First tracks of newborn straight-tusked elephants (Palaeoloxodon antiquus)
The MTS proboscidean tracks and trackmakersRounded-to-elliptical tracks, with an axial length range from 9.6 to 54.5 cm (pes), were found mostly isolated and as manus-pes couples, or associated forming at least eight short trackways (see Table 1). They reveal good preservation in one 6-footprint trackway (see below), two converging trackways and some couples, showing anteriorly directed, wide, short and blunt toe impressions (Figs. 2, 3 and 4). Toe impressions are not commonly visible in elephant footprints9,13, (but see27), which attests to cases of exceptional preservation in Matalascañas tracks. Preservation as true tracks is identified through expulsion marginal rims (e.g., Fig. 4a, g) and possible ejecta (Fig. 3b,e). Large and flat sole surfaces sometimes show evidence of pockmarks23 (Fig. 4f).Table 1 Measurements of Proboscipeda tracks, ordered from smallest to largest in length.Full size tableFigure 2Proboscidean tracks (Proboscipeda panfamilia) attributed in the MTS to straight-tusked elephants. (a–h) Morphological features of small-sized tracks produced by calves and juveniles. Examples of manus impressions in (a) PAT/MTS/011a, (b) PAT/MTS/016 and (f) PAT/MTS/015x, and for further interpretation of (a) see Fig. 3; the latter two with drag marks made during the foot-off event. (c) and (g) PAT/MTS/002a,b: Manus-pes couple found isolated showing heteropody and different number of toe impressions (interpretation as left-side tracks by peak pressure deformation in the left side of the track according to27); interpretation in (c). (d) PAT/MTS/014 and (e) PAT/MTS/007a: Calf-sized pes with three toe impressions. (h) PAT/MTS/011 h: Badly preserved manus of a calf. Scale bar = 5 cm.Full size imageFigure 3Photograph, outline, high-resolution 3D and false-coloured 3D images of the PAT/MTS/0011a track representing the best preserved manus of a juvenile-sized Proboscipeda track. (a) and (c) From the photograph and high-resolution images, five toe impressions in the anterior part of the rounded track are clear (especially toes I–IV). (b) and (f) The false coloured images in orthogonal (b) and oblique angle views (f) highlight the deepening of the track fore- and outwards, thus revealing a peak pressure pattern typical of left forefoot (toes III–IV), as well as a possible ejecta mound in front of the track. The poorly evident and narrow expulsion rim developed around the track is the result of the high cohesiveness and plasticity of the clayey fine-sand substrate. (d) Contour map supporting previous interpretation. (e) The cross-section of the track details the anterior migration of the foot pressure during its rotation, creating a peak pressure in the foot-off event that is represented in the deepest part of the track. Scale bars are 10 cm.Full size imageFigure 4Large-sized Proboscipeda tracks attributed to P. antiquus adults. (a) to (d) PAT/MTS/001: Right manus showing clearly 5 toe impressions and the frontal and lateral displacement rims (morphological interpretation based on the orthogonal (b) and oblique (d) depth and contour (c) maps). (e) and (f) PAT/MTS/010e: Deeper manus with pockmarks; toe pad impressions indicated (I–III). (g) PAT/MTS/004a,b: large manus-pes couple where the hind foot deformed the fore foot during overstepping, and revealing a typical elephantine gait; the toe impressions in both tracks indicate the direction of movement. Scale bar = 10 cm.Full size imageIrrespective of the track size, pes are elliptical to sub-rounded, with the length axis larger than the width and manus are circular or elliptical, with the width axis larger than the length (Figs. 2c and 4d, g for small and large size tracks, respectively). The safest way to differentiate between pes and manus is through the orientation of the track provided by the toe impressions, or by the orientation of the longer axis in trackways. When arranged in trackways, manus-pes couples show the typical elephantine gait, showing a short pace resulting from the fore- and hind feet on the same side swinging forward simultaneously below the body, as it is known from modern elephant gait28. In some cases, the partial impression of a pes overstepping the proximal part of a manus can be seen (Fig. 2c, g). Based on similar preservational style and opposing directions of movement without overlapping at the meeting point, a converging pair of trackways was apparently produced contemporaneously by an adult and a rather small juvenile. Sharp edges of the toe impressions indicate the presence of nails. These are found mostly in well preserved, smaller-sized tracks (Fig. 2a, d, e) because nails are commonly worn down in adult elephants and not always shown in their tracks13. These morphological features allow us to attribute the MTS trackways to the ichnospecies Proboscipeda panfamilia used previously for describing, among other tracksites, those tracks attributed confidently to the straight-tusked elephant Palaeoloxodon antiquus in the paleogeographical context of southern Europe11,14 (see supplementary Table S1).Manus-pes couples, when showing overstepping, were not considered in Table 1 (Fig. 2c, g). Overstepping depends on the speed of walking; at faster speeds the overstepping is only partial or there is no overstepping; elephants maintain the footfall pattern at all speeds, shifting toward a calculated 25% phase offset between limbs as they increase speed28 (Fig. 2g). The smallest tracks usually do not show overstepping possibly because of the greater activity, with longer pace and stride lengths, demonstrated by calves and juveniles when compared to adults. Manus or pes showing a large width-length ratio (below 0.80–0.96 sensu25) were not considered for the estimates since they represent slippage.Younger elephants have more pliable skin and musculature than adults. Also, the greater expansion and distribution of the weight in heavier adult animals is enough to reduce or negate toe impressions in some types of sediments, such as compacted substrates24,29. Interpreting the sedimentological data for the paleosol where MTS was developed15,17,30, suggests a drying clayey-sandy substrate14 that was still plastic enough to absorb the impact of the limbs during the locomotion of the elephants (presence of expulsion rims and absence of radial pressure cracks), and preserving, in many cases, the morphological details of the feet in good condition (Figs. 2a, 3, 4a; see Fig. 2h for a badly preserved example).Ichnological inference about the height, body mass and age of Palaeoloxodon antiquus in the MTSSeveral methods have been proposed for estimating the height at the shoulders for proboscideans, and the relationship between body mass and age with shoulder height 1,31,32. A linear relationship between foot length and shoulder height was confirmed by Lee and Moss33 from extant elephants and compared with fossil examples by Pasenko24. Pes length has been especially used in studies as an indicator of shoulder height21,34,35,36. Among Asian elephants, manus circumference has been shown to have a similar predictive relationship with shoulder height33. These parameters were determined for each isolated track (or representative track in a trackway), including manus and pes (Table 1), using equations previously proposed31,33 (see Methods). A similar approach has been applied to mammoth track studies in North America21,27, where modern ontogenetic and body-mass data has been used to provide age and size estimates from fossil tracks.From the skeletal record, sexual dimorphism of P. antiquus was observed to be more accentuated than in extant elephants, especially in terms of size differences1. During the first 10 years of life, both male and female African bush elephant foot lengths increase rapidly, with the fastest growth shown in the first two years for calves33,37. In P. antiquus, males would have continued to grow until their fifties according to bone data1, while females would have been much smaller as result of energy expenditure with reproduction, flattening the growth curve just after puberty. That is why the equations of Lee and Moss33 that discriminates the shoulder height from tracks for males and females have been applied. However, by comparison with the study of Marano and Palombo32 (based on the progress of eruption and degree of wear of teeth compared to extant elephants), and the body mass correlation of Larramendi et al.1 for calculating the age of P. antiquus, our MTS ages obtained from the application of the regression curve of Lee and Moss33 are underestimated and must be analysed as minimum age approximations for track lengths corresponding to adolescent and adult animals, especially for males. The obtained estimations from tracks are subject to a level of uncertainty related to biotic and abiotic factors that can distort the data (i.e., taphonomy) as it happens also with the calculations taken from skeletal proportions. Therefore, McNeil et al.21 even included data from frozen mammoth carcasses on the growth curve of Lee and Moss33 for correcting size discrepancies along ontogeny. For P. antiquus, our best data for comparison comes, however, from the flesh reconstructions1.Ontogenetic implicationsBased on the best fossil site found for this species in Europe, corresponding to 70 individual Palaeoloxodon antiquus specimens recovered in Geiseltal, Germany, Larramendi et al.1 developed the best reconstruction, so far, of the life appearance of this species and discussed size, body mass, ontogeny and sexual dimorphism. The Neumark-Nord bone site may be contemporary or slightly older than MTS, corresponding to late Middle Pleistocene-to-Eemian interglacial period1. The authors found that the body mass of P. antiquus males was up to three times more that of male Asian elephants and twice that of extant male African bush elephants. The large size determined for straight-tusked elephants (with an estimated > 400 cm shoulder height in the flesh and body mass of 13 tonnes) and a later complete epiphyseal-diaphyseal fusion of limb bones (not yet totally fused at an estimated age of 47 years), in comparison with extant elephants, suggests that this species had a longer lifespan of 80 years or more1. Sexual dimorphism of P. antiquus was observed to be more accentuated than in extant elephants, with females generally not exceeding 300 cm at the shoulders with an estimated weight of not more than 5.5 tonnes, while males continued to grow until their fifties1. Males in extant elephant species grow more rapidly than females after puberty (i.e., around 7 years in age), which are affected by a trade-off between growth and reproduction. Under normal nutritional conditions, the growth rate is generally higher in males than females leading to a marked difference in size between sexes at already around 10 years in age33,37,38,39.The ontogenetic variation in growth projected for the MTS, when compared to what we known from extant proboscideans, is expressed in the track size distribution plot, with the definition of five age classes (Fig. 5; see also Table 1): calves under 2 years in age (when extant elephants experience fastest growth rates in both sexes), juveniles between 2 and 7 years in age (up to when elephant females reach their sexual maturity and therefore experience a strong reduction of growth rate in comparison to males), 7–15 years in age which include pre-puberty males and young female adults, over 15 years in age and More
