Utetitan zellaguymondeweyae

Utetitan zellaguymondeweyae was a large titanosaurid sauropod dinosaur that lived in what is now central Utah, USA, during the Late Cretaceous period, approximately 69–66 million years ago (late Maastrichtian stage). It was discovered in the lower North Horn Formation, a sedimentary deposit known for its fluvial and overbank facies preserving diverse Late Cretaceous vertebrates near the Cretaceous-Paleogene boundary.

The genus name Utetitan honors the original Ute peoples of central Utah upon whose ancient lands the fossil was found, combined with Ancient Greek “titan” (giant). The species name zellaguymondeweyae honors the author’s maternal grandmother Zella Guymon Dewey (1901–2002), born and raised in Huntington, Utah, near the discovery site.

As a member of Titanosauridae within Sauropoda, Utetitan represents a distinct North American titanosaurid taxon, coexisting with Alamosaurus sanjuanensis but differentiated by stratigraphic and anatomical evidence.

Etymology

The genus name “Utetitan” derives from the Ute peoples of central Utah, where the holotype was found, combined with Ancient Greek “titan,” meaning “giant.” The species name “zellaguymondeweyae” honors Zella Guymon Dewey (1901–2002), the author’s maternal grandmother, who was born and raised in Huntington, Utah, approximately 30 km (19 mi) northeast of the holotype quarry, and who referred to the Wasatch Plateau as “the hills of home.”

Physical Characteristics

Utetitan zellaguymondeweyae was a large titanosaurid, estimated at 22.3 m (73 ft) in length and 22 tonnes in mass for the holotype individual. Larger referred specimens (e.g. BYU 9087) suggest up to 31 tonnes”. The holotype is a partial skeleton preserving axial and appendicular elements.

Key anatomical features include:

• Scapula with a strongly prominent acromion process, strongly sinuous posterior profile forming a prominent triangular glenoid process, and paired fairly prominent tubercles dorsal to the glenoid, widely separated; base of blade cross-section asymmetrically concave with thinner anterior and thicker posterior margin; antero-dorsal blade edge nearly straight, postero-dorsal edge flaring posteriorly.
• First caudal centrum biconvex with circumferential depression limited to ventral half of anterior condyle, multiple large foramina piercing lateral surface.
• Ilium with small indentation on dorsal rim.
• Ischium with modest-sized lateral process and moderately concave-arced postero-lateral profile.
• Femur robust.

These traits distinguish it from Alamosaurus sanjuanensis, supporting generic separation.

Diet and Feeding Habits

As a herbivorous sauropod, Utetitan zellaguymondeweyae fed on a variety of plants in its Late Cretaceous ecosystem. Its diet likely included:

• Conifers, ferns, and horsetails, abundant in Maastrichtian floodplains.
• Flowering plants (angiosperms), increasingly dominant.
• High vegetation, accessed via its long neck.

Like other titanosaurs, it used a broad beak for cropping and gastroliths for digestion.

Habitat and Distribution

Utetitan zellaguymondeweyae lived in what is now central Utah, USA, specifically in the lower North Horn Formation of the Wasatch Plateau.

Its environment was characterized by:

• Fluvial channels and overbank deposits, with seasonal flooding.
• Humid subtropical climate supporting lush vegetation.
• Diverse fauna including hadrosaurs, ceratopsians, and theropods.

The North Horn Formation preserves a terminal Cretaceous assemblage.

Paleoenvironment

The site was a dynamic floodplain during the late Maastrichtian, with rivers and wetlands near the Cretaceous-Paleogene boundary. The assemblage reflects a productive habitat for large herbivores like Utetitan.

Behavior and Social Structure

No direct evidence exists for Utetitan zellaguymondeweyae, but titanosaurs likely lived in herds. Possible behaviors include:

• Gregarious living for protection and migration.
• Bulk feeding on floodplain vegetation.
• Egg-laying in clutches, with rapid reproduction favoring diversification.

As a titanosaurid, it occupied large herbivore niches in a predator-rich landscape.

Discovery and Research

The holotype of Utetitan zellaguymondeweyae (USNM 15560) was discovered in the lower North Horn Formation near Joe’s Valley, Wasatch Plateau, Emery County, Utah. Collected in the early 20th century, it was described in 1946 as Alamosaurus sanjuanensis but reclassified in 2025 based on stratigraphic and anatomical distinctions.

Referred specimens include BIBE 45958 (partial left scapula, Texas) and tentatively BYU 9087, 11392, 11393 (proximal humerus/femur, anterior caudals).

Phylogenetic analyses support a distinct North American titanosaurid subclade.

Discovery Context

Unearthed from fluvial sediments dating to ~69–66 million years ago, the partial skeleton (dorsal ribs, caudals 1–30, 25 chevrons, both sternals, right scapulocoracoid, humerus, radius, ulna, metacarpus, both ischia, osteoderm) is the most complete late Maastrichtian titanosaurid known.

Significance and Interesting Facts

• Distinct from Alamosaurus sanjuanensis, indicating multiple titanosaurid taxa in late Maastrichtian North America.
• Part of a potential “Utetitan miniclade,” evolving in semi-isolation.
• Estimated 22 tonnes, not among the largest titanosaurs.
• Honors Ute peoples and a local historical figure.
• Supports rapid sauropod evolution, with species lasting <1 million years.

Conclusion

Utetitan zellaguymondeweyae was a large titanosaurid from the late Maastrichtian of Utah, exemplifying North American sauropod diversity near the Cretaceous-Paleogene boundary. Distinguished from Alamosaurus by scapular and caudal features, it highlights rapid evolution in isolated clades. This 2025 reclassification clarifies Laramidian titanosaur biogeography.