TYPE SPECIES: Matthewichnus velox Matthew 1905

DIAGNOSIS: Quadrupedal, plantigrade tetrapod trackway with tetradactyl manus and pentadactyl pes imprints. Manus is approximately as wide as long and is much smaller than pes. Manus digits II and III approximately equal in length and elongate, slightly more than half the length of the entire print. Digits I and IV of manus approximately one third the length of the entire print. Pes digits increase in length from I to IV, with digits III and V being approximately equal in length. (Modified from Haubold et al. 2005.).

REMARKS: Matthewichnus is similar in gross morphology to the ichnogenera Batrachichnus and Limnopus. Voigt and Lucas (2015), for example, noted that Matthewichnus caudifer is indistinguishable from Batrachichnus salamandroides based solely on the morphology of the tracks. Fillmore et al. (2012), however, noted that Matthewichnus can be differentiated from Batrachichnus and Limnopus by the much smaller manus track than pes track. Further morphological differences between Matthewichnus and Batrachichnus include the presence of elongate digits II and III on the manus and a more irregular placement of manus and pes imprints within the trackway (Kohl and Bryan 1994; Haubold et al. 2005; Fillmore et al. 2012; Voigt and Haubold 2015; Voigt and Lucas 2015).

• 1930 Megapezia longipes; Willard and Cleaves, p. 324, pl. 4, fig. 1.

• 1970 ? Megapezia longipes; Haubold, p. 94.

• 1971 ? Megapezia longipes; Haubold, p. 14.

• 1995 Megapezia longipes; Cotton et al., p. 201, fig. 8B, table 1.

• 2007 Megapezia; Lucas, p. 9.

MATERIAL: Holotype ― MCZ 140, housed at the Museum of Comparative Zoology at Harvard University.

OCCURRENCE: Late Carboniferous (Pennsylvanian), Rhode Island Formation.

DESCRIPTION: Two tracks are preserved side by side in concave epirelief on the slab. They are separated by a nearly continuous area of rock at the same elevation as the rest of the slab. The track on the right is smaller, tetradactyl, and slightly longer than wide. It exhibits a long, rectangular imprint proximal to the digit imprints. The track measures 40 mm long (26 mm without the proximal rectangular imprint) by 22 mm wide. The digit imprints, which are counted from right to left, are in general straight, although the third digit exhibits a bend along its length. All have rounded distal terminations. Digits I and IV measure 5 mm long, or about one-fifth the length of the track. Digits II and III 10 mm and 12 mm, or 38–46% the length of the track. The total angle of divarication between the digits is 113°, and the angles of divarication between the successive digits are as follows: I–II = 54°, II–III = 33°, and III–IV = 28°. The track exhibits a pronounced transverse wrinkling that is especially well developed in the region between the digits and the rectangular imprint.

The larger track, on the left, is pentadactyl and is nearly twice the length of the tetradactyl track. It measures 50 mm long by 20 mm wide. A broad, ovate imprint measuring 23 mm long by 18 mm wide is located proximal to the digit imprints, which are less well defined than those in the other track. Digit imprints are counted from right to left. Digits I and V are short, measuring approximately 5 and 4 mm long, respectively. Both appear as small, subtly rounded projections on the sides of the track. Imprints of digits II, III and IV are relatively long, measuring 11 mm, 15 mm, and 16 mm, respectively. The imprint of digit II is approximately straight, whereas those of digits III and IV curve and are concave to the left. All of the digits are of relatively uniform thickness and have rounded terminations. The total angle of divarication is 60°; the angles of divarication between the successive digits are as follows: I–II = 28°; II–III = 21°; III–IV = 10°; and IV–V= 9°.

REMARKS: The two tracks on MCZ 140 are here interpreted as having been made by a manus and pes. The smaller, tetradactyl track is inferred to be that of the manus, whereas the larger pentadactyl track is that of the pes. The serial increase in length from right to left in the first four digits of the pes track indicates that it was made by a left foot. Considering the close proximity of the tracks, along with their similar orientation, it seems likely that they were made by the same animal as a manus-pes set. If that were true, then the tetradactyl track would be that of the left manus. It is possible, however, that the tracks were made by two different individuals traveling in the same direction. The tracks were probably made in wet mud, as is evidenced by the presence of the rectangular imprint (which was likely made by the antebrachium, or forearm) associated with that of the manus, the deep digit imprints of the manus, and the poorly defined and strongly curved digits of the pes imprint. Similar tracks from Argentina have also been interpreted as being produced in saturated sediment (Melchor and Sarjeant 2004).

When Willard and Cleaves (1930) described Megapezia longipes, they interpreted the fossil to include only a single, pentadactyl imprint that they thought was made by the left hind foot. These authors clearly stated that they thought the manus imprint was lacking. Considering that the ichnogenus Megapezia was established for a trackway with tetradactyl pedal imprints (Matthew 1903), Willard and Cleaves’ specimen should not have been assigned to Megapezia. The reinterpretation of the specimen here as being composed of both manus and pes imprints does nothing to change this assessment since the correctly identified pes is also pentadactyl.

Rather than Megapezia, the tracks on MCZ 140 are more similar in morphology to the ichnogenera Batrachichnus, Limnopus, and Matthewichnus, all of which have tetradactyl manus tracks and pentadactyl pes tracks (Marsh 1894; Woodworth 1900; Baird 1952; Haubold 1970; Kohl and Bryan 1994; Tucker and Smith 2004). Various features of the tracks, however, suggest that they are more appropriately assigned to Matthewichnus. For example, the manus track is much smaller than the pes track. Furthermore, digit impressions II and III of the manus are relatively long, although not as long as is often cited for Matthewichnus. Finally, as in some specimens of Matthewichnus (e.g., Voigt and Lucas 2015, fig. 2B), the pes imprint on MCZ 140 is situated anterolaterally to that of the manus. The poor definition of the tracks and the lack of a complete trackway, however, argue against a definitive ichnogeneric reassignment (Bertling et al. 2006). Consequently, the tracks on MCZ 140 are provisionally reassigned to Matthewichnus. The ichnospecies cf. Matthewichnus longipes is, however, not synonymized with any other ichnospecies within Matthewichnus pending the collection of additional material that can be compared with other species within the ichnogenus.