because of the lack of knowledge of intermediate forms. Internalistic and orthogenetic development (“Typostasis”) can only be favoured by neglecting possible functions of morphological characters. Preprogrammed extinction of “degenerated” clades (“Typolysis”) is unlikely when ruling out anthropocentric views regarding ammonoid morphology. In terms of evolution of Palaeozoic ammonoids, there is no basis for the preference of the “Typostrophe Theory” or some of its composing elements, including the “Type Concept” and “Proterogenesis”, over the Darwinian evolutionary model and the Modern Synthesis.

Early late Visean ammonoid faunas from the Jadar Block (NW Serbia)

Early late Visean ammonoid faunas from the Jadar Block (NW Serbia) The outcrop at Milivojevića Kamenjar in Družetić (Jadar Block, Vardar Zone, NW Serbia), which exposes a fossiliferous limestone olistolith, is one of the key sites for Carboniferous stratigraphy and paleogeography in the Balkan Peninsula. Its age has been debated several times, and re-examination of the succession was required. Based on ammonoids and conodonts, an interval spanning from the latest Devonian to the basal Serpukhovian is represented. From the early late Visean portion of the section, the new ammonoid genus and species Ubites filipovici gen. nov. et sp. nov. is described. Entogonites tetragonus (Kullmann, 1962), a formerly misinterpreted ammonoid species, is revised.