19-4 DINOSAURS AND MORE! THE FOSSIL VERTEBRATE FAUNA OF THE HALL LAKE FORMATION (CAMPANIAN/MAASTRICHTIAN: SIERRA COUNTY, NEW MEXICO)

Session: The Campanian Crucible: A synthesis of vertebrate paleobiogeography and ecosystem dynamics in Laramidia

Presenting Author:

Spencer Lucas

Authors:

Lucas, Spencer G¹, Fiorillo, Anthony R², Dalman, Sebastian G³, Lichtig, Asher J⁴

(1) Department of Cultural Affairs, New Mexico Museum of Natural History & Science, Albuquerque, NM, USA, (2) Department of Cultural Affairs, New Mexico Museum of Natural History & Science, Albuquerque, NM, USA; Hokkaido University Museum, Sapporo, Japan, (3) Montana State University, Bozeman, MT, USA, (4) Department of Cultural Affairs, New Mexico Museum of Natural History & Science, Albuquerque, NM, USA,

Abstract:

Non-marine, Cretaceous sedimentary rocks are abundantly exposed in southern New Mexico in central Sierra County. From these exposures, an important vertebrate fossil record is emerging in the Hall Lake Formation (HLF). The HLF had formerly been placed in the McRae Formation and defined as one of two members, but later stratigraphic work elevated the McRae to group status and elevated the HLF to formation status. The HLF is at least 700 m thick and perhaps over 1 km thick. It is primarily red-bed mudrock with some interbeds of volcanic litharenite sandstone and lenticular conglomerate. Deposition was by fluvial processes, and numerous calcisols and vertic calcisols reflect periods of intermittent moisture and landscape stability. Fossil palm wood indicates that prolonged freezing did not occur. U/Pb ages on ash bed zircons and detrital zircons range from about 73 to 69 Ma, placing the lower HLF in the late Campanian/early Maastrichtian.  Turtle and dinosaur fossils are found in the lower HLF, 24-141 m above its base. The turtles are assigned to Compsemys, Denazinemys and a bothremydid, characteristic Campanian-Maastrichtian turtle taxa. Theropod, sauropod, ceratopsid and hadrosaurid dinosaur fossils are present, largely as disarticulated and isolated postcrania, dental and cranial material. Previous taxonomy of the dinosaurs identified Tyrannosaurus rex, Alamosaurus and Torosaurus, suggesting a Lancian (late Maastrichtian age). However, the tyrannosaur has been reassigned to the new species Tyrannosaurus mcraeensis, and the ceratopsian fossils are now identified as Sierraceratops turneri. These endemic dinosaur taxa are consistent with, but not demonstrative of a late Campanian/early Maastrichtian age. Among the collected dinosaur fossils, ceratopsids (not hadrosaurids) are most abundant, and this could reflect the inland depositional setting of the HLF, which was about 400 km from the late Campanian shoreline to the northeast. This frequency parallels the pattern of the younger Prince Creek Formation in northern Alaska, yet contrasts with the fossil record of Alberta, suggesting that habitat preferences should not be applied at high taxonomic levels.  The large HLF tyrannosaur also complicates the biogeographic model of an Asian origin for the group. We build on earlier work and present new information on recently collected fossils from the HLF attributable to titanosaurid, ceratopsid, and hadrosaurid dinosaurs.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

doi: 10.1130/abs/2026RM-13978

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