A diverse mammal-dominated, footprint assemblage from wetland deposits in the Lower Cretaceous of Maryland

Abstract

A newly discovered assemblage of predominantly small tracks from the Cretaceous Patuxent Formation at NASA’s Goddard Space Flight Center, Maryland, reveals one of the highest track densities and diversities ever reported (~70 tracks, representing at least eight morphotypes from an area of only ~2 m2). The assemblage is dominated by small mammal tracks including the new ichnotxon Sederipes goddardensis, indicating sitting postures. Small crow-sized theropod trackways, the first from this unit, indicate social trackmakers and suggest slow-paced foraging behavior. Tracks of pterosaurs, and other small vertebrates suggest activity on an organic-rich substrate. Large well-preserved sauropod and nodosaurs tracks indicate the presence of large dinosaurs. The Patuxent Formation together with the recently reported Angolan assemblage comprise the world’s two largest Mesozoic mammal footprint assemblages. The high density of footprint registration at the NASA site indicates special preservational and taphonomic conditions. These include early, penecontemporaneous deposition of siderite in organic rich, reducing wetland settings where even the flesh of body fossils can be mummified. Thus, the track-rich ironstone substrates of the Patuxent Formation, appear to preserve a unique vertebrate ichnofacies, with associated, exceptionally-preserved body fossil remains for which there are currently no other similar examples preserved in the fossil record.

Introduction to Adalatherium hui (Gondwanatheria, Mammalia) from the Late Cretaceous of Madagascar

“…Indeed, the footprints of C. angolanus appear to be very similar in size to those of Mustela nigripes, the blackfooted ferret (both manus and pes tracks are 2.9 cm long and 3.2 cm wide; Halfpenny and Biesiot, 1986), which has a body mass of only 645-1,125 g (Anderson et al, 1986). Furthermore, as pointed out by Stanford et al (2018), several other Early Cretaceous trackmakers were as large or larger than C. angolanus.…”

“…Tracks of Mesozoic mammaliaforms are, in general, rare (see recent summaries in Lockley et al [2017], Mateus et al [2017:fig. 8], and Stanford et al [2018]), and many of them "are of questionable value" because they are isolated, of mediocre preservation, and/or lack comparative material in the form of skeletal remains (Lockley et al, 2017:221). Nonetheless, if correctly attributed, some comparative statements can be made about their size.…”

“…This conclusion clearly relates to the various species of Ameghinichnus from the Late Triassic/Early Jurassic of South Africa (Olsen and Galton, 1984) and the Jurassic of South America (Casamiquela, 1964;de Valais, 2009), North America (Szajna and Silvestri, 1996), and Europe (Gierlinśki et al, 2004); Ameghinichnus? ichnospecies from the Jurassic of South America (de Valais, 2009); Catocapes angolanus from the Early Cretaceous of Angola (Marzola et al, 2014;Mateus et al, 2017); Brasilichnium elusivum and Brasilichnium saltatorium from the Early Cretaceous of Brazil (Leonardi, 1981;Buck et al, 2017b); Koreasaltipes jinjuensis from the Early Cretaceous of Korea (Kim et al, 2017); Morphotype B from the Early Cretaceous of Maryland (Stanford et al, 2018); Schadipes crypticus and Schadipes, ichnosp. indet., from the Late Cretaceous of Colorado (Lockley and Foster, 2003); ichnogenus indet.…”

“…These include ichnogenus indet. from the Middle Jurassic of Argentina (de Valais, 2009); Sederipes goddardensis (Morphotype A) from the Early Cretaceous (Aptian) of Maryland (Stanford et al, 2018); Aracoaraichnium leonardii from the Early Cretaceous of Brazil (Buck et al, 2017a); and ichnogenus indet. from the Late Cretaceous of Colorado (Lockley et al, 2017).…”

“…The preservation of both the skull and feet of A. hui motivates us to make additional observations about Mesozoic mammaliaform trackmakers and their relative body sizes. Marzola et al (2014:181) estimated that the Early Cretaceous Angolan trackmaker, later named Catocapes angolanus, by Mateus et al (2017) (the validity of which has been questioned by Stanford et al, 2018), represented an animal "as big as a modern raccoon … comparable in size to Repenomamus." Mateus et al (2017:230) reiterated the comparison with the extant raccoon (Procyon lotor) but went further and identified C. angolanus as the "largest known mammaliamorph from the Early Cretaceous … unmatched in size by the coeval skeletal fossil record," thus implying that C. angolanus was larger than even R. giganticus.…”