Dimetropus, Romer and Price, 1940

Voigt, Sebastian & Ganzelewski, Michael, 2010, Toward the origin of amniotes: Diadectomorph and synapsid footprints from the early Late Carboniferous of Germany, Acta Palaeontologica Polonica 55 (1), pp. 57-72 : 64-66

Dimetropus ichnosp. indet.

Figs. 5–7, Appendices 3, 4.

1963 Herpetichnus erini Schmidt, 1963: 179–184 , fig. 1, pl. 13.

1971 Pseudobradypus (Herpetichnus) erneri [sic]; Haubold 1971: 27, fig. 16: 3.

1982 Pseudobradypus erini ; Fichter 1982: 50–59, figs. 7–10, 15, tables 1, 4.

1984 Pseudobradypus (Herpetichnus) erini ; Haubold 1984: 64, fig. 42: 2.

Material.— DBM 060003260001 to DBM 060003260005, original material, preserved as convex hyporelief; DBM 060003357001 and DBM 060003357002, plaster casts. All of the original material belongs to the same slab, 0.5 × 2.3 m in size, composed of five segments showing 27 monospecific imprints of one trackway ( Fig. 5).

Locality.—Former coal−mine „Erin“ at Castrop, Ruhr area, western Germany ( Hahne 1957).

Horizon.—Above seam “Sonnenschein”, lower Bochum Formation, Westphalian A2, Langsettian, Late Carboniferous.

Lithology.—Based on the original material the tracks were made in dark grey mudstone. Natural casts are preserved in light grey siltstone to fine−grained sandstone. The track−bearing surface is partially covered by imprints of macrofloral remains and isolated tetrapod footprints of other trackways that are indeterminable due to faint or incomplete preservation (cf. Schmidt 1963).

Description.—Trackway of a quadrupedal tetrapod with pentadactyl, plantigrade footprints; manus and pes imprints are similar in shape but different in size ( Figs. 5, 6A, B , Appendix 3). Imprints are always longer than wide, with distally pointed slender digits and proximally elongated heel. Sole of the pes may cover almost half of the length of the imprint. In both manus and pes imprints, digits I to IV exhibit a serial increase in length; digit V is longer than II in the pes imprint, but shorter than II in the manus imprint. Pes imprints measure up to 14 cm being about 50% larger than the manus imprints. Manus imprints up to one third longer than wide, with a total digit angulation of 77.5° on average. Pes imprints more than 50% longer than wide with a mean total digit angulation of 61.5°.

The trackway pattern shows a more or less alternating arrangement of imprints, where the manus imprint of the one side of the trackway is nearly opposite a contralateral pes. In some parts of the trackway the manus−pes couples are so closely spaced that the trackway pattern merges into an alternating arrangement of inversely coupled imprints similar to that described for I. praesidentis . The trackway pattern is characterised by a relatively short stride, wide gait, and low pace angulation. Deviation of the manus and pes imprints from trackway midline is highly variable; both imprints may be significantly pointed inwardly or outwardly; on average, however, the imprints are placed almost parallel to the trackway midline (Appendix 3).

Remarks.— Schmidt (1963) compared the Castrop tracks with Ichnium acrodactylum Pabst, 1895 from the Early Permian of central Germany. He recognised an overall similarity regarding the relative length of the digits, their specific shape with pointed digit tips, and the posteriorly extended sole of the imprints. Following the proposal of Nopcsa (1923), who had assigned Ichnium acrodactylum to Herpetichnus Jardine, 1850 from the Early Permian of Scotland, Schmidt (1963) introduced Herpetichnus erini but without an appropriate ichnospecies definition. Haubold (1971) did not accept the synonymy of Herpetichnus Jardine, 1850 and Ichnium acrodac−

doi:10.4202/app.2009.0021

tylum (= Dimetropus leisnerianus (Geinitz, 1863) according to Haubold 1971) and assigned the Castrop tracks to Pseudobradypus Matthew, 1903 from the Early Pennsylvanian of Nova Scotia. Fichter (1982) supported this referral on the basis of trackway parameters and the extended heel of the pes imprints. On the other hand, he conceded that the type material of the generic type, Pseudobradypus unguifer (Dawson, 1872) , is lacking crucial details of the imprint morphology. The ambiguity of the Pseudobradypus type material is expressed by the wide morphological range of Carboniferous tracks that have recently been assigned to this ichnotaxon ( Lucas et al. 2004; Falcon−Lang et al. 2007; Wood and Miller 2007).

Our results support the determination of Schmidt (1963) who firstly stressed the similarity of the Castrop footprints with Dimetropus Romer and Price, 1940 ( Herpetichnus in the usage of Schmidt 1963). Neither imprint morphology nor trackway pattern support ichnogeneric separation. As Schmidt (1963) observed, the digit imprints of these tracks are characterised by their narrow middle part. This is a typical feature of Dimetropus tracks, whose most deeply impressed segments are the metatarsal− and metacarpal−phalangeal pads and the claws (cf. Voigt 2005). Fichter (1982: 58) and even Haubold (1971: 27) noted the striking similarity between the Castrop tracks and Dimetropus ; the reservation of these authors on the ichnogeneric identity might be due to stratigraphic age discrepancies.

Eleven ichnospecies of Dimetropus have been named, of which three were accepted as valid taxa by Haubold (2000): D. leisnerianus (Geinitz, 1863) , D. berea ( Tilton, 1931) , and D. nicolasi Gand and Haubold, 1984 . Lacking reliable criteria for ichnotaxonomic discrimination other than stratigraphy and locality, Voigt (2005, 2007) argued for a monospecific ichnogenus Dimetropus . The trackway pattern of the Castrop specimen is similar to trackways of D. leisnerianus made by slow−moving individuals ( Fig. 7A–D ). All differences of the trackway pattern, including the shorter length of stride, the higher manus−pes distance, and the lower pace angulation, can be explained by a reduced speed of gait. Coincidence is given for the relative length of pace and the deviation of the pes imprints from midline ( Fig. 7E, F ). The Castrop specimen differs from D. leisnerianus in having relatively longer manus and pes imprints ( Fig. 7F ); minor differences concern the digit proportions ( Fig. 7G, H ). The relatively higher length of imprints and the relatively shorter digit V of the pes imprint of the Castrop tracks might be of ichnospecific relevance. Because consistency of the features can not be assessed by a single specimen, the Castrop tracks are tentatively referred to Dimetropus with open nomenclature on the ichnospecies level.

Nomenclature.—The Castrop tracks are indistinguishable from Dimetropus Romer and Price, 1940 and are consequently assigned to that ichnogenus. Dimetropus is a valid, widely accepted, and well characterised taxon for Palaeozoic tetrapod footprints. Herpetichnus was erroneously used for Dimetropus tracks from the Early Permian of central Germany by Nopcsa (1923), but there is no synonymy of that ichnotaxa as repeatedly stated by several authors ( Haubold 1971, 1984, 1996, 2000; Fichter 1982; Voigt 2005).

Osteological interpretation.—The Castrop tracks were referred to an amniote trackmaker by all previous authors ( Schmidt 1963; Haubold 1971, 1973, 1984; Fichter 1982). Whereas Schmidt (1963) and Fichter (1982) proposed an association with early edaphosaurids, Haubold (1971) compared the specimen with footprints of supposed captorhinomorph affinity, but conceded that the tracks could also have been made by basal synapsids (“pelycosaurs”) due to the large size of the imprints.

Dimetropus is usually referred to Eupelycosauria, which include edaphosaurids, sphenacodontids, ophiacodontids, and varanopids ( Tilton 1931; Romer and Price 1940; Haubold 1971, 1984, 1996, 2000; Fichter 1976, 1979, 1983; Gand 1988; Haubold et al. 1995; Voigt 2005; Voigt et al. 2005). With the exception of some large caseids such as Cotylorhynchus , the manus and pes structure of basal synapsids (“pelycosaurs”) are generally similar ( Reisz 1986). Varanopids are unique in that the fourth digit appears to be extended showing digital proportions similar to the tracks of Tambachichnium Müller, 1954 (cf. Voigt 2005). At the current state of knowledge, the ichnogenus Dimetropus subsumes tracks of a wide range of non−therapsid synapsids.

Figures

Fig. 6. Comparison between tracks of Dimetropus sp. from the Late Carboniferous Bochum Formation, Ruhr area, Germany, DBM 060003260001 (A) and DBM 060003260003 (B) and Dimetropus leisnerianus (Geinitz, 1863) from the Early Permian Tambach Formation, Thuringian Forest, Germany, MNG 1762 (C). Scale bars 100 mm.

Fig. 7. Comparison of trackway pattern and selected parameters of Dimetropus sp. from the Late Carboniferous Bochum Formation, Ruhr area, Germany, DBM 060003260001 to DBM 060003260005 (A) and Dimetropus leisnerianus from the Early Permian Tambach Formation, Thuringian Forest, Germany, MNG 1762 (B), MNG 1828 (C), MNG 13490 (D). E, F. Derived trackway and imprint parameters graphically expressing the mean, minimum, and maximum values. G, H. Relative lengths of the manus and pes imprint digit lengths expressed as a percentage of the length of digit IV. Data source for E–H, see Appendix 4.