Jonathan E. Jeffery, Glenn W. Storrs, Timothy Holland, Clifford J. Tabin, and Per E. Ahlberg (2018)
Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning.
Proceedings of the National Academy of Sciences (advance online publication)
The fossil fish Rhizodus hibberti, a member of the tetrapod stem group, shows a unique skeletal pattern in the pelvic fin. Rather than the highly conserved one-to-two pattern of a femur, tibia, and fibula (seen in all known tetrapods, including the extinct, fishlike members of the group), the fin of Rhizodus comprises a femur articulating distally with three bones, each with a distinct morphology. This reveals an early stage in the evolution of limb development, in which the processes patterning the proximal parts of the embryonic fin/limb (the stylopod and zeugopod) were not constrained in the way seen in living tetrapods and could produce more varied skeletal patterns in the adult.
All living tetrapods have a one-to-two branching pattern in the embryonic proximal limb skeleton, with a single element at the base of the limb (the humerus or femur) that articulates distally with two parallel radials (the ulna and radius or the tibia and fibula). This pattern is also seen in the fossilized remains of stem-tetrapods, including the fishlike members of the group, in which despite the absence of digits, the proximal parts of the fin skeleton clearly resemble those of later tetrapods. However, little is known about the developmental mechanisms that establish and canalize this highly conserved pattern. We describe the well-preserved pelvic fin skeleton of Rhizodus hibberti, a Carboniferous sarcopterygian (lobe-finned) fish, and member of the tetrapod stem group. In this specimen, three parallel radials, each robust with a distinct morphology, articulate with the femur. We review this unexpected morphology in a phylogenetic and developmental context. It implies that the developmental patterning mechanisms seen in living tetrapods, now highly constrained, evolved from mechanisms flexible enough to accommodate variation in the zeugopod (even between pectoral and pelvic fins), while also allowing each element to have a unique morphology.
A new specimen of coelacanth based on a new specimen from the Meride Limestone Formation of the UNESCO World Heritage area of Monte San Giorgio is described. It represents the first occurrence of an actinistian in this formation. The newly discovered specimen shares many characters with the poorly known Heptanema paradoxum Bellotti, 1857 from the Ladinian Perledo Formation of Northern Italy. A comparison with the holotype and only existing specimen of H. paradoxum supports the assignment of the new specimen to the genus Heptanema. Some anatomical differences between the two specimens are most probably due to different ontogenetic stages, while few may support the erection of a new species; since the specimen is a juvenile it is preferred not to erect a new species, but to classify the specimen as Heptanema sp. New available data from both the holotype of H. paradoxum and form the new specimen allows an attempt to assess the phylogenetic relationships of Heptanema.
Kenneth Carpenter (2018)
Soft-bodied fossil of a lizard from the Parachute Creek Member, Green River Formation (Eocene), Utah.
Geology of the Intermountain West 5: 263-269
A rare specimen of soft tissue preservation of a lizard from the Parachute Creek Member of the Eocene Green River Formation, Uinta Basin, Utah, is described. The preservation is unusual in that it is a minerÂalized body lacking the skeleton. This, and other small boneless vertebrate specimens also from the ParaÂchute Creek, indicate occasional demineralizing conditions in Lake Uinta, but not apparently in the other two lakes of the Green River FormationâFossil Lake and Lake Gosuite.