Ancestry, Origin and Early Evolution of Ammonoids

First report of Devonian bactritids (Cephalopoda) from South America: paleobiogeographic and biostratigraphic implications

“…Given that we have no data about the apical part of these specimens, we cannot clarify their relations with the Emsian material. The presence of those traits considered "bactritid-like," such as the ventral lobe of the suture and the small, ventral siphuncle in "Bactrites" bohemicus Ristedt (in Ristedt, 1981), was considered homeomorphism by Kröger and Mapes (2007) and Klug et al (2015). However, we believe that the fact that they were found within one stratigraphic section suggests these pre-Emsian specimens are most probably bactritids as well.…”

“…However, determining which orthocerid group is the bactritid ancestor and when it occurred is an issue of ongoing discussion. The synapomorphies linking Orthocerida and Bactritida are a small subspherical to ovoid initial chamber, the straight to slightly bent conical shell, and the narrow siphuncle (Klug et al, 2015).…”

“…In addition to the interesting phylogenetic aspects concerning their origin, their important role in the evolutionary history of cephalopods has been especially debated (e.g., Erben, 1964;Kröger and Mapes, 2007;Kröger et al, 2011;Klug et al, 2015). The present consensus assumes they evolved from an orthocerid group during the latest SilurianEarly Devonian, and, in turn, gave rise to ammonoids during the Devonian and to coleoids in the Mississippian (Erben, 1966;Mapes, 1979;Doguzhaeva, 2002;Kröger and Mapes, 2007;Klug et al, 2015).…”

Evolutionary Patterns of Ammonoids: Phenotypic Trends, Convergence, and Parallel Evolution

“…This pattern has even been documented in the earliest history of ammonoids: during the Early Devonian, ammonoids showed a progressive coiling from straight orthocerids via curved bactritoids to coiled ammonoids (Erben 1966;Wiedmann 1966;Klug 2001;Klug and Korn 2004;De Baets et al 2009. This increasing shell involution (protracted closure of the umbilicus by increasing overlap of the whorls) is also the most commonly described long-term morphological trend among more derived ammonoids (e.g., Hyatt 1889;Schindewolf 1940;Haas 1942;Guex 1973Guex , 1981Guex , 1992McGhee 1984, 1985;Dommergues 1990;De Baets et al 2009Klug et al 2010Klug et al , 2015aMonnet et al 2010Monnet et al , 2012Monnet et al , 2013; see also Monnet et al 2011 and references therein). Note that uncoiling of ammonoids has also been documented repeatedly (e.g., Wiedmann 1969;Cecca 1997).…”

Ammonoid Buoyancy

“…Possibly, the phragmocone is the most important autapomorphy of the Cephalopoda (for cephalopod ancestry and origin see Klug et al 2015a;Kröger et al 2011;Ax 2001). With respect to cephalopod buoyancy, a hypothesis suggested by Dzik (1981) has to be mentioned: According to Pojeta (1980), the siphuncle of the earliest cephalopods, i.e.…”