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| general view of the cephalic region of an orthocerid in an attacking position. The Aptychopsis is working as a protective shield, the dorsal plate is displaced to open the mouth with a radula on a short proboscis. The number of arms (ten) is based on Shigeno et al. 2008, 2010. The presence of well-developed eyes in orthocerids is based on the molecular study of Nautilus eyes (Ogura et al. 2013). in Mironenko, 2021. DOI: 10.1111/let.12414 |
Abstract
Cephalopoda is the only class of molluscs in which virtually all its modern representatives have a pair of powerful jaws. There is little doubt that jaws have contributed to the evolutionary success of cephalopods, but their origin still remains a mystery. Though cephalopods appeared at the end of the Cambrian, the oldest unequivocal jaws have been reported to date from the Late Devonian, though they were initially interpreted as phyllopod crustaceans of the suborder Discinocarina. After their relation with ammonoids was proven, they were considered as opercula, and only later their mandibular nature was recognized and widely accepted. Finds of discinocarins from Silurian deposits are still considered as opercula of ammonoid ancestors - nautiloids of the order Orthocerida. However, according to modern ideas, there is no place within their soft body for the location of such large opercula. Moreover, the repeated appearance of very similar structures in the same evolutionary line at least twice, but in different places of the body and for different purposes seems highly improbable. A new hypothesis is proposed herein, in which the Silurian fossils, earlier assigned to Discinocarina, are not specialized opercula, but protective shields, to defend orthocerids not from the predators, but from their own prey. The chitinous plates around the mouth likely appeared in the Silurian orthocerids for protection from such damage and later, during Silurian and Devonian, most likely gradually evolved into the jaws.
Keywords: Anaptychi, aptychi, Aptychopsis, Cephalopoda, Discinocarina, jaw apparatus
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| Hypothetical reconstruction of Silurian Orthocerida with Aptychopsis as proto-jaws. A, general view of the cephalic region of an orthocerid in an attacking position. The Aptychopsisis working as a protective shield, the dorsal plate is displaced to open the mouth with a radula on a short proboscis. The number of arms (ten) is based on Shigeno et al. 2008, 2010. The presence of well-developed eyes in orthocerids is based on the molecular study of Nautilus eyes (Ogura et al. 2013). B, various views of Orthocerida with Aptychopsis. A small formation on the top of the orthocerid's head is an anterior part of a collar. In modern Nautilus it is a part of the protective hood (Shigeno et al. 2008), but in ancient cephalopods most likely it served to connect the head to the shell and to support collar folds (see Mironenko 2015). (Andrey Atuchin, based on the sketch by A. A. Mironenko). |
Aleksandr A. Mironenko. 2021. Early Palaeozoic Discinocarina: A Key to the Appearance of Cephalopod Jaws. Lethaia: an international journal of palaeontology and stratigraphy. 54(4); 457-476. DOI: 10.1111/let.12414