EXCEPTIONALLY PRESERVED HYDROZOANS (CHONDROPHORINES) FROM THE LOWER PERMIAN ROBLEDO MOUNTAINS FORMATION OF SOUTH-CENTRAL NEW MEXICO

We report here an unusual occurrence of hydrozoan chondrophorine fossils from two localities within the Lower Permian (Wolfcampian) Robledo Mountains Formation in the Robledo Mountains, Doña Ana County, south-central New Mexico. These localities are in siliclastic red-bed tidal flat facies associated with the Hueco seaway. Specimens from the first locality consist of numerous medusae and fewer hydroids. These are preserved on mud-draped sandstone slabs as circular, positive hyporelief impressions seen in oral aspect. Diameters range from 2 to 13 mm. Several medusae show soft-bodied, subumbrella anatomy including velum, oral arms and capitate tentacles. One well-preserved hydroid shows a central, cone-shaped manubrium surrounded by spindle-shaped gastrogonozooids and numerous tentacular dactylozooids. The exceptional preservation of soft-bodied anatomy and the overlap of many specimens precludes their being sedimentary pseudofossils (“scratch circles”), misidentification of which has clouded the chondrophorine fossil record. The second locality extends over an outcrop area of tens of meters and yields a large number of less well-preserved specimens. These consist of similar discoidal impressions, some of which display concentric ornamentation that we interpret as representing pneumatophores.

The Robledo occurrence likely represents a Lagerstätte deposit recording a mass stranding of chondrophorines along an Early Permian coastal tidal flat. Cnidarian mass strandings are commonplace today but are rare in the fossil record. This is the first report of chondrophorine fossils from the Permian.