INTRODUCTION

This publication is to clarify the nomenclature of the type species of the genera Idoceras and Subnebrodites and thereby preserve the use of Subnebrodites planula and Idoceras balderum as index species of the top biostratigraphical zone of the Upper Oxfordian and the bottom biostratigraphical zone of the Lower Kimmeridgian, respectively, in the Submediterranean Province.

Idoceras and Subnebrodites are central to attempts to correlate and fix a unified Oxfordian-Kimmeridgian boundary for the Boreal and Tethyan Realms. Especially during the last two decades, they have been used in a large number of papers, memoirs, and monographs dealing with the Upper Oxfordian and Lower Kimmeridgian in Europe and elsewhere.

During revisions currently in progress for the Jurassic Ammonoidea volume of the Treatise on Invertebrate Paleontology, ambiguity appeared about the type species of Subnebrodites Spath, 1925, long neglected or considered to be a synonym of Idoceras Burckhardt, 1906. The search for an answer led us to question the validity of the type species of Idoceras. Resolution of these problems is important because the type species of both genera are currently used as index species of zones of the Upper Oxfordian and Lower Kimmeridgian in the Submediterranean Province.

IDOCERAS AND SUBNEBRODITES AND THEIR TYPE SPECIES

The genus Idoceras was proposed by Burckhardt (1906, p. 38) for 13 new species in Mexico belonging to the two mainly European groups of Ammonites planula Hehl in Zieten, 1830 in 1830–1833, and Ammonites balderus Oppel, 1863, but without fixing a type species. In discussing Burckhardt's genus, Spath (1925, p. 129) referred to “the true Idoceras of the balderum group,” a statement that has been accepted as a valid subsequent designation of Ammonites balderus Oppel as the type species of Idoceras. Spath also noted that the “early evolute planula ‘group’ and the late involute balderum ‘group’ are separable … not only morphologically but also stratigraphically,” and he therefore proposed the new genus Subnebrodites with the following text: “It is thus advisable to distinguish the planula-group by a new name Subnebrodites, nov., the type to be the species to which belongs a Wüttemberg example (848) from the collection of the late Dr. T. Wright, agreeing with Ammonites planula (Hehl), Zieten, as figured by Quenstedt (Ammoniten des Schwäbischen Jura, 1888, p. 974, pl. 108, fig. 2 only). In spite of the differences in the drawings, this appears to be Hehls original type form”.

Figure 1.

Subnebrodites planula Spath. A–B, lectotype, microconch, Wasseralfingen, Württemberg, Germany, ×0.72;A, Ziegler, 1959, pl. 1,6; B, Geologisches Universität Tübingen, Germany, Ce 5/108/2; C–D, paralectotype, NHMUK C.26579, probably a macroconch, Württemberg, Germany, ×0.72 (The Natural History Museum, London, England).

Quenstedt identified the specimen figured in his pl. 108,2, as Ammonites planula Zieten gigas, and in selecting pl. 108,2 only, Spath was deliberately misidentifying the type species. Such cases are covered by Article 67.13.1 (and the related Articles 11.10, 50.1.2 and 70.4) of the Code of Zoological Nomenclature (ICZN, 1999, p. 69), which states that a type species fixed by the deliberate misidentification of a previous established name “is deemed to be a new nominal species” proposed by that author. So the type species of Subnebrodites is S. planula Spath, 1925, nom. nov. pro Ammonites planula Zieten gigas Quenstedt, 1888 in 1887–1888, pl. 108,2, non Ammonites planula Hehl in Zieten, 1830 in 1830–1833, p. 9, by original designation.

Figure 2A–C.

Ammonites planula Hehl in Zieren, holotype, Donzdorf, Württemberg, Germany, ×0.6 (Zieten, 1830, pl. 7,5a–c).

As proposed by Spath, the type series of Subnebrodites planula Spath, 1925 consists of two specimens: the lectotype (Fig. 1A, 1B) designated herein, figured by Quenstedt, 1888 in 1887–1888, pl. 108,2 (the original specimen is in the Geologisches Institut Universität Tübingen, Ce 5/108/2), and the paralectotype (Fig. 1C, 1D) a Wüttemberg specimen in the collection of Dr. T. Wright (no. 848; now in The Natural History Museum, London, England, register number NHMUK C.26579), which closely resembles the lectotype. S. planula Spath is evolute, with oval whorl sides and a rounded venter; moderately strong, straight primary ribs, mostly bifurcate at the ventro-lateral shoulder into secondaries that curve gently forwards on the venter, ending at a smooth, mid-ventral band; a few primary ribs remain single. This is the identity of the species that has given its name to the Subnebrodites planula Zone of the Upper Oxfordian in the Submediterranean Province.

The identity of Ammonites planula Hehl in Zieten, 1830 in 1830–1833, p. 9, pl. 7,5 remains to be established. Zieten's figures (Fig. 2A–C) are of an ammonite that is more involute than Subnebrodites, has higher, more quickly expanding whorls, slightly curved or sinuous primary ribs dividing lower on the upper half of the whorl into two or three secondaries that curve more strongly forwards, no constrictions, and small, radially elongated tubercles on the primary ribs at the umbilical edge. The whorl section is trapezoidal towards a narrowly rounded venter (Fig. 2C) or a bluntly angled venter (Fig. 2B). This comparison suggests that Zietens ammonite is at least specifically different from Subnebrodites planula Spath, 1925, and it could belong to a different genus (possibly Idoceras). For these reasons we disagree with Spath's (1925, p. 129) statement that “In spite of the differences in the drawings, this [Quenstedt, 1888 in 1887–1888, pl. 108,2] appears to be Hehl's original type form.” But the form of the venter in Zietens drawings is inconsistent and suggests that the original specimen might have been crushed or distorted. According to Schairer (1989, p. 101), Hehl's collection was purchased by Quenstedt and housed in the University of Tubingen, but the specimen cannot now be found there and must be considered lost. In the absence of the original specimen and the inconsistency of Zieten's drawings, our opinion is that Ammonites planula Hehl in Zieten, 1830 in 1830–1833, is not identifiable from the original description and figures and is therefore a nomen nudum.

Figure 3.

Idoceras balderum (Oppel), Aargau, Bremgarten, Switzerland; A–B, holotype × 1 (Oppel, 1863, pl. 67,2); C, plaster cast of holotype, × 1 (Wegele, 1929, pl. 9,7).

Idoceras Burckhardt, 1906 (type species Ammonites balderus Oppel, 1863) is more involute than Subnebrodites, with higher whorls, an oval whorl section, rounded venter, and straight to gently curving ribs, some single, others dividing on the upper half of the whorl into forwardly curving secondaries that end at the smooth central part of the venter; there are about three constrictions per whorl, which follow the line of the ribs (Fig. 3A, 3B).

So, it is important to preserve Subnebrodites planula Spath, 1925 (nom. nov. for Ammonites planula Quenstedt, 1888 in 1887–1888, pl. 108,2 only, non Ammonites planula Hehl in Zieten, 1830 in 1830–1833) as the type species of Subnebrodites on account of its long use and particularly as index species of the Planula Zone Upper Oxfordian; and to confirm and stabilize the use of I. balderum (Oppel) as index species of the Balderum Zone Lower Kimmeridgian.

SUBNEBRODITES PLANULA AND IDOCERAS BALDERUM AS ZONAL INDEX SPECIES IN THE STANDARD ZONAL SCHEME OF THE SUBMEDITERRANEAN PROVINCE

The Planula Zone was first used by Engel (1908, p. 404), but its later proposal by Wegele (1928 in 1928–1929, p. 145) was the source of the universal use of both Planula Zone and Planula Subzone in the Upper Oxfordian for the past 85 years. The index species is usually identified as Ammonites planula Hehl in Zieten, 1830 in 1830–1833, but the figured specimens to which authors always refer are: Ammonites planula gigas Quenstedt, 1888 in 1887–1888, pl. 108,2 (a microconch) and Ammonites planula cornutus Quenstedt, 1888 in 1887–1888, pl. 108,13 (a macroconch), as first pointed out by Énay (1966, p. 570–571). Now that the original of Quenstedt, 1888 in 1887–1888, pl. 108,2 has been designated as the lectotype of Subnebrodites planula Spath, 1925, the Planula Zone is protected against being referred back to Ammonites planula Hehl in Zieten, 1830 in 1830–1833, of which Subnebrodites planula Spath, 1925, is not a homonym.

The Balderum Zone was also first used by Engel (1908, p. 404), but its modern universal usage in the past 50 years is based on the later proposal by Karvé-Corvinus (1966, p. 130). If it is considered that Ammonites planula Hehl in Zieten, 1830 in 1830–1833, might be the macroconch dimorph of Idoceras balderum, the only way to prevent the Balderum Zone from being replaced by another Planula Zone in the Lower Kimmeridgian is to either declare Hehls species unidentifiable and therefore a nomen nudum, or apply to the ICZN for a ruling that would suppress Hehl's species for the Purpose of Homonymy. As has been shown above, Hehl's species cannot be satisfactorily identified from the original description and figures, and the original specimen is lost, so application to the ICZN for a ruling is not necessary.