Crouching Theropods in Taxonomic Jungles: Ichnological and Ichnotaxonomic Investigations of Footprints with Metatarsal and Ischial Impressions

Abstract

Tracks and traces of crouching theropods are rare, known from only three specimens from the Lower Jurassic of New England and the Lower to ?mid Jurassic of China. The New England specimens reveal manus, metatarsal and sub-crescentic ischial callosity impressions associated with Grallator-like tracks. The Chinese traces reveal metatarsal traces and a sub-triangular ischial callosity impression associated with Eubrontes-like tracks. All reveal symmetrical crouching postures. Theropod crouching traces should not be confused with ornithopod crouching traces, often assigned to Anomoepus. The ichnotaxonomy surrounding all these traces and associated footprints is very complex, and over split. Suggestions for simplification allow recognition that the North American and Chinese ichnofaunas have many similarities. The ichnotaxonomy is not as complex and confused as it may at first appear.

Elongated theropod tracks from the Cretaceous Apenninic Carbonate Platform of southern Latium (central Italy

“…Taking into account the features described above, the tridactyl footprints from Riomartino are best attributed to a medium-sized, non-avian, theropod trackmaker (Lockley et al, 2003). The traces of the metatarsals are, in fact, quite elongated and, therefore, more similar to the osteological structure of theropods rather than with that of ornithopods.…”

“…The traces of the metatarsals are, in fact, quite elongated and, therefore, more similar to the osteological structure of theropods rather than with that of ornithopods. The latter, according to Lockley et al (2003), would leave shorter metatarsal impressions. This is in accordance with the "morphodynamic rule" with shorter feet and longer legs in ornithopods and longer feet and shorter legs in theropods (Lockley et al, 2003; see also Thulborn, 1990).…”

“…In the literature, the term "crouching" has been used several times to indicate both true resting tracks, with parallel metatarsals and traces of ischial callosity and hands, and for single tracks with elongated metatarsals, either isolated or in succession in a trackway, so not parallel but representative of a squatting walking (see Lockley et al, 2003;Milan et al, 2008). Alternatively, according to Lockley et al (2003) the definition of crouching is more restrictive and requires both the impression of the metatarsals and of ischial callosity in the resting phase. In this case, the authors used the term "complete crouching".…”

“…In this case, the authors used the term "complete crouching". Lockley et al (2003) then considered true crouching as a different form of behaviour compared to a continuous walk in a squatting position, possibly linked to a firmer substrate (at least in the known examples) with the dinosaur stopping to rest at least for a certain period of time. Milan et al (2008) considered crouching or "full crouching" the condition in which parallel metatarsal traces, the ischial callosity and also the manus are impressed.…”

“…Tetrapod footprints can also be analyzed within a framework of "reciprocal illumination" (sensu Carrano and Wilson, 2001; see also (Romano et al, 2015), where the integrated and refined study of osteology and the type of impressions may shed light on the biomechanics, functionality, and complex locomotor cycle of a putative trackmaker (see Voigt et al, 2007;Romano and Citton, 2014;Sacchi et al, 2014;Romano et al, 2015. Dinosaur footprints with metatarsal traces are common in the dinosaurian ichnological record; they are often reported as isolated tracks and, less frequently, as part of a trackway segment. Previously described material has been attributed both to ornithischian and theropod trackmakers (e.g., Kuban, 1989 and reference therein; Lockley et al, 2003;Nicosia et al, 2007;Milán et al, 2008;Gierlinski et al, 2009;Milner et al, 2009;Wilson et al, 2009). Recently, new dinosaur footprints with metatarsal impressions were discovered on an isolated block of the pier of Porto Canale-Riomartino, a few kilometres from the city of Latina (Latium, Central Italy).…”