A conch with a collar: early ontogeny of the enigmatic fossil bivalveMyoconcha

Abstract

<jats:p>Larval and juvenile growth stages of myoconchid bivalves (family Kalenteridae) are illustrated and described for the first time. Excellently preserved shells of<jats:italic>Myoconcha crassa</jats:italic>J. Sowerby, 1824 from the middle Bathonian (Middle Jurassic) clay/silt deposits of southern Poland reveal that the prodissoconch in this taxon is large (exceeding 350 µm in length) and characterized by a prominent, collar-like structure arising from the demarcation to the dissoconch. Similar prodissoconch morphology has not been described previously in any other bivalve genus. The large size and absence of a prodissoconch II strongly suggest a non-planktotrophic development of the larvae, indicating either simple lecithotrophy or long-term brooding and parental care for the larvae in this species. Consequently, the new discovery constitutes the earliest well-documented record of non-planktotrophic development in the Bivalvia. The shell of<jats:italic>Myoconcha crassa</jats:italic>is composed of an outer cross-lamellar layer and an inner complex cross-lamellar layer. This composition suggests that the Kalenteridae may be included in the Carditida rather than the Palaeoheterodonta or Anomalodesmata. The genus<jats:italic>Pseudomyoconcha</jats:italic>Rossi Ronchetti and Allasinaz, 1966 is rejected herein and considered as a junior synonym of<jats:italic>Myoconcha</jats:italic>J. Sowerby, 1824.</jats:p>

Four new species of the Jurassic to Cretaceous seep-restricted bivalveCaspiconchaand implications for the history of chemosynthetic communities

“…Kalenterids were previously classified within the Pholadomyoida (e.g., Waller, 1990; Fang and Morris, 1997; Kelly et al, 2000; Griffin and Pastorino, 2006), Palaeoheterodonta (Cox et al, 1969; Hautmann, 2001, 2008), or Carditida (e.g., Chavan, 1954, 1969; Newell, 1957, 1969; Nevesskaja, 2009). Here we follow the interpretation of Kaim and Schneider (2012) who placed the kalenterids among the Carditida based on the presence of complex cross-lamellar shell microstructure and the lack of a nacreous layer in two genera of kalenterids: Caspiconcha (Jenkins et al, 2013) and Myoconcha J. Sowerby, 1824 (Kaim and Schneider, 2012).…”

Patellogastropoda and Vetigastropoda (Mollusca, Gastropoda) from the marine Jurassic of Patagonia, Argentina

“…Ancient species of Calliotropis are abundant in the fossil record and well known from the Triassic of Europe, Asia and South America as family Eucyclidae. These fossil representatives of the group are very similar to their extant counterparts, and Kaim (2004) has observed that the only difference between extant calliotropids and the Jurassic forms is the absence of umbilicus among the latters. Though several Jurassic calliotropid species have been united in genus Riselloidea Cossmann, Kaim (2004) suggested that actually there are no serious taxonomic 568 S.M.…”

A highly diverse bivalve fauna from a Bithynian (Anisian, Middle Triassic) Tubiphytes‐microbial buildup in North Dobrogea (Romania)

“…The ‘actinodont’ hinge of the Kalenteridae indicates their affiliation to the Modiomorphoidea and Palaeoheterodonta (Hautmann 2001a). In contrast, Kaim & Schneider (2012) placed the Kalenteridae in the Carditida within Heteroconcha, because the inner shell layer of Myoconcha and Caspiconcha is cross‐lamellar, rather than nacreous. However, the crossed‐lamellar shell structure developed several times independently, for example in Pectinida and Heterodonta (e.g.…”

“…Hodges (2000) suggested an endobyssate mode of life for oval modioliform specimens of Myoconcha , whereas Kaim & Schneider (2012) suggested an epifaunal mode of life for mytiliform Myoconcha specimens. Because of its incomplete preservation the exact mode of life for the Dobrogea specimen remains unclear.…”