Aktautitan, TRACKWAYS

MIHLBACHLER, MATTHEW C., LUCAS, SPENCER G., EMRY, ROBERT J. & BAYSHASHOV, BOLAT, 2004, A New Brontothere (Brontotheriidae, Perissodactyla, Mammalia) from the Eocene of the Ily Basin of Kazakstan and a Phylogeny of Asian ‘‘ Horned’ ’ Brontotheres, American Museum Novitates 3439 (1), pp. 1-43 : 19-21

POSSIBLE AKTAUTITAN TRACKWAYS

Aktautitan ? tracks are exposed at Kyzyl Murun near Aktau Mountain (fig. 1) at UTM zone 44, 362307E, 4873406N (datum: WGS 84). They are in the Kyzylbulak Formation in the upper part of unit 26 of the measured section of Lucas et al. (1997: fig. 3) (see fig. 2 of this paper). The track­bearing stratum is a 0.1­m­thick bed of light greenish­gray (5 GY 8/1), very fine­grained calcareous silty sandstone that is ~ 0.5 m above the bonebed dominated by the complete, articulated skeletons of Aktautitan hippopotamopus .

At Kyzyl Murun, about 100 tracks are preserved as ‘‘potholes’’ in sandstone (fig. 17). The footprints are preserved in concave epirelief, generally lack clear orientation, and are crowded and superimposed to indicate a trampled surface. All are round, ovoid, or oblong in shape and have diameters of ~ 0.2 m and depths of up to ~ 0.1 m. They lack clear indications of digits, pads, or hooves and obviously are underprints. A single partial trackway indicates the trackmaker was a quadruped with a gleno­acetabular length of ~ 1.2 m and a trackway width of ~ 0.4 m.

The mammal tracks reported here closely resemble those previously reported Paleogene mammal tracks attributed to brontotheres or rhinoceroses. Thus, the tracks attributed to large perissodactyls and described by Hamblin et al. (1998, 1999) from the Uintan (middle Eocene) of Utah are approximately the same size and shape as those from Kyzyl Murun. Sarjeant and Langston (1994: p. 40–41, pls. 4, 24) described and illustrated large perissodactyl tracks from the Chadronian (late Eocene) of Texas that are larger than, but otherwise very similar to, the Kyzyl Murun tracks. Other tracks attributed to Chadronian brontotheres (e.g., Chaffee, 1943) are also larger than, but similar to, the Kyzyl Murun tracks.

The Kyzyl Murun tracks are undertracks that poorly record the foot shape and other anatomical details of the trackmaker. Thus, a precise identification is impossible, though a large perissodactyl trackmaker seems most likely. Body fossils from the underlying strata of the Kyzylbulak Formation belong to the brontothere Aktautitan hippopotamopus and the amynodont rhinocerotoid Sharamynodon , both possible trackmakers. Although it is difficult to reconstruct the general body proportions of A. hippopotamopus from the material at hand, Rhinotitan mongoliensis is of similar size and is known from a mounted skeleton and can therefore be used for size estimates ( Wang, 1982). Thus, femur lengths suggest A. hippopotamopus is about 70% the size of Rhinotitan , which would give A. hippopotamopus an estimated gleno­acetabular length of about 1.1 m and manus and pes diameters (minus any fleshy pads) of 14–19 cm. Based on Osborn (1936), Sharamynodon has a gleno­acetablular length of 1.4 m and manus and pes diameters (minus fleshy pads) of 16–19 cm. Thus, based on size and foot shape, either Aktautitan or Sharamynodon are plausible trackmakers of the Kyzyl Murun footprints. The abundance of the brontothere and the relative rarity of the amynodont lead us to suggest that the brontothere was the more probable trackmaker.

The Kyzyl Murun tracks are the first report of fossil mammal tracks from Kazakstan. They fit well into what is known of Paleogene mammal tracks, namely that they are mostly the footprints of primitive large ungulates and carnivores. Paleogene tracks are known mostly from North America and are dominantly the footprints of primitive perissodactyls, artiodactyls, and carnivores (e.g., Chaffee, 1943; Curry, 1957; Sarjeant and Wilson, 1988; Lucas and Williamson, 1993; Sarjeant and Langston, 1994; Hamblin et al., 1998, 1999). Records from outside of North America—from China, Peru, western Europe, and Iran —also fit this pattern (e.g., Lockley et al., 1999; Ataabadi and Sarjeant, 2000).