AN UNEXPECTEDLY ADVANCED KUFENGOCERATIN AMMONOID FROM THE PIPELINE SHALE MEMBER, BRUSHY CANYON FORMATION (UPPER ROADIAN STAGE)

The Pipeline Shale Member of the Brushy Canyon Formation commonly contains ammonoids of the Subfamily Cyclolobinae that morphologically lie near the transition from Demarezites to Waagenoceras. These specimens, such as phragmocones, are typically 50-60mm in diameter, and 85-100mm diameter specimens with the intact body chamber are not unusual. These specimens are well-preserved in the hard black limestone nodules of the lower Pipeline Shale, and have been known since the 1960s, although they have received little attention. The specimens were historically considered to be a primitive species of Waagenoceras, and they have most commonly been referred to as Waagenoceras cf. W. dieneri.

This is the first report of an ammonoid specimen recently collected from the type Pipeline Shale Member by Dr. Katherine Giles. The specimen comprises just over a quarter whorl of the phragmocone, which is estimated to have been approximately 85mm in diameter as a complete specimen, plus body whorl. It has a suture typical of Subfamily Kufengoceratinae, characterized by a directly transverse suture line with typical kufengoceratin lobes. The width of the ventral lobe is relatively thin in comparison to the first lateral lobe. It is a mature phragmacone based on the evidence of septal crowding, but no mature modification is preserved on the specimen.

The specimen is surprising in several ways. The suture is more advanced compared to the kufengoceratin Mexicoceras, which is relatively common in overlying Guadalupian strata. The conch has a subglobular shape, whereas Mexicoceras is more spherocone in shape. The umbilicus is small, but not well-preserved. There are 7 pairs of external lateral lobes, whereas typical Mexicoceras specimens have 5. The specimen has more lateral lobes than any currently known kufengoceratin. The lobes are digitate in the manner expected for kufengoceratins. All these characters point to an advanced morphology for a taxon preceding that of Mexicoceras, the only kufengoceratin common in west Texas.

In conclusion, this specimen challenges our current understanding of the phylogenetic relationships in Subfamily Kufengoceratinae, and could contest our assumptions regarding relationships within the Family Cyclolobidae as well.