Thursday 24 October 2019

Stegosaurs from the Late Jurassic Morrison Formation of Montana.

The Stegosauria is a group of Ornithischian (Bird-hipped) Dinosaurs that is characterised by the possession of a series of plates and spikes that extend from the neck to the tail in two parasagittal rows. The best known genus of the group, Stegosaurus, is from the Upper Jurassic of the western USA, but members of the group are known from all continents except for Antarctica and Australia, and their temporal range extends from the Middle Jurassic to the late Early Cretaceous. Despite the iconic status of Stegosaurs among Dinosauria, they are poorly known as fossils, with most genera being known from single partial skeletons and/ or fragmentary remains. Consequently, they have received relatively little attention, and much remains unknown about their palaeobiology.

In a paper published in the journal Acta Palaeontologica Polonica on 5 August 2019, Cary Woodruff of the Great Plains Dinosaur Museum and Field Station, the Department of Ecology and Evolutionary Biology at the University of Toronto, and the Royal Ontario Museum, David Trexler of the Two Medicine Dinosaur Center, and Susannah Maidment of the Department of Earth Sciences at the Natural History Museum, describe two sets of Stegosaur remains from the Late Jurassic Morrison Formation of Montana, an area further north than any that have previously produced described Stegosaur remains.

The Upper Jurassic Morrison Formation of the western United States comprises the deposits of rivers and floodplains that were laid down in a broad shallow basin over about 10 million years in the Kimmeridgian (157.3-152.1 million years ago and Tithonian (152.1-145.0 million years ago). The deposits host a diverse and well-preserved Dinosaurian fauna that includes such well-known taxa as Apatosaurus, Diplodocus, and Stegosaurus. The Dinosaurs of the southern part of the Morrison Basin, from Colorado, Utah, and southern Wyoming, have been intensively studied for about 150 years, but those from northern outcrops in northern Wyoming and Montana are much less exploited. An increasing body of evidence suggests that northern Morrison faunas were somewhat different to their southern counterparts but further exploration in this part of the basin is needed.

The first specimen described by Woodruff et al., GPDM 205, was collected in 2004 by the Judith River Dinosaur Institute from the 5E Ranch, owned by the Hein family, in the Little Snowy Mountains of Fergus County, Montana. The specimen is now housed at the Great Plains Dinosaur Museum in partnership with the Judith River Foundation. GPDM 205 was collected in close proximity to Camarasaurus sp. specimen GPDM 220. Unfortunately, neither field notebooks nor quarry maps exist for the excavation of this specimen, so Woodruff et al. do not possess any documentation regarding taphonomy. However, Susanneh Maidment visited the 5E Ranch quarry in 2005 and was told by the excavation crew that the specimen was found disarticulated, and stratigraphically above the Camarasaurus remains. 

The material assigned to GPDM 205 comprises a prepared humerus, ulna, and radius, plus an unprepared coracoid, fragmentary scapula, humerus, and ulna. During the 2005 field season, at the time of Susanneh Maidment’s visit, a carpal and some poorly preserved dermal plates from this specimen were also recovered. Several other Stegosaur specimens were discovered during the 2005 field season at the 5E Ranch quarry, and were excavated over subsequent years. These specimens currently reside in a private collection, and thus are unavailable for study.

Left humerus of Stegosaur GPDM 205 from the 5E Ranch, Little Snowy Mountains, Fergus County, Montana, USA; in anterior (A₁), posterior (A₂), lateral (A₃), medial (A₄), dorsal (A₅), and ventral (A₆) views. Woodruff et al. (2019).

The second specimen, GPDM 178, was collected in 2003 and 2004 by the Judith River Dinosaur Institute near the town of Giffen, Cascade County, Montana, on land belonging to the Yurek family. Elements were initially found eroding out of a hill adjacent to the Yurek’s house. The distal end of the right tibia was the first element recovered, and it was not immediately recognised as a fossil. For a period of time, this 'black rock'  served as a door stop until construction of a pole barn on the hill started, and further elements were discovered below the surface. At this time the Yurek family contacted the Judith River Dinosaur Institute, and the excavation commenced. The specimen is now housed at the Great Plains Dinosaur Museum in partnership with the Judith River Foundation 

 Partial right tibia of Stegosaur GPDM 178, near Giffen, Cascade County, Montana; in medial (A₁, A₆), lateral (A₂, A₇), anterior (A₃, A₅), dorsal (A₄), and ventral (A₈) views. (A₁)–(A₄) proximal end, (A₅)–(A₈), distal end. Woodruff et al. (2019).

GPDM 178 consists of a partial skeleton including both crania and post-crania. Several field jackets that are labelled as dorsal ribs or vertebrae await preparation. Only those elements prepared to-date are described, though the skeletal reconstruction for this specimen includes all elements collected as recorded in the quarry log.

Skeletal reconstruction of the stegosaurs GPDM 178 (A) and GPDM 205 (B) scaled together based on humeral lengths. Black, elements preserved in each specimen; gray, missing elements. Human scale is the Artemision Bronze featuring the deity of Zeus as 1.83 m in height. Woodruff et al. (2019).

Three genera of Stegosaur have previously been described from the Morrison Formation, Alcovasaurus, Hesperosaurus, and Stegosaurus, though unfortunately neither of the Montana specimens is complete enough to assign them to a specific genus, as useful diagnostic features are not preserved.

The specimens GPDM 178 and 205 represent the northernmost Stegosaurs found to-date, and GPDM 178 is the northernmost Dinosaur fossil thus far recovered from the entire Morrison Formation. Interestingly, according to a Works Progress Administration report from 1938, a 'Stegosaurus' was found at Sheep Creek, 25 miles north of Great Falls, Montana. If true, then this specimen is the northernmost Morrison Formation dinosaur, as GPDM 178 was found near equivalent distance southeast of Great Falls, Montana. Unfortunately, this report does not mention what material was collected or where it was sent. Searching for this material in Montana institutions with historic palaeontological collections and the National Museum of Natural History has yielded no results. Therefore Woodruff et al. believe that these remains, if true, have subsequently been lost or discarded. Thus by default, GPDM 178 is the northernmost record of the Stegosauria.

(A) Map of the USA with indicated the state of Montana, were the Stegosaur specimens GPDM 178 and GPDM 205 were found. (B) Map of Montana with the extent of the Morrison Formation in green, and the general locations of GPDM 178 and GPDM 205 indicated by the Stegosaurus skull silhouettes. Woodruff et al. (2019).

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