Background. Solemydidae is an enigmatic clade of stem turtles, which is primarily represented by fragmentary shell material found in Upper Jurassic and Cretaceous deposits of Europe and North America. Specimens are typically identified by a distinct surface ornamentation comprised of tall, narrow tubercles, which are often incompletely preserved due to taphonomic processes. The only currently recognized North American species of Solemydidae, Naomichelys speciosa was erected in 1908 on the basis of a nearly complete entoplastron from the Lower Cretaceous Cloverly Formation of Montana; however, a far more complete specimen from the Lower Cretaceous Antler Formation of Texas (FMNH PR273) was recently referred to this species and has shed light on the morphology of Naomichelys. Here, I describe a partial skeleton (MOR 941) that was discovered in 1995 near the type locality in Montana, which allows for the first look at variation in many of the skeletal elements of the genus. Methods. Morphological variation of Naomichelys was determined by comparison of MOR 941 to the holotype specimen and FMNH PR273 from the literature. Results. MOR 941 is referable to Solemydidae based on a large, diamond shaped entoplastron, which exhibits the distinctive solemydid surface ornamentation and an entoplastral scute, and is referable to Naomichelys because of its near identical morphology to that of the holotype. This specimen consists of the carapace, plastron, four cervical vertebrae, both shoulder girdles, and the right femur. Preserved carapacial elements are the right costals and peripherals in articulation, albeit somewhat crushed, disarticulated left costals, and a single neural. Using peripheral and costal sutures as well as marginal sulci as landmarks, peripherals 1 – 4 are identified, which indicates that the nuchal notch is either reduced or lacking completely. The plastron is well preserved with only portions of the left bridge and xiphiplastra missing and a central plastral fontanel is completely lacking. Cervical vertebrae 5 – 8 are preserved and show varying states of completeness with C7 being the most complete. Cervical 5 preserves a ventral keel on the centrum that is well developed and projects well beyond the central articulations. The shoulder girdle elements are similar to FMNH PR273 but are less well preserved and are not sutured together. The femur is complete and almost identical to FMNH PR273; however, the distal condyles are equally developed. Discussion. The morphological differences between MOR 941 and the Texas material suggest two plausible conclusions. 1) They demonstrate two different ontogenetic stages of development, with MOR 941 representing a skeletally more mature, albeit smaller individual; or 2) there are two distinct species of Naomichelys from the Lower Cretaceous of North America.
Morphological variation in the hydromedusa genus Polyorchis on the west coast of North America
