Embryonic Development of Primitive Ammonoids and the Monophyly of the Ammonoidea

Ammonoid Embryonic Development

“…Wrinkle-like creases are also present on the shell and extend perpendicular to the lirae, giving the shell a reticulate appearance ( Fig. 5.7e; Klofak et al 1999). The lirae are symmetrical in cross section and represent periodic thickenings on the shell surface.…”

“…Occasionally, the outer prismatic layer doubles back along the inside surface of the primary varix forming a short return section (Kulicki 1979). A primary varix has not been documented in the Agoniatitida (Klofak et al 1999(Klofak et al , 2007. In the Bactritina, a nacreous layer appears gradually on the straight part of the shell adapical of the primary constriction (Doguzhaeva 2002).…”

“…In more derived taxa such as Mimagoniatites and Archanarcestes (Mimagoniatitidae) as well as Anarcestes and Sellanarcestes (Anarcestidae; Fig. 5.2i,j), the embryonic shell coil is about one-half whorl, but the umbilical window is still partially surrounded by the post-embryonic whorls (Klofak et al 1999(Klofak et al , 2007De Baets et al 2012). In more derived taxa, the umbilical window becomes fully enclosed by the embryonic shell coil (e.g., Praewernoceras: More derived taxa from the Middle Devonian to Late Cretaceous typically have a barrel-to spindle-shaped initial chamber, where the succeeding whorls are closely coiled around the initial chamber leaving a shallow to deep dorsal impression in the whorl profile as seen in transverse cross section.…”

“…Landman et al (1996) pointed out that only little was known about the embryonic development of Paleozoic ammonoids compared with Mesozoic ones. Numerous studies have recently focused on the embryonic shell structure and development of Paleozoic ammonoids (House 1996;Landman et al 1999;Klofak et al 1999Klofak et al , 2007Shigeta et al 2001;Tanabe et al 2001;Kulicki et al 2002;Landman 2010, 2012;De Baets et al 2012. Additional studies on Mesozoic ammonoids, particularly on embryonic shell structure, have also come out since 1996 (Sprey 2001(Sprey , 2002Landman et al 2001;Shigeta et al 2001;Rouget and Neige 2001;Tanabe et al 2003Tanabe et al , 2008Tanabe et al , 2010.…”

“…Early ontogenetic development played a central role in the discussion of the derivation of ammonoids from coiled nautiloids (e.g., Spath 1933Spath , 1936Böhmers 1936;Donovan 1964) or straight bactritoids (e.g., Schindewolf 1932Schindewolf , 1933Schindewolf , 1934. Mainly due to the work of Erben (1960Erben ( , 1962aErben ( , b, 1964Erben ( , 1965Erben ( , 1966 and Bogoslovsky (1969), it is now well accepted that ammonoids can be traced back to a bactritoid ancestor (Dzik 1981(Dzik , 1984Klofak et al 1999;Doguzhaeva 2002;Klug and Korn 2004;De Baets et al 2009;Kröger et al 2011;Ritterbush et al 2014), which in turn can be traced back to an orthoceratid ancestor (Kröger and Mapes 2007;compare Ristedt 1971, Dzik 1984, Holland 2003Klug et al 2015). This is supported by the presence of a similar early shell with a small, elliptical initial chamber and a transitional series linking all intermediate forms from straight orthocerid nautiloids, through straight to slightly curved bactritoids, to coiled ammonoids, which is also consistent with their stratigraphic appearance (Kröger and Mapes 2007;De Baets et al 2013b).…”