Early ontogeny, anomalous growth, and healed injuries in the Silurian nautiloid Ophioceras Barrande - Implications for hatching and the autecology of the Tarphycerida

Abstract

Our study of the early ontogeny of the Silurian Ophioceras has led to the revision of the current concept that many juvenile tarphycerids possessed a coiled conch upon hatching and thus resembled adults in habit as is in extant Nautilus. In fact, there is no evidence that any Early Palaeozoic coiled nautiloid possessed an embryonic conch exceeding half a whorl. A change in conch coiling, occasionally accentuated by a dorsolateral groove analogous to the nepionic constriction and the appearance of conspicuous growth anomalies indicate that, after hatching, Ophioceras possessed a cap-shaped, slightly curved conch, usually approximately a quarter whorl long. A hatchling thus differed substantially from the likely nektonic late juveniles with coiled conchs and their obliquely oriented aperture as in Nautilus. A relatively large first phragmocone chamber and very short body chamber possibly resulted in positive buoyancy and a planktonic habit of hatchlings. The embryonic conch size is highly variable and the height of the first chamber varies between 1.2-2.6 mm. Changes in sculpture across the embryonic/juvenile conch boundary are sometimes gradual, but frequently, hatching is manifested by an abrupt increase in growth line spacing and the appearance of longitudinal ridges. The cicatrix is here documented in the Tarphycerida for the first time. A distinct chamber length decrease, commonly present close to the end of the first whorl, is not linked with hatching. Anomalous conch structures in Ophioceras including healed injuries, atypical shapes of ribs, atypical courses of septa and pits occurring in late juvenile growth stages are described in the light of the autecology of Ophioceras and the determination of early post-embryonic growth anomalies. •

Silurian tarphyceridDiscoceras(Cephalopoda, Nautiloidea): systematics, embryonic development and paleoecology

“…A planktonic habit for Ordovician tarphycerid juveniles was suggested by Holland (1985). This has been documented by Turek and Manda (2016) in Silurian Ophioceras , an evolutionarily younger tarphycerid, displaying several evolutionary novelties unknown in other tarphycerids, even those from the Silurian. Thus, to expand the hypothesis of a planktonic habit of juveniles to other tarphycerids, more evidence from evolutionarily basal tarphycerids was needed.…”

“…Whorl width index (WWI) is used as defined by Korn and Klug (2003) as the ratio of whorl width and whorl height. RI defined by Stridsberg and Turek (1997) is the proportion of whorl height at any point to whorl height of the previous whorl exactly one turn earlier; revolving index is used because it is also applicable to fragmentary shells and has been used in Silurian tarphycerids (Turek and Manda, 2016).

Figure 1Schematic drawing explaining shell morphology of Discoceras .

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“…graftonense and the origin of four species of Discoceras in the latest Sheinwoodian and early Homerian represented the last weak diversification and dispersion event of tarphycerids, terminated by the mid-Homerian extinction (Mulde and Lundgreni events; Calner, 2008). Only a single tarphycerid, Ophioceras Barrande, 1865, survived this extinction, including two long-ranging species; the youngest of these became extinct just below Silurian-Devonian boundary (Stridsberg and Turek, 1997; Turek and Manda, 2016). No tarphycerid species originated following the middle Homerian; there is no evidence of a splitting lineage or a reason to split a lineage into a series of successive chronospecies.…”

“…Although the change in septal spacing is often linked with hatching in Nautilus , the intraspecific variability in the chamber length pattern is high (Stenzel, 1964). Except in Ophioceras (Turek and Manda, 2016), the variability of septal spacing in early ontogeny has not been studied in early Paleozoic nautiloids. A change in septal spacing in the first whorl in Ordovician tarphycerids was linked with hatching by Schindewolf (1934).…”

Cephalopod embryonic shells as a tool to reconstruct reproductive strategies in extinct taxa

“…belemnoids) or cannot be defined with certainty (some nautiloids s.l .). The following features were used for determination of the ES: ( i ) nepionic constriction – some bactritoids and Nautilida are characterised by the presence of a wide constriction in the early part of the shell; ( ii ) septal approximation (decrease in the distance between septa) – this is known in many nautilids and is commonly used for recognition of the ES (Stumbur, ; Shimansky, ), but as in recent Nautilus the septal approximation lies some one‐third of a whorl before the end of the ES (Landman, ), such measurements may be slight underestimates; ( iii ) changes in microsculpture are rarely seen in fossil cephalopods as this relies on excellent preservation, but they were recently reported in tarphycerids by Turek & Manda () who also emphasised very high variability in the ES size of the taxa studied; ( iv ) shell thickening as a kind of hatching scar and a shift in the shell axis as in some Belemnoidea (Hewitt & Jagt, ); ( v ) the ES of ammonoids, the so‐called ammonitella, is finished by a constriction known as the primary constriction, which is accompanied by the nacreous primary varix, indicating appearance of the nacreous layer.…”