Amphibiopus patagonicus, Casamiquela, 1961

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 : 9-12

Ameghinichnus patagonicus Casamiquela, 1961

Figures 3 , 4

Ameghinichnus patagonicus Casamiquela, 1960: 11 (nomen nudum); Casamiquela, 1961: 226; Casamiquela, 1964: 90; Bonaparte, 1978: 451; Bonaparte, 1983: 81; Leonardi & de Oliveira Lima, 1990: 220, 222, 225; Leonardi, 1994: 27; Rainforth & Lockley, 1996: 266; Sarjeant, 2000: 160; Casamiquela, 2002: 434; Melchor et al., 2004: 51.

Holotype. MLP 60-X-31-14 A, trackways with six manus-pes sets with opposite arrangement about the midline (associated with the paratype MLP 60-X-31-14 B) (trackway 1 in Figs. 3 b, 3e).

Paratypes. MLP 60-X-31-1, 60-X-31-3, 60-X-31-4, 60-X-31-8, 60-X-31-10, 60-X-31-14 B, 60-X-31-15, PVL 2300, 2301, 2302, 2303, 2304 ( Figs. 3 a–d,f,g).

Additional material. MACN 18528, 18616, 18617, 18618, 18619, 18620, 18621, 18622, 18623, 18525b, 3823, MACN-SC 3824, 3825, 3826, 3827, 3828, 3829, 3830, 3831, 3833, 3834, 3835, 3836, 3837, 3838, 4354, PVL 2742, 2743, 3684, 3685, 3686, 3688, 3689, 3691, 3692, 3693, 3694, 3697, 3698, 3699, 4354, MLP 65-XI-12-1 (more than thirty slabs with the same collection number), 65-XI-12-4, 65-XI-13-1 (two slabs with the same collection number), one slab with no collection number, MPEF-IC 1015, 1016, 1023, 1030, 1031, 1033, 1034, 1035, 1036, 1037, 1038, 1040, 1041, MPM-PIc-173, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 184, 185, 186, 187, 188, 189, 194, 195, 196, 197, 198, 199, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 3763, 3764, 3765, 3766, 3767, 3768, 3769, 3770, 3771, 3772, 3773, 3774, 3775, 3776, 3952, 3953, 3954, 3955, 3956 ( Fig. 4 ).

Emended diagnosis. Ameghinichnus preserved as manus and pes tracks with a length:width ratio of 0.7. Manus digit impressions are subequal in length, with a relative digit length: I<II=V<IV<III, angles between III-IV<I-II≈II-III≈IV-V, I-V averages 151º. Manus impressions display an inward rotation of about -16º in relation to the midline, and are located aligned-to-posterior to pes impressions. Pes digit impressions are subequal in length, with relative length: I<IV≈III=V<II, angles between III-IV<II-III<I-II<IV-V, I-V averages 152º. Pes display an outward rotation of about 31º in relation to the midline. Claw marks are absent and there are no marked phalangeal pads. When manus-pes sets display alternate arrangement about the midline, manus and pes pace angulation is about 143º and 114º, respectively, and manus and pes stride length is about 83 mm and 81.2 mm, respectively.

Remarks and comparisons. The ichnospecies Ameghinichnus mirabilis ( Ellenberger, 1970) nov. comb., from the Elliot Formation in the Karoo basin, is composed of large-sized, symmetric manus and pes impressions subequal in size, with digit imprints splayed and subequal in length. Ameghinichnus patagonicus displays a track morphology comparable to A. mirabilis , but as it lacks the trackway parameters and is larger in size. It is not possible to assure the synonymy between both ichnospecies.

Ameghinichnus patagonicus can be distinguished from the specimens from the Newark basin, USA, mentioned as Ameghinichnus n. isp. by Olsen and Rainforth (2001: fig. 59) by their lack of the sharp claw marks, as well as being almost half the size.

The specimen presented by Gierliński et al. (2004, 2005) from the Lower Jurassic of Sołtyków, Poland, is interpreted here as a left track. It lacks sufficient details to make a comparison with Ameghinichnus patagonicus to define an ichnotaxonomical relationship.

Description. The specimens of Ameghinichnus patagonicus are represented by numerous tracks and trackways produced by a quadrupedal trackmaker. Table 1 summarizes the measurements from 28 trackways accounting for 405 imprints.

The trackways are composed of manus-pes sets, both pentadactyl tracks, which are nearly symmetrical in relation to the major imprint axis. Manus and pes digit impressions are slender, with a maximum width up to 1.5 mm and may be parallel-sided. They lack phalangeal pads and claw marks, though have a distal swelling.

The average width and length of the hand impressions are 11.5 mm and 8.2 mm, respectively. Digit impressions are subequal in size and shape, with an average length, in decreasing order, of: I: 2.8 mm, II and V: 3.2 mm, IV: 3.4 mm, and III: 3.5 mm. The angles formed by them are relatively equidistant, having an average of: I-II: 42º, II-III: 42º, III-IV: 37º, and IV-V: 42º; the I-V displays an angle with a mean of 151º. Manus imprints show an outward (negative) rotation relative to the midline, with a mean of -16º.

The average width of the footprints is 13 mm, while the average length is 9 mm. Digit impressions are subequal as in the case of the manus, with an average length of: I: 3.0 mm, IV: 3.6 mm, III and V: 3.7 mm, and II: 4.0 mm. The average angles formed by them are as follows: I-II: 42º, II-III: 30º, III-IV: 26º, and IV-V: 58º; the I–V displays an angle with a mean of 152º. Pes imprints show an inward (positive) rotation relative to the midline with an average of 31º.

Both manus and pes sole display metacarpal and metatarsal imprints, a posterocentered depression, and a posterior bilobate outline which is only visible in the best- preserved tracks. When this posterior bilobated outline is present, the length of the track (named here as total length, see figure 2) is about 9.2 mm for hand prints and 10.9 mm for footprints ( Figs. 3 d, 4g).

The manus-pes sets can display two arrangements: either ipsilateral with the manus and pes imprints on the same side of the body of the trackmaker in alternate arrangement (sensu Trewin 1994; e.g. Figs. 3 a,g), or both sets accounting the four limbs of the trackmaker in a transverse group as opposite arrangement (sensu Trewin 1994), with one manus and pes set on one side, opposite the corresponding pair on the other side (e.g. Fig. 3 e, trackway 2 in 4b). In both cases, the footprints are located externally and overstep or slightly overlap the manus.

When the sets have an alternate arrangement, the trackways display an average pace angulation of 143º for the manus, and of 114º for the pes. The average stride length of the manus and pes imprints is 83 mm and 81.2 mm, respectively. The inner trackway width between manus imprints averages 2.7 mm, and the width between footprints averages of 17.5 mm, whereas the average outer trackways width is 23.6 mm for manus and 38.8 mm for pes.

When the sets have a transverse or opposite arrangement, the distance between two consecutive groups, named herein as a hop or jump, displays a mean of 91.6 mm.

The trackways may show tail marks, in a continuous and curved or sinuous pattern when the sets have an alternative arrangement (e.g. Fig. 4 c, trackway 1 in 4b), or discrete (average length of 7.7 mm) and straight when they are opposite (e.g. Fig. 4 e, trackway 2 in 4b). Both kinds of tail marks have an average width of 4.1 mm.

Figures

FIGURE 3. Photographs of the type series of Ameghinichnus patagonicus. A, MLP 60 - X- 31 - 1; B, MLP 60 - X- 31 - 14 B, holotype (right trackway: 1) and MLP 60 - X- 31 - 14 A, paratype (left trackway: 2), the black arrows indicate the moving direction of the trackmaker; C, PVL 2303; D, PVL 2302, the white arrow indicates the bilobate outline; E, detail of the two first manus-pes sets of MLP 60 - X- 31 - 14 B; F, MLP 60 - X- 31 - 15; G, MLP 60 - X- 31 - 3. A-C, D, F and G, paratypes. Scale bars: 2 cm in D and E, 5 cm in the rest. m, manus imprint; p, pes imprint.

FIGURE 4. Photographs of specimens of Ameghinichnus patagonicus. A, MPM-PIc- 3768 (first manus-pes set to the left) and 3769 (first manus-pes set to the right); B, MPEF-IC 1016 at left, indicated with a 1, MPEF-IC 1015 at rigth, indicated with a 2; C, MPEF-IC 1023; D, MACN 18619; E, MPEF-IC 1041; F, MACN-SC 4354; G, MACN 18622, the white arrows indicate the bilobate outline; H, detail of the third manus-pes sets of MPM-PIc- 3952; I, MPM-PIc- 3952. Scale bars: 2 cm in C, E, F, and H, 5 cm in A, D, G, and I, and 10 cm in B. The black arrow in C, E, and F indicates the tail mark. m, manus imprint; p, pes imprint.