Abstract
A second nearly complete, articulated specimen of the basal troodontid Mei long (DNHM D2154) is reported from the Early Cretaceous (Hauterivian-Valanginian) lower Yixian Formation, Liaoning Province, China. New diagnostic features of Mei long are identified, including: a uniquely shaped maxilla, low with small, low maxillary fenestra; sacrum with an extremely wide caudal portion and elongate 4th and 5th sacral processes; and a large distal articular surface on the tibiotarsus which continues caudally on the tibia. A phylogenetic analysis including new data from the second specimen recovered Mei as a basal troodontid, in keeping with previous analyses. Although the skeleton exhibits several juvenile-like features including free cervical ribs, unfused frontals and nasals, and a short snouted skull, other attributes, full fusion of all neurocentral synostoses and the sacrum, and dense exteriors to cortical bone, suggest a small, mature individual. Microscopic examination of tibia and fibula histology confirms maturity and suggests an individual greater than two years old with slowed growth. Despite being one of the smallest dinosaurs, Mei long exhibits multi-year growth and cortical bone consisting largely of fibro-lamellar tissue marked by lines of arrested growth as in much larger and more basal theropods. This Mei long specimen lies in a similar but mirrored sleeping position to that of the holotype, strengthening the hypothesis that both specimens were preserved in a stereotypical life position. Like many Liaoning specimens, the new specimen also lacks extensive taphonomic and stratigraphic data, making further behavioral inference problematic.
Highlights
IntroductionThe Jehol Group is composed of the stratigraphically conformable Yixian and Jiufotang Formations and is exposed in western portions of Liaoning Province, China
The Jehol Group is composed of the stratigraphically conformable Yixian and Jiufotang Formations and is exposed in western portions of Liaoning Province, China. These formations are dominated by laminated and finely bedded siliciclastic sediments interspersed with extrusive basalts and tuffs [1]. They have produced a widely acclaimed fossilized fauna that includes a wide diversity of fish, invertebrates, plants [1], mammals [2,3], diapsids [4,5] including dinosaurs [6,7,8] with many specimens displaying exceptional soft-tissue and integument preservation [9,10,11]
These spectacular fossils are predominantly strongly compressed [12] and come from the siliciclastic sediments thought to have been deposited by a series of inland, freshwater lacustrine environments [1]
Summary
The Jehol Group is composed of the stratigraphically conformable Yixian and Jiufotang Formations and is exposed in western portions of Liaoning Province, China These formations are dominated by laminated and finely bedded siliciclastic sediments interspersed with extrusive basalts and tuffs [1]. They have produced a widely acclaimed fossilized fauna that includes a wide diversity of fish, invertebrates, plants [1], mammals [2,3], diapsids [4,5] including dinosaurs [6,7,8] with many specimens displaying exceptional soft-tissue and integument preservation [9,10,11]. Several specimens have emerged from the lower beds with three-dimensional preservation provides the opportunity for behavioral inferences [13,15,16]
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