No abstract

Fossil penguins (Aves, Spheniscidae) are widely recorded throughout the Southern Hemisphere and are especially well represented from the Eocene to present in Patagonia. There are currently three Miocene specimens that include preservation of part of the skull and mandibles: Paraptenodytes antarcticus, Madrynornis mirandus and MLG 3400 (cf. Spheniscus). Osteological appraisals relative to extant taxa and a morphometric analysis were employed to infer possible dietary preferences. The results suggest that Paraptenodytes antarcticus and cf. Spheniscus were piscivores but used different strategies for prey capture. Madrynornis, on the other hand, might have had a mixed diet comprising both small fish and crustaceans.

No abstract

Embryonic muscular activity (EMA) is involved in the development of several distinctive traits of birds. Modern avian diversity and the fossil record of the dinosaur-bird transition allow special insight into their evolution. Traits shaped by EMA result from mechanical forces acting at post-morphogenetic stages, such that genes often play a very indirect role. Their origin seldom suggests direct selection for the trait, but a side-effect of other changes such as musculo-skeletal rearrangements, heterochrony in skeletal maturation, or increased incubation temperature (which increases EMA). EMA-shaped traits like sesamoids may be inconstant, highly conserved, or even disappear and then reappear in evolution. Some sesamoids may become increasingly influenced in evolution by genetic-molecular mechanisms (genetic assimilation). There is also ample evidence of evolutionary transitions from sesamoids to bony eminences at tendon insertion sites, and vice-versa. This can be explained by newfound similarities in the earliest development of both kinds of structures, which suggest these transitions are likely triggered by EMA. Other traits that require EMA for their formation will not necessarily undergo genetic assimilation, but still be conserved over tens and hundreds of millions of years, allowing evolutionary reduction and loss of other skeletal elements. Upon their origin, EMA-shaped traits may not be directly genetic, nor immediately adaptive. Nevertheless, EMA can play a key role in evolutionary innovation, and have consequences for the subsequent direction of evolutionary change. Its role may be more important and ubiquitous than currently suspected.