[D] Limusaurus inextricabilis [sG] [T]
Xu, Clark, Mo, Choiniere, Forster, Erickson, Hone, Sullivan, Eberth, Nesbitt, Zhao, Hernandez, Jia, Han, & Guo, 2009
Jurassic Late Oxfordian
Saurischia Theropoda Ceratosauria
Shishugou Formation, Wucaiwan area, Junggar Basin, Xinjiang, China
Theropods have traditionally been assumed to have lost manual digits from the lateral side inward, which differs from the bilateral reduction pattern seen in other tetrapod groups. This unusual reduction pattern is clearly present in basal theropods, and has also been inferred in non-avian tetanurans based on identification of their three digits as the medial ones of the hand (I-II-III). This contradicts the many developmental studies indicating II-III-IV identities for the three manualdigits of the only extant tetanurans, the birds.
Here we report a new basal ceratosaur from the Oxfordian stage of the Jurassic period of China (156–161 million years ago), representing the first known Asian ceratosaur and the only known beaked, herbivorous Jurassic theropod. Most significantly, this taxon possesses a strongly reduced manual digit I, documenting a complex pattern of digital reduction within the Theropoda.
Comparisons among theropod hands show that the three manual digits of basal tetanurans are similar in many metacarpal features to digits II-III-IV, but in phalangeal features to digits I-II-III, of more basal theropods. Given II-III-IV identities in avians, the simplest interpretation is that these identities were shared by all tetanurans. The transition to tetanurans involved complex changes in the hand including a shift in digit dentities, with ceratosaurs displaying an intermediate condition.
Limus, Latin for mud or mire; saurus, Latinization of Greek for lizard; inextricabilis, Latin for impossible to extricate. This name is in reference to the specimens’ inferred death in a mire.
Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) V 15923 is an articulated, nearly complete skeleton.
IVPP V 15924 is a semi-articulated skeleton missing the skull; it is 15% larger than the holotype.
Locality and horizon
Wucaiwan area, Junggar Basin, Xinjiang; Oxfordian upper part of the Shishugou Formation.
Small ceratosaur with the following autapomorphies: short skull (half as long as the femur); skull and mandible toothless; nasal with a lateral shelf; premaxilla with a convex buccal edge; short and wide nasal less than one-hird of skull roof length and only twice as long as wide; ventral process of lacrimal strongly inclinednteriorly; slender jugal with rod-like sub-orbital and sub-temporal rami; large external mandibular fenestra about 40% of mandibular length; flange on anterior margin of scapular blade; radius tightly adhering to ulna, and longer than the latter bone; olecranon process absent; metacarpal II much more robust than other metacarpals; metacarpal III with sub-triangular proximal articular surface and non-ginglymoidal distal end; metacarpal I highly reduced and carrying no phalanges; phalanx II-1 with distinct lateral process proximodorsally; pubis with laterally ridged, prominent posterior boot;metatarsus forming a strong transverse arch; robust ventral process at medial margin of proximal end of metatarsal III; metatarsal IV nearly straight, appressed against lateral surface of metatarsal III for nearly its whole length; and pedal digit I small, only 17% as long as metatarsal I