Paleobiology and taphonomy of fishes from the Late Cretaceous marine ecosystems of northeastern Mexico

Abstract

The platy limestone deposit of Vallecillo in the northeastern Mexican state of Nuevo León is well known for its exceptionally well preserved faunal assemblage of vertebrate in addition to invertebrate fossils of Late Cretaceous (Latest Cenomanian-middle Turonian) age. Large marine reptiles are associated with cartilaginous as well as sarcopterygian and actinopterygian fishes that populated an open marine environment, at least 300 km away from the nearest coast. The most common fishes in the Vallecillo platy limestone member are the halecomorph needle fish Rhynchodercetis yovanovitchi, the teleost Tselfatia formosa, the pycnodont Nursallia gutturosum and the pachyrhizodont fish Goulmimichthys roberti. <br /> Goulmimichthys roberti is a fusiform taxon that resembles modern tunas or mackerels. The fish reached a maximum length of 650 mm and is interpreted as a sustainable high-speed swimmer that formed schools in the pelagic. 177 individuals from Vallecillo were reviewed including 11 specimens from deposits near Múzquiz in Coahuila, about 400 km to the north of Vallecillo. These latter deposits reflect significantly more shallow and more near-shore environments extending the stratigraphical and environmental range of the taxon previously known from the Cenomanian to Santonian. Goulmimichthys roberti may thus have migrated between shallow environments and the pelagic. Taphonomic analyses point to articulated and complete specimens in both deposits, Vallecillo and Múzquiz, excluding a phase of carcass transportation. Different embedding positions suggest that the sea bottom of the northern (Múzquiz) deposits must have been more soften. Specimens from Vallecillo are majorly preserved with open mouth and spread-out fins which is interpreted as a result of respiratory stress associated with anoxic sea bottom conditions. The oxygen-deficient bottom also excluded scavengers and favored preservation. Specimens from Múzquiz were rapidly phosphatized preserving additional details, and corpses were covered in a soft mud sediment that prevented the carcass to turnover into a lateral position as seen in Vallecillo. <br /> During the revision of G. roberti, the first Cretaceous isopod was revealed on a specimen from Vallecillo. The isopod is directly attached to the skull of the fish host and slightly twisted exposing the right body side. The isopod is nearly complete with most of the appendages allowing for an assignation to a new species of Cymothoidae named Mothocya vallecillae. A parasitic lifestyle is based on hook-like distal articles on all appendages (dactyli) as well as the position on the host, close to the gills. Modern Mothocya populate the branchial cavity of the host and feed on tissue, blood and mucus. A scavenging mode of life appears unlikely for M. vallecillae due to the anoxic bottom conditions reported from Vallecillo and the preservation of host and parasite which exclude the interference of scavengers. Mothocya vallecillae is of special interest because of the direct association to G. roberti which belongs to an pelagic ecosystem. Parasitic isopods directly attached to the fish host are presently only reported from Solnhofen in Germany. <br /> The teleost Tselfatia formosa is a frequent member of the Vallecillo fish assemblage. The fish resembles modern fan fishes and is characterized by extraordinary long dorsal and anal fins, pseudo fulcra on the main rays of the unpaired fins, and small pectorals above the ventral column. A review of 149 specimens revealed two different body shape types which are interpreted as sexual shape dimorphism excluding ontogenetic variation. Deeply keeled specimens are interpreted as females that are twice as long as high compared to torpedo-shaped males which are three times longer than wide. The teleost is frequently articulated and complete and the unpaired fins are retracted behind modified scales which allow for comparison to extant fan fishes (e.g. Pteraclis and Pterycombus). A membrane between the fin rays is concluded due to fin preservation and arrangement. Taphonomic analyses suggest that Tselfatia formosa inhabited deeper water environments similar to recent fan fishes.