The origin and evolution of Longisquama “feather-like structures”

 

Figure 1. Click to enlarge. The origin and evolution of Longisquama's "feathers" - actually just an elaboration of the same dorsal frill found in Sphenodon, Iguana and Basiliscus. Here the origin can be found in the basal tritosaur squamate, Huehuecuetzpalli and becomes more elaborate in Cosesaurus and Longisquama.

Figure 1. Click to enlarge. The origin and evolution of Longisquama’s “feathers” – actually just an elaboration of the same dorsal frill found in Sphenodon, Iguana and Basiliscus. Here the origin can be found in the basal tritosaur squamate, Huehuecuetzpalli and becomes more elaborate in Cosesaurus and Longisquama.

A year ago
New Scientist covered a new paper on the dorsal appendages of Longisquama (Buchwitz and Voigt 2012) who wrote in his abstract, “We explain the existing feather similarity by their development from a filamentous primordium and a complex sequence of individual processes, some of which are reminiscent of processes observed in feather development. Such an interpretation is in agreement with a set of homologous mechanisms of appendage morphogenesis in an archosauromorph clade including Longisquama and feather-bearing archosaurs but does not necessarily require that the appendages of Longisquama themselves are feathers or high-level feather homologues.”

Here a larger and more parsimonious phylogenetic analysis
nests Longisquama with fenestrasaur, tritosaur squamates, derived from sisters to Huehuecuetzpalli and Cosesaurus (Fig. 1). With this lineage the origin and the development of the single line of dorsal plumes becomes easy to visualize. They were small originally and, through evolution, became enlarged. Among living reptiles, the tuatara (Sphenodon) and the iguana (Iguana) bear similar and homologous small structures.

Buchwitz reported in New Scientist, “The strange skin appendages of Longisquama are neither scales nor feathers,” says Michael Buchwitz of the Freiberg University of Mining and Technology, Germany. “They are perhaps linked to the early evolution of dino and pterosaur fuzz, though. Longisquama‘s skeleton is too incomplete to work out its exact evolutionary position, but Buchwitz says the little reptile was probably part of the lineage that gave rise to pterosaurs, crocodiles, dinosaurs and birds. Many of these groups later evolved their own skin appendages, including filaments on pterosaur wings, quills on the tails of some plant-eating ornithischian dinosaurs, and the proto-feathers of theropod dinosaurs. Longisquama shows that evolution was experimenting with the genes that gave rise to feathers long before any of these animals appeared on the scene.”

Tomorrow we’ll take a close look at the metatarsus of Longisquama courtesy of Buchwitz and Voigt (2012).

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
New Scientist article here
Buchwitz M and Voigt S 2012. The dorsal appendages of the Triassic reptile Longisquama insignis: reconsideration of a controversial integument type. Paläontologische Zeitschrift, Issue 3, pp 313-331

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