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Ammonoid Articles

DWhatley

Cthulhu
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#1
Paleobiology of the Latest Tithonian (Late Jurassic) AmmoniteSalinites grossicostatum Inferred from Internal and External Shell Parameters
Patrick Zell, Wolfgang Stinnesbeck 2016

Ontogenetic sequence reconstruction and sequence polymorphism in extinct taxa : an example using early tetrapods (Tetrapoda: Lepospondyli) Jennifer C. Olori 2013

The earliest Cenomanian ammonoid Tanabeceras yezoense (Shigeta) from the Hobetsu area, Hokkaido (PDF)
Paleontological Research, vol. 16, p. 208–218. Spath, LF, 1927: Revision of the
Jurassic cephalopod fauna of Kachh (Cutch), part 1. Memoires of the Geological Survey
of India, Palaeontologia Indica (new series), vol. 9, p. 1–71.

A New Species of the Heteromorph Ammonoid Phylloptychoceras from the Lowest Maastrichtian of Hokkaido, Japan
Yasunari Shigeta [SUP]1[/SUP] and Tomohiro Nishimura [SUP]2[/SUP] [SUP]1[/SUP] Department of Geology and Paleontology, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan (e-mail: shigeta@kahaku.go.jp)
[SUP]2[/SUP]Hobetsu Museum, 80-6, Hobetsu, Mukawa, Hokkaido 054-0211, Japan
Abstract.

A new Cretaceous heteromorph ammonoid, Phylloptychoceras horitai sp. nov., is described from the lowest Maastrichtian of Hokkaido, Japan. Its shell is ornamented with very weak, broadly rounded ribs and its suture line is characterized by a deeply incised, trifid dorsal lobe and three bifid lateral saddles with minor indentations. This occurrence suggests that Phylloptychoceras evolved in the North Pacific during late Campanian or early Maastrichtian time and then achieved worldwide distribution during late Maastrichtian time.
 
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DWhatley

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#2
Ammonite aptychi: Functions and role in propulsion
Horacio Parent, Gerd Westermann, John A. Chamberlain 2013 (subscription)

Abstract
Seven previous proposals of aptychus (sensustricto) function are reviewed: lower mandible, protection of gonads of females, protective operculum, ballasting, flushing benthic prey, filtering microfauna and pump for jet propulsion. An eighth is introduced: aptychi functioned to actively stabilize the rocking produced by the pulsating jet during forward foraging and backward swimming. Experiments with in-air models suggest that planispiral ammonites could lower their aperture by the forward shift of a mobile cephalic complex. In the experiments, the ventral part of the peristome is lowered from the lateral resting (neutral) position by the added “ballast” of a relatively thin Laevaptychus to an angle <25° from horizontal with adequate stability to withstand the counter-force produced by the jet of the recurvedhyponome. However, of the shell forms tested, only brevidomes with thick aptychi, e.g.,the Upper Jurassic Aspidoceratidae with Laevaptychus and average whorl expansion rates, were stable enough to swim forward by jet propulsion at about Nautilus speed (∼25 cm/s).We propose that aptychus function most commonly combined feeding (jaw, flushing, filtering) with protection (operculum), and, more rarely, with locomotion (ballast, pump, diving and stabilizing plane). Aptychi may thus have been multi-functional.
 

Architeuthoceras

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#4
@Architeuthoceras, After I posted this, I noticed you had referenced it on Facebook but I beat you to posting it here :grin:.
This old thread talked about aptychi and how they may have been opercula, I just thought it nice that the last few words of the abstract were "Aptychi may thus have been multi-functional"
 

DWhatley

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I had to laugh at that statement as well but did not realize that is the only reason you mentioned it on FB :oops:
 

DWhatley

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#6
Fossil Focus: Ammonoids
Kenneth De Baets, René Hoffmann, Jocelyn A. Sessa, Christian Klug 2016 (full article)

Nice layman readable article

Introduction:
Ammonoids (Ammonoidea) are an extinct group of marine invertebrates with an external shell. They were cephalopods, and hence closely related to modern cuttlefish, squid, octopuses and the pearly nautilus. In a non-scientific context, they are commonly called ammonites, but that term really includes only Jurassic and Cretaceous forms in its stricter scientific sense. The Ammonoidea as a whole lived from the Early Devonian to the earliest Palaeogene period, covering a timespan of about 350 million years. Normally, only their shells, also called conchs, or their internal moulds are found in the fossil record. Conchs from adult ammonoids range from about 5 millimetres to 2 metres in diameter.

Due to the large diversity (taxonomic richness), disparity (morphological richness), nearly global distribution and abundance of their shells in the fossil record, ammonoids have been valued by geologists, palaeontologists, biologists and fossil collectors alike. They have been particularly useful for studies of biodiversity and for correlating and assigning relative ages to rocks (the field of biostratigraphy). Ammonoids have also proved valuable for studying the processes and patterns of evolution, because they repeatedly evolved towards more coiled, larger and/or more complex conchs. They probably had a large variety of life modes and reproductive strategies, but despite the widespread attention that they have received, there are still several controversies concerning the group’s palaeoecology, anatomy and evolutionary relationships.
I particularly like this simple description of the conch shell parts
Whatever the shape, the conch of all ammonoids can be subdivided into a body chamber containing the soft parts, and a phragmocone consisting of individual chambers separated by walls called septa and connected by an organic tube called the siphuncle (Figs. 1, 2). The phragmocone of ammonoids and other chambered, now-extinct, cephalopods functioned as a buoyancy apparatus, as in extant Nautilus or Spirula, and implies a mode of life of floating in the water column.
 
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