Nodocephalosaurus (meaning "knob headed lizard") is a monospecific genus of ankylosaurid dinosaur from New Mexico that lived during the Late Cretaceous (late Campanian to early Maastrichtian stage, 73.49 to 73.04 Ma) in what is now the De-na-zin member of the Kirtland Formation.[1][2] The type and only species, Nodocephalosaurus kirtlandensis, is known only from a partial skull.[1] It was named in 1999 by Robert M. Sullivan.[1] Nodocephalosaurus has an estimated length of 4.5 metres (15 feet) and weight of 1.5 tonnes (3,306 lbs).[3] It is closely related and shares similar cranial anatomy to Akainacephalus.[2]
In 1995, a partial skull of an ankylosaur was discovered weathering out of a grey mudstone a few hundred metres west of a new Parasaurolophus site in the De-na-zin member of the Kirtland Formation, New Mexico. Robert M. Sullivan and Thomas E. Williamson left the partial skull in the field due to the apparent fragmentary nature of the skull and time constraints. A year later, the skull was obtained from the site by Sullivan with the help of Kesler Randall. The specimen was subsequently described in 1999 by Robert M. Sullivan. The holotype specimen, SMP VP-900 , consists of an incomplete skull and associated cranial fragments. The holotype specimen is currently housed at the State Museum of Pennsylvania, Harrisburg.[1]
The generic name, Nodocepahlosaurus, is derived from the Latin word "nodus" (knob or swelling ) and the Greek words "kephale" (head) and "sauros" (lizard), in reference to the many bulbous osteoderms fused to the top of the skull. The specific name, kirtlandensis, refers to the Kirtland Formation, the formation from which the holotype came.[1]
In 2000, Tracy Ford assigned NMMNH P-20880, a single pup-tent scute with a hollowed-out base, as a potential specimen of Nodocephalosaurus.[4] In 2006, Robert M. Sullivan and Denver W. Fowler referred four additional specimens: SMPVP-1957, two cranial osteoderms; SMP VP-1870, a left cervical spine; SMP VP-1149, the seventh or eighth caudal vertebrae; and SMP VP-1743, the first caudal vertebra.[5] The specimens were referred to Nodocephalosaurus as, at the time, it was the only named ankylosaurid from the Kirtland Formation.[5] In 2011, Michael Burns and Robert M. Sullivan referred three more additional specimens: SMP VP-1632, an incomplete minor tail club plate; SMP VP- 1646, an incomplete tail club that consists of an incomplete major plate and minor plate; and SMP VP2074, a nearly complete left plate, partial right plate and numerous miscellaneous tail club fragments.[6] These three additional specimens were referred to the genus based on the similar surface texture to the holotype specimen.[6] However, Arbour & Currie (2015) assigned all referred specimens, with the exception of SMPVP-1957, to Ankylosauridae indet. due to the discovery of Ahshislepelta in the Hunter Wash Member, and Ziapelta from the De-na-zin Member.[7]
In 2016, Gregory S. Paul gave Nodocephalosaurus an estimated length of 4.5 metres (15 feet) and a weight of 1.5 tonnes (3,306 lbs).[3]
Sullivan (1999) originally diagnosed Nodocephalosaurus based on the presence of semi-inflated to bulbous polygonal osteoderms fused to the nasal, frontal and supraorbital regions of the skull, a prominent quadratojugal horn that is directed anteroventrally, and a prominent post-maxillary/lacrimal osteoderm.[1] However, Arbour & Currie (2015) later diagnosed Nodocephalosaurus based on the presence of a bulbous, ridge-like loreal caputegulum, a quadratojugal horn with an anteriorly positioned apex, conical frontonasal caputegulae with circular bases, lacks V-shaped upraised area of frontals, and a lacrimal caputegulum that is smaller, more square than in Ankylosaurus, Anodontosaurus, Euoplocephalus.[7]