|mhorn||Дата: Thursday, 30.09.2010, 23:57 | Сообщение # 1|
|Hoffmann R. (2010) New insights on the phylogeny of the Lytoceratoidea (Ammonitina) from the septal lobe and its functional interpretation // Revue de Paléobiologie. Vol.29. no.1. P.1-156. |
Lytoceratites together with Phylloceratites are often described as “conservative”. However, the origin and monophyly of lytoceratid ammonites as well as their role in the evolution of all ammonites is under constant debate. In this work, the Lytoceratoidea investigated on generic level for the presence of a septal lobe. Included into this investigation is the verification of relevant literature and the registration of the collected literature in a database system. Furthermore material from public and private collections was studied. The investigation reveals that all Lytoceratoidea are characterised by the presence of a septal lobe which constitutes their monophyly (cp. Arkell et al., 1957). The septal lobe represents a consistent morphological character that appeared in the lowermost Jurassic and is present in the uppermost Cretaceous members of Lytoceratoidea. It was noticed that the septal lobe appeared earlier in ontogeny and
was stronger developed in most of the stratigraphical younger representatives compared to Jurassic members (acceleration). In addition, further characters have been investigated and tested for their phylogenetic significance. It turned out that most of these characters e. g. whorl section, course and number of constrictions are homoplastic or highly variable within one genus. Finally, six characters (septal lobe, parabolic ribs, fimbriation, shape of adult lateral lobe, primary suture, jaw type) have been kept for stratocladistic analyses. With this method the phylogeny within the Lytoceratoidea was enlightened as far as possible. Within the family Lytoceratidae five monophyletic subfamilies are retained: Ectocentritinae, Pleuroacanthinae, Lytoceratinae, Alocolytoceratinae and Megalytoceratinae. The monophyly of the second family Tetragonitidae is based on the distribution of the quinque- and sixlobate primary suture line and the development of a rhynchaptychus (Engeser & Keupp, 2002). Within the Tetragonitidae two subfamilies (Gaudryceratinae and Tetragonitinae) and one tribus Gabbioceratini are retained. This work has been the first critical-systematical review for the whole group within the last forty years. As a main result of this investigation fifteen genera have been excluded from Lytoceratoidea, seventeen genera have been regarded as junior subjective synonyms. A sexual dimorphic couple is presented for Anagaudryceras (M) and Anagaudryceras (Zelandites [m]) for the first time in addition to the two well known of Lytoceras (M). Thirty genera have been retained as valid lytoceratid taxa. For some genera e. g. Lobolytoceras and Protetragonites the stratigraphic distribution has been extended significantly. “Trachyphyllites” costatus is included into Analytoceras hermanni and represents a typical Lower Jurassic lytoceratid member. It appears that the Lytoceratoidea are derived from psiloceratids at the Triassic-Jurassic-boundary as already stated by Guex (1987). The author assents the statement of Guex (1987) and regards the Lytoceratoidea as members of Ammonitina. Tetragonitidae are derived from Protetragonites. Both taxa develop non-fimbriate ribs in contrast to contemporary lytoceratids. Protetragonites fraasi from the Oxfordian (Upper Jurassic) represents the oldest member of that genus. Judging from the two characters sixlobate primary suture and septal lobe, discussed above, Lytoceratoidea represent the most advanced ammonoid group. In the second part of this thesis the functional significance of the septal lobe was investigated. It was planned to calculate the ratio of chamber volume against its surface. For this purpose a Gaudryceras specimen in hollow preservation was scanned using computer tomography at the “Bundesanstalt für Materialforschung und -prüfung” (Berlin). The ammonite was digitalised at “Konrad-Zuse-Institut” (Berlin) using AM IRA®. Unfortunately, the resolution was too low for the project. The positive effect of the ratio of chamber volume against its surface caused by the septal lobe can therefore only be assumed. It is supposed that the septal lobe increases the efficiency of the hydrostatic apparatus.
Middle Jurassic - Lower Cretaceous ammonites & aptychi