RODENTIA MINUS SCIUROMORPHS

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 71

RODENTIA MINUS SCIUROMORPHS (78)

Node Calibrated. The divergence of Muroidea + Ctenohystrica (see above and Churakov et al., 2010) to the exclusion of sciuromorphs.

Fossil Taxon and Specimen. Chapattimyidae such as Birbalomys sondaari (Geological Survey of Pakistan, Howard University Collection, H-GSP 92161 of Thewissen et al., 2001) from the Kuldana Formation of the Ganda Kas area, Pakistan.

Phylogenetic Justification. Phylogenetic analysis by Marivaux et al. (2004) placed Eocene chapattimyids within the Ctenohystrica, close to hystricognaths and to the exclusion of diatomyids. They are therefore nested within the unnamed crown clade comprising Muroidea and Ctenohystrica.

Minimum Age. 47.6 Ma

Soft Maximum Age. 59.2 Ma

Age Justification. The Kuldana Formation of Pakistan has been dated as early–middle Eocene,

and ages may differ in different regions of northern India, and mammals occur in different horizons. Birbalomys is noted as occurring at 51 Ma by Marivaux et al. (2004, p. 132), thus well within the Ypresian. Hence, we assign a minimum constraint for rodents excluding the squirrel-like clade at the top of the Ypresian marine stage, hence 47.8 Ma ± 0.2 Myr = 47.6 Ma ( Gradstein et al., 2012).

The soft maximum constraint might be taken as equivalent to the age of the ischyromyids and other entirely extinct rodent groups from the late Paleocene (base of the Thanetian) of North America and Europe, 59.2 Ma ± 0.0 Myr = 59.2 Ma.

Discussion. Fossils associated with the mouse-related stem group are younger, including Eocene dipodids such as Ulkenulastomys , Blentosomys , and Aksyiromys from the Obayla Svita of the Zaysan Basin, Kazakhstan (Shevyreva, 1984). Lucas (1998) assigned a younger, Irdinmanhan age to this site based on biostratigraphic comparisons, changing its previous stratigraphic interpretation from early Eocene to the base of the middle Eocene.