Aptychus

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Aptychus in the living room
Ammonite Oppelia in Solnhofen limestone (Upper Jura, Bavaria)
Ammonite of the genus Oppelia with aptychus of the form genus Lamellaptychus in the living room, Solnhofen limestone , width of the image section: approx. 10 cm

Aptychs ( Sing. Aptychus ) are bilaterally symmetrical fossils , which are mostly isolated, but sometimes found in very close connection with the remains of "modern" ammonites (Neoammonoidea). Hence, they are considered body parts of these extinct cephalopods. They appear in the Upper Lower Jurassic ( Toarcian ) a relatively short time after the first neoammonoids in the fossil record and disappear together with all ammonites at the Cretaceous-Paleogene border .

Etymology and history

Scheuchzer (1702) by Meyer (1829)
Left: Isolated valves of Laevaptychus longus , depicted as mussel valves (“ Tellinoides ”) in Scheuchzer's Specimen Litographiæ Helveticæ Curiosæ (1702).
Right: “ Aptychus laevis latus ” (today Laevaptychus latus ) in the living room of an indistinctly preserved ammonite, depicted in von Meyer's formative work from 1829.

The name "Aptychus" was coined in 1829 by the then relatively young, important German paleontologist Hermann von Meyer . It is a Latinized compound from Alpha privativum and the Greek word πτυχός ptychos , which means something like 'butterfly'. The compound word therefore means 'unfaltering'. Meyer wanted to use this name to express the two-sided formation of these fossils, which are reminiscent of certain invertebrates, especially mussels , but which he recognized as different.

Already well before that, in the early 18th century, but also at the beginning of the 19th century, various naturalists actually described aptyches as shells under names such as Tellinoides * ( Scheuchzer ) and Tellinites * ( Schlotheim ). Meyer (1829) assigned "his" aptychs to an independent, extinct branch of the molluscs , but admitted that his observations on pieces from the Solnhofen limestone lead to the alternative conclusion that the "biscuit-shaped" formations were the result , which he interpreted as the soft body of the "aptych animal" could be ammonites cut across. Other contemporary authors ** at least interpreted the aptyches as hard parts of cephalopods, although not ammonites. Still others thought they were skull elements of bony fish and therefore called them "Ichthyosiagones" (from ἰχθύς ichthys 'fish' and σιαγών siagon 'jaws').

It was the French Philippe Louis Voltz who in 1837 was the first to publicly support the hypothesis that aptychs are body parts of ammonites with a certain degree of conviction, and he substantiated this by showing, among other things, that certain aptych species are only associated with certain ammonite species that the ratio of the size of an Aptychus to the size of the ammonite with which it is associated is always roughly the same. While this view and the term "aptychen" became generally accepted in the following decades, a controversy arose about which parts of the ammonite's body it actually was and what function they fulfilled. In the 1880s in particular, the interpretation of paired nidamental glands (eggshell glands) in the mantle cavity as a protective cover was intensely discussed among experts. Today they are generally considered to be modified lower jaws, but there is still no agreement on their exact functions (see below ).

* These two names indicate that the respective authors interpreted the pieces in question as Tell mussels .
** Details and general brief overviews of the various assumptions on the taxonomic affinity of the aptych up to at least the early 1830s can be found in Meyer (1831), Bronn (1838) and Trauth (1927)

Features, nomenclature and function

Schematic representation of an aptych valve with description of the morphology elements

Unless they are separated from one another by taphonomic processes, aptychs form a pair of curved, almost unequal-triangular "plates" (flaps, valves) whose concave inner surfaces are smooth and whose convex outer surfaces can be sculptured in various ways, depending on their origin. Since the valves have a certain thickness, there are narrow edge surfaces called facets . With regard to the three “sides” of a valvum, the facet on which the two valvas of an aptychus lie next to each other is called the symphysial † or harmonic facet. It is clearly delimited from the two adjacent facets by two sharp "kinks", the terminal and apical edge . The apical edge separates the symphysial facet from the internal facet . The latter is followed by a relatively sharp bend, the umbonal or umbilical edge , the lateral facet, followed by a slight bend, the marginal edge , by the external facet , which is separated from the symphysial facet by the terminal edge . The internal facet thus corresponds to the shortest, lateral and external facets, which can flow smoothly into one another, to the longest side of the unequal triangle. The lines of intersection of facet and outer surfaces of the hot Valven edges (engl. Margins ) corresponding to the facet and inner surfaces are edges (engl. Edges given). At least the inner surface usually shows concentric growth strips that run parallel to the lateral and external edges. If the marginal edge is relatively clearly pronounced, a ridge, which is called the keel , extends on the outer surface, regardless of any sculpting, between the apical and marginal corners . The inclined from the keel line to Symphysialrand surface Kiel slope (engl. Also adharmonic slope ) called the trailing face is to Lateralrand edge slope mentioned. It can still between keel line and flank slope edge depression be formed. This relief is reproduced as a negative from the inner surface.

Overall, a completely preserved aptych is reminiscent of a gaping shell . However, the aptychs lack characteristic shell features such as the lock (“valve joint”) and the scars of the sphincter muscles and the surface line on the inside of the valves .

In contrast to the aragonitic housing of the ammonites, aptyches are calcitic and therefore usually handed down as originals. However, since the preservation of an aptychus together with the case in situ is relatively rare, the systematic classification (see Etymology and History ) and later the function of the aptychs was completely unclear for a long time.

Schematic reconstruction of the soft body of an ammonite in a longitudinal section showing the aptychus as both a functional lower jaw and a housing cover

In extremely good preservation, aptychs in the living chamber of ammonites have been handed down together with organic, sometimes weakly mineralized, hook-beak-like structures that are interpreted as upper (dorsal) jaw elements, with remnants of the radula between the two . This is considered to be relatively unequivocal evidence that aptychs are anatomically the lower (ventral) elements of the jaw apparatus of ammonites. In addition, because the size and shape of aptychs correspond to those of the case mouths of the ammonites with which they were found together, a secondary function as an operculum (case lid) is likely. Furthermore, a secondary function as ballast (to align the mouth downwards or to shift the center of gravity of the animal as far down as possible (ventrad) and thus increase its stability in the water column), as a hydrodynamic organ for damping , was and is discussed the rotational movements of the housing as an undesirable side effect of the recoil drive or for the generation of a turbulent flow, by means of which the sediment surface was swirled up in order to transport animals on it into the water column for the purpose of prey (English flushing ), as well as a combination of primary and / or several secondary Functions. A reduction in the upper jaw found in some species suggests that at least in these, the function of the aptychus as a lower jaw has largely been lost and that it only fulfilled secondary functions, in particular as a housing cover.

† From “symphysis” as a term for a relatively mobile, common interface of paired skeletal elements (cf. symphysis in the vertebrate anatomy ).

Anaptychs

In addition to the fossilized Calicic plates, an aptychus originally also consists of a chitinous structure on which the two plates rest. This structure is sometimes preserved as a blackish coating on the unsculptured inner surface of the chalky aptych. It is homologous to the unmineralized, large lower jaws, which are known from several, often pre-Jurassic ammonoids and are known as anaptychen . Anaptychen with pronounced rostrum, i.e. with a hook-shaped tip, are to be distinguished from those with greatly reduced rostrum, the latter being known exclusively from post-Triassic. In the narrower sense, only the unmineralized lower jaws with strongly reduced rostrum are called anaptychs ("Anaptychus type morphotype"), while those with pronounced rostrum are called "normal lower jaws" ("normal type morphotype"). Since aptychs are characterized by the reduction of the rostrum, they should be derived from such anaptychs. These probably also functioned as a secondary housing cover. Calcitic aptychs and organic anaptyches are also summarized as aptychs sensu lato , whereby the designation diaptyches was proposed for the calcitic aptyches sensu stricto .

Systematic benefit

Inclusion of characteristics of the aptychs in cladistic analyzes of the neoammonoids revealed that all aptych-bearing forms form a monophyletic group that was called aptychophora. In addition, the inclusion of the aptych features led to a better resolution of the relationships within certain subgroups of the neoammonoids.

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General

  • Ulrich Lehmann, Gero Hillmer: Invertebrates of the past. 4th revised and expanded edition. Enke, Stuttgart 1997, ISBN 3-8274-1258-7 , p. 144 f.
  • Aptychen in the spectrum online lexicon of geosciences

Individual evidence

  1. ↑ a b Herrmann von Meyer: The genus Aptychus. Nova Acta Physico-Medica Academiae Caesareae Leopoldino-Carolinae Naturae Curiosum. Vol. 15, Part 2, 1829, pp. 125–170 ( BHL )
  2. ^ Johann Jakob Scheuchzer: Specimen Litographiæ Helveticæ Curiosæ. Book printer David Gessner, Zurich 1702, doi: 10.3931 / e-rara-12116 , p. 21 .
  3. ^ Ernst Friedrich von Schlotheim: The Petrefactenkunde on their current position. Becker'sche Buchhandlung, Gotha 1820 ( HathiTrust ), p. 182 ff.
  4. ^ A b Heinrich Georg Bronn: Ichthyosiagones. S. 86 in: AG Hoffmann (Ed.): General Encyclopedia of Sciences and Arts. Section Two: H - N. Fifteenth Part: Ibaba - Jesztreb. FA Brockhaus, Leipzig 1838 ( GDZ )
  5. ^ Philippe Louis Voltz: Second lecture on the genus Aptychus. Yearbook for mineralogy, geognosy, geology and petrefacts. 8th year, 1837, pp. 432–402 ( BSB OPACplus )
  6. ^ A b c Friedrich Trauth: Aptychenstudien. I. About the aptychs in general. Annals of the Natural History Museum in Vienna. Vol. 41, 1927, pp. 171-259 ( PDF 9.0 MB).
  7. ^ Herrmann von Meyer: The genus Aptychus. Yearbook for mineralogy, geognosy, geology and petrefacts. 2nd year, 1831, pp. 391–402 ( BSB OPACplus )
  8. ↑ Lucie Měchová, Zdeněk Vašíček, Václav Houša: Early Cretaceous ribbed aptychi - a proposal for a new systematic classification. Bulletin of Geosciences. Vol. 85, No. 2, 2010, pp. 219-274 ( online ), Fig. 2
  9. Jump up ↑ Patrick Zell, Wolfgang Stinnesbeck, Seija Beckmann: Late Jurassic aptychi from the La Caja Formation of northeastern Mexico. Boletín de la Sociedad Geológica Mexicana. Vol. 68, No. 3, 2016, pp. 515-536 ( PDF 8.5 MB), Fig. 4
  10. ↑ The latter is favored by Günter Schweigert, at least for species with strongly calcified aptychi in the families Aspidoceratidae and Oppeliidae, excluding a function as an operculum: First three-dimensionally preserved in situ record of an aptychophoran ammonite jaw apparatus in the Jurassic and discussion of the function of aptychi. P. 321–330 in: Rolf Kohring, Frank Riedel, Kerstin Zobel (eds.): On the 60th birthday of Helmut Keupp, Berlin, 2009. Berliner Paläobiologische Abhandlungen, Vol. 10. Freie Universität Berlin, 2009 ( PDF 750 kB) .
  11. ↑ Horacio Parent, Gerd EG Westermann, John A. Chamberlain Jr: Ammonite aptychi: Functions and role in propulsion. Geobios. Vol. 47, No. 1–2, 2014, pp. 101–108, doi: 10.1007 / s13358-015-0102-1 (alternative full text access : ResearchGate ).
  12. ↑ Horacio Parent, Gerd EG Westermann: Jurassic ammonite aptychi: functions and evolutionary implications. Swiss Journal of Palaeontology. Vol. 135, No. 1, 2016, pp. 101-108, doi: 10.1007 / s13358-015-0102-1 .
  13. ↑ Helmut Keupp: Complete ammonoid jaw apparatuses from the Solnhofen plattenkalks: implications for aptychi function and microphagous feeding of ammonoids. New Yearbook of Geology and Paleontology - Treatises. Vol. 245, No. 1, 2007, pp. 93-101, doi: 10.1127 / 0077-7749 / 2007 / 0245-0093 .
  14. ↑ a b Kazushige Tanabe, Isabelle Kruta, Neil H. Landman: Ammonoid Buccal Mass and Jaw Apparatus. Pp. 429–484 in Christian Klug, Dieter Korn, Kenneth De Baets, Isabelle Kruta, Royal H. Mapes (eds.): Ammonoid Paleobiology: From Anatomy to Ecology. Topics in Geobiology, Vol. 43. Springer 2015, ISBN 978-94-017-9629-3 , doi: 10.1007 / 978-94-017-9630-9_10 (chapter; alternative full-text access: ResearchGate ), Fig. 10.4 and p. 456 ff.
  15. ↑ Calvin J. Frye, N. Rodney M. Feldmann: North American Late Devonian cephalopod aptychi. Kirtlandia. Vol. 46, 1991, pp. 49-71 ( BHL ), p. 50 .
  16. ^ Theo Engeser, Helmut Keupp: Phylogeny of the aptychi-possessing Neoammonoidea (Aptychophora nov., Cephalopoda). Lethaia. Vol. 35, No. 1, 2002, pp. 79-96, doi: 10.1111 / j.1502-3931.2002.tb00070.x .

Web links

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