Hoploscaphites constrictus johnjagti Machalski, 2005a

Machalski, Marcin, Jagt, John W. M., Alekseev, Alexander S. & Jagt-Yazykova, Elena A., 2012, Terminal Maastrichtian ammonites from Turkmenistan, Central Asia, Acta Palaeontologica Polonica 57 (4), pp. 729-735 : 731-734

Hoploscaphites constrictus johnjagti Machalski, 2005a

Figs. 3A–H , 4A, B, E, F, D ?.

Material.—NHMM 2011 033–040, 2011 042–044, and, possibly, 2011 046, comprising at least two macroconchs, three

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microconchs and around ten phragmocones, Sumbar River section, topmost part of the upper Maastrichtian.

Description.—Material quite variable in ribbing style, onset of tuberculation and size and shape of tubercles; phragmocones involute, with tiny umbilicus (e.g., Fig. 4E, F ); most specimens highly compressed with flat−sided phragmocone; flexuous primary ribs arising at umbilical seam (e.g., Fig. 3G ), being either feebly concave or near−straight and prorsiradiate on inner flank, convex at mid−flank, concave on outer flank and ventrolateral shoulder and weakly convex over venter; primary ribs dividing at various heights on flank, intercalatories inserting on inner to outer flank; ventrolateral tubercles develop at variable phragmocone diameters ( Fig. 3A, D ); umbilical bullae developed both in micro− and macroconchs ( Fig. 3G, H ), with shaft of body chamber coarsely ribbed and ventrolateral tubercles either continuing almost to aperture or disappearing earlier ( Fig. 3B, F–H ). NHMM 2011 035, a well−preserved microconch, measures 31 mm in greatest length, while NHMM 2011 040, a macroconch, attains an approximate length of 36 mm.

Specimen NHMM 2011 046 ( Fig. 4D ) is tentatively assigned here. It was not collected in the field, but recognised in a fragment of marly matrix in sample bag SM4/12 during preparation for microfossil analysis at Moscow University. It is a limonitised, fragmentary mould of a phragmocone, found in the Danian portion of the section, in the interval 22–24 cm above the base of the boundary clay. It was referred to as “ Pachydiscidae gen. et sp. indet.” by Alekseev et al. (1988).

The phragmocone is slightly compressed and filled with a porous aggregate of ferro−hydroxides which probably are products of pyrite oxidation. In cross section, the thick ferruginous crust with smooth outer surface is visible and the ribs of the ammonite are underneath. The ventral portion of the body chamber is crushed and poorly preserved. The greatest diameter of the preserved part of the shell is 16 mm. The shell reveals flexuous primary ribs and intercalatories on the outer flank and is indistinguishable in this respect from nuclei of Hoploscaphites constrictus johnjagti .

Associated aptychi (NHMM 2011 047–048) are of the general scaphitid type and correspond closely to material illustrated by Birkelund (1993: pl. 17: 2–4) and Machalski (2005a: fig. 26).

Discussion.—These specimens clearly represent the youngest evolutionary stage of the Hoploscaphites constrictus lineage, which is defined by the presence of ribbing all over the body chamber in macroconchs (see Machalski 2005a: 667, figs. 7C, 8, 12). This subspecies, Hoploscaphites constrictus johnjagti , is restricted to the upper upper Maastrichtian in northwest and central Europe, as documented by Machalski (2005a, b), who listed material from eastern (Stevns Klint, Sjaelland) and northern (Jylland) Denmark, as well as southern Sweden, central and eastern Poland, northeast Belgium, and the southeast Netherlands. Machalski (2005a, b) also noted that macroconchs ranged in maximum length between 36 and 51 mm, microconchs between 27 and 33 mm, and that in numerous macroconchs ventrolateral tubercles persisted until the apertural margin. Thus, the complete macroconch

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(NHMM 2011 040; Fig. 3H ) is relatively small, while the other, incomplete specimen (NHMM 2011 039; Fig. 3B ) would appear to be more closely comparable to material from Denmark, Poland, and the southeast Netherlands. The elongate appearance of the ventrolateral tubercles in both micro− and macroconchs (see Fig. 3B, F–H ) is interpreted as a taphonomic feature, the tops of the tubercles having been eroded.

Stratigraphical and geographical range.—Uppermost Maastrichtian of Denmark, southern Sweden, Poland, northeast Belgium, the southeast Netherlands, France, southwest Turkmenistan, Mangyshlak ( Kazakhstan), and Crimea ( Ukraine), as well as lowermost Danian of Denmark and the southeast Netherlands ( Machalski and Heinberg 2005; Machalski et al. 2009; Jagt 2012). The record from Crimea is based on unpublished material collected by the late Professor Dmitry P. Naidin and by one of us (ASA) in the uppermost Maastrichtian, and currently housed in the VSEGEI collections (EAJ−Y and JWMJ, unpublished data).

Figures

Fig. 3. Scaphitid ammonoid Hoploscaphites constrictus johnjagti Machalski, 2005a, Sumbar River section, Turkmenistan, upper Maastrichtian, Upper Cretaceous, all collected by ASA, with indications in brackets of centimetres below K/Pg boundary (where known).A. NHMM 2011 037, external mould of phragmocone. B. NHMM 2011 039, partial body chamber of macroconch. C. NHMM 2011 036, nucleus. D. NHMM 2011 034, partial phragmocone of?macroconch (15 cm). E. NHMM 2011 038, nucleus (5 cm). F. NHMM 2011 033, near−complete?microconch (18 cm). G. NHMM 2011 035, microconch (5 cm). H. NHMM 2011 040, macroconch.

Fig. 4. Ammonites from the Sumbar River section, Turkmenistan, upper Maastrichtian, Upper Cretaceous, collected by ASA and Mikhail A. Nazarov, with indications in brackets of centimetres below K–Pg boundary (where known). A, B, E, F. Scaphitid ammonoid Hoploscaphites constrictus johnjagti Machalski, 2005a. A. NHMM 2011 044, microconch body chamber. B. NHMM 2011 043, apertural portion of body chamber, with epizoic craniid brachiopod (arrowed). E. NHMM 2011 042b, pair of phragmocones (10–15 cm). F. NHMM 2011 042a, set of phragmocones (10–15 cm). C. Baculitid ammonoid Baculites cf. vertebralis Lamarck, 1801, NHMM 2011 041a, partial body chamber. D. Scaphitid phragmocone from the Danian portion of the Sumbar River section, NHMM 2011 046, probably Hoploscaphites constrictus johnjagti Machalski, 2005a (= Pachydiscidae indet. sensu Alekseev et al. 1988: fig. 1).