Amphibiopus manantialensis

Valais, Silvina De, 2009, Ichnotaxonomic revision of Ameghinichnus, a mammalian ichnogenus from the Middle Jurassic La Matilde Formation, Santa Cruz province, Argentina, Zootaxa 2203, pp. 1-21 : 13-15

Ameghinichnus manantialensis isp. nov.

Figures 5a–e

Ameghinichnidae: Casamiquela, 1964: 139; Leonardi, 1994: 29. “A gigantic form: Leonardi & de Oliveira Lima, 1990: 225.

Holotype. MPEF-IC 1028, trackways with three manus-pes sets with alternate arrangement about the midline ( Fig. 5a ).

Paratypes. MLP 60-X-31-13, 65-XI-12-1 (three slabs with the same collection number), MPM-Pic-2098 ( Figs. 5b–e ).

Etymology. Manantialensis (Latin) , in reference to the Estancia Laguna Manantiales (Santa Cruz province, Argentina), the locality where the footprints were discovered.

Diagnosis. Ameghinichnus preserved as manus and pes tracks with a length:width ratio of 0.7 and 0.6 respectively. Manus digit impressions are subequal in length, with relative digit length: I<V<IV<II≈III, angles between between II-III<III-IV<I-II˜IV-V. Manus impressions display an inward rotation of about -38° in relation to the midline, and are located aligned to anteriorly to pes. Pes digit impressions are subequal in length, with relative length: I<III<IV≈V<II, angles between III-IV<II-III<I-II<IV-V. Pes impressions display an outward rotation of about 26° in relation to the midline. Both I–V angles for pes and manus are about 107°. Claw marks are absent and there are no evident phalangeal pads. When manus-pes sets display an alternating arrangement about the midline, manus and pes pace angulation averages 156° and 110º respectively, and manus and pes stride length is about 116 mm and 119 mm respectively.

Remarks and comparisons. The morphology of these tracks allows them to be included in the ichnogenus Ameghinichnus: quadrupedal trackways with symmetric pentadactyl manus and pes impressions, manus slightly smaller than pes, subequal and splayed digit impressions. But given its particular features, this group of tracks is different enough from the other specimens of the ichnogenus to deserve a separate ichnospecies level designation. A. manantialensis differs from A. patagonicus by having different digit length proportions and divarication angles, and the manus located aligned-to-anterior to the pes impression. In addition, the impressions of the hands are, on average, 63 % longer and 60 % wider relative to Ameghinichnus patagonicus , whereas the footprints are, on average, 50 % longer and 72 % wider.

The tracks assigned to Ameghinichnus isp. from the Lower Jurassic Newark basin of USA (see Olsen & Rainforth 2001, and references therein) differ from A. manantialensis in the presence of clear sharp claw marks in some of the digits. This could be preservation differences due to sediment texture (e.g. mud or sand), and more specimens are needed to define the assignation of the tracks.

Ameghinichnus mirabilis ( Ellenberger, 1970) nov. comb., from the Upper Triassic-Lower Jurassic Karoo basin of South Africa, differs from the new ichnospecies in having different proportion in the length of the digit imprints. However, an ichnotaxonomical assignment is uncertain, taking into consideration that the trackway parameters of A. mirabilis are unknown.

The left footprint from the Lower Jurassic of Sołtyków, Poland ( Gierliński et al. 2004, 2005) is similar to Ameghinichnus manantialensis due to the angles between the digit impressions, but as a result of the scarse details, it is not possible to make a confident comparison with the new ichnospecies.

Description. The specimens are represented by three trackways, with a total of seventeen tracks, and two isolated manus-pes sets, produced by a quadrupedal trackmaker. Their measurements are summarized in Table 1 and detailed in Table 2.

The manus-pes sets, both pentadactyl and digitigrade to semidigitigrade tracks, are nearly symmetrical in relation to the major imprint axis. Manus and pes digit impressions are slender, with a maximum width up to 2 mm, and may display a parallel outline, exhibiting a distal swelling. They lack phalangeal pads and claw marks. Plantar and sole impressions display similar features to Ameghinichnus patagonicus (i.e. with metacarpal or metatarsal imprints, a posterocentered depression, and a posterior bilobate outline which is rarely visible).

The hand impressions range from 17.8 mm to 20.6 mm wide, with an average of 18.9 mm, from 12.6 mm to 15.1 mm long, with an average of 13.7 mm, and about 16 mm total length. Digit impressions are subequal in size and shape, with an average length, in decreasing order, of: I: 4.2 mm, V: 5.2 mm, IV: 5.7 mm, II: 6.3 mm, and III: 6.4 mm. The angles formed by the digit imprints are relatively equidistant, having an average of: I-II: 35°, II-III: 22°, III-IV: 26°, and IV-V: 34°; the I-V angle ranges from 102° to 110°, with an average of 107°. Manus imprints show an outward rotation relative to the midline, which ranges from -32° to -40°, with a mean of -38°.

The footprints range from 20.5 mm and 25.8 mm wide, with an average of 22.3 mm, from 12.6 mm to 18.0 mm long, with an average of 14.3 mm, and about 17.1 mm total long. Digit impressions are subequal as in the case of the manus, but with an average length of: I: 5.0 mm, III: 5.5 mm, IV≈V: 5.8 mm and 5.9 mm, respectively, and II: 6.1 mm. The angles formed average: I-II: 35°, II-III: 29°, III-IV: 16°, and IV-V: 40°; the I- V displays an angle from 101° to 117°, with an average of 107°. Pes imprints show an inward rotation relative to the midline, which ranges from 8° to 41°, with a mean of 26°.

The specimens display only one arrangement, with the ipsilateral manus and pes imprints in alternating configuration. There is no trackway displaying an opposite arrangement, possibly due to preservation bias in this small sample. The hand imprints are located aligned or anterior and medial to the footprints and may even touch them. The inner trackway width between opposite manus imprints is about - 8 mm, and the width between pes is about 20.5 mm. The average outer trackway width is 28.7 mm and 61.4 mm for hand imprints and footprints, respectively. The pace angulation ranges from 146° to 160°, with an average angle of 156° for the manus, and only one measurement of 110° for the pes. The stride length ranges from 99 mm to 119 mm, with a mean of 110.4 mm for the manus, and 109.5 mm for the pes. The inner trackway width between manus and pes is about - 8 mm, and 20.5 mm, respectively. The outer trackway width is 28.7 mm for manus and 61.4 mm for pes.

Figures

FIGURE 5. A – E, photographs of specimens of Ameghinichnus manantialensis. A, MPEF-IC 1028, holotype; B, MLP 65 - XI- 12 - 1; C, MLP 65 - XI- 12 - 1; D, MLP 60 - X- 31 - 13, the black arrow indicates the specimen; E, MPM-Pic- 2098; B - E, paratypes; F, MPM-PIc- 3957, Ameghinichnus? isp. Scale bars: 2 cm in F, and H, and 5 cm in the rest. m, manus imprint; p, pes imprint.