[D] Propanoplosaurus marylandicus [Su]
Stanford, Weishampel & Deleon 2011
Cretaceous Early Aptian
Ornithischia Thyreophora Eurypoda Ankylosauria Nodosauridae
Patuxent Formation, Maryland, US
Abundant and diverse dinosaur footprints have been discovered recently on bedding surfaces of the Lower Cretaceous Patuxent Formation of Maryland and Virginia. Found along with those ichnofossils is a fossil preserved partially as natural casts and partially as natural molds of a baby nodosaurid ankylosaur so small as to justify interpreting it as a hatchling. Despite the rather unusual type of preservation, the find is properly termed a body fossil and not an ichnite, per se, because it records not the action of an organism, but the body form and bone structure (including partial articulation) of a dinosaur.
We here name it Propanoplosaurus marylandicus and provide a description of its diagnostic characteristics. Although actual skeletal remains referable to P. marylandicus have not been found in the Patuxent Formation, other nodosaurids recognized from skeletal remains are known from both the Lower and Upper Cretaceous strata of the Western Interior of North America and Europe. P. marylandicus represents the only diagnostic nodosaurid from the Early Cretaceous of the eastern U.S.A., provides information on growth patterns among nodosaurids, and is the first direct evidence of a dinosaur hatchling and, deductively, nesting, on the entire eastern seaboard.
USNM 540686, was found in the Patuxent Formation, dating from the late Aptian. It consists of the impressions of the back of the head together with a \\\\\\\\\\\\\\\"natural\\\\\\\\\\\\\\\" cast of the ribcage, some vertebrae, the right forelimb, the right femur and a right foot. The animal is shown laying on its back. The authors rejected the possibility the specimen represented a pseudofossil or an embryo. The specimen is the first nodosaurid skeleton from the eastern seaboard from which previously only nodosaurid teeth named as Priconodon were known, and the first neonate dinosaur from that region.
The generic name combines the Latin prefix pro~ with the name of the genus Panoplosaurus because the new species lived earlier than — but resembled much — this already described nodosaurid. The specific name refers to Maryland.
The specimen has a preserved length of thirteen centimetres. The total length of the individual was estimated as between twenty-four and twenty-eight centimetres. Only the skull shows osteoderms and the authors suggest this was a common developmental stage of all nodosaurids, together with a long middle section of the snout which is characterised by a unique cross-pattern of the bone plates, probably formed by the triangular osteoderms of the maxillae.
Propanoplosaurus was by the describers assigned to the Nodosauridae.