Longisquama’s Long Scales

A new paper by Buchwitz and Voight (2012) sheds new light on the dorsal frill (plumes, appendages) of Longisquama (Sharov 1971). From their abstract: “On the basis of comparative description of the individual morphology of all yet known Longisquama specimens we address aspects of taphonomy, development, and function and define to what extent Longisquama’s appendages share characteristics of avian vaned feathers. 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.”

Not Plants
Buchwitz and Voight (2012) further established that the frills were not from coincident plants. Several appendages were shown to articulate with vertebrae and no similar plants are known.

Origin
With regard to the appendages origin, Buchwitz and Voigt (2012) reported, these plumes, “can be explained by formation in an elaborate developmental process that began with a filamentous appendage primordium.” They considered, “the ability to form simple filamentous and more complex elongated skin appendages as the synapomorphy of an archosauromorph clade including Longisquama, crocodylians, pterosaurs, non-avian dinosaurs, and birds.”

Basic Problem
Buchwitz and Voigt (2012) did not consider the possibility that the frill was homologous to those found in various lizards like Iguana (Fig. 1) and Sphenodon (Fig 2). The present large reptile tree found Longisquama to be nested within the tritosaur lizards, not far from the base of the Lepidosauria (which includes Sphenodon and Iguana as phylogenetic brackets). They did not establish another closest sister taxon to Longisquama within the Archosauromorpha, nor did they offer any results from a phylogenetic analysis, like this one. Their family tree (Buchwitz and Voigt 2012, fig, 10) graphically nested Longsiquama between “Lepidosauromorpha” and “basal archosauromorpha” within a monophyletic “diapsida,” which is false and extremely vague at best. They referenced Peters (2000) which proposed a relationship between Macrocnemus, JesairosaurusLangobardisaurus, Cosesaurus, Sharovipteryx, Longisquama and pterosaurs, then “but see”-d it with a reference to Hone and Benton (2007 and by inference 2008) which deleted Sharovipteryx and Longisquama from its database and deleted 75% of the traits of Cosesaurus to eliminate rather than test these taxa in phylogenetic analysis.

Blame the Preservation
Instead they blamed the state of preservation and the false paradigm of a partial skeleton for their uncertainty. They observed no clear indication of a antorbital or mandibular fenestra. They observed “no skull bone substance” and the “sediment relief seems to be of limited informativeness.” They did not attempt to perform DGS (digital graphic segregation on this flattened fossil. Some of this is understandable as Longisquama is one of the few fossil reptile specimens in which the soft tissue is more apparent than much of the hard tissue (largely due to coverage by the soft tissue).

The spines of Iguana.

Figure 1. The spines of Iguana. These occur much more frequently than the underlying vertebrae.

Iguana spines
The dorsal spines of a Green iguana are noticeably longer and thicker in males (Fig. 1). Thus these are considered secondary sexual characteristics. In Iguana the spines occur in multiples relative to the each vertebrae, rather than one-to-one, as in Longisquama.

The dorsal spines of Tuatara (Sphenodon).

Figure 2. The dorsal spines of Tuatara (Sphenodon).

Sphenodon Spines
The dorsal spines in Sphenodon (Fig. 2) are short, white and there is little color in this otherwise drab taxon. Nevertheless, there they are. They’re not fibers, they’re modified scales, as Sharov (1971) originally proposed. Some appear one-to-one with the underlying vertebral spines. Others are two-to-one.

Functional Analog to Squamates
Buchwitz and Voigt (2012) reported, “the series of dorsal appendages in Longisquama could be a functional analogue to the dorsal crests of elongated scales in extant squamates”. 

Analogs, yes, AND homologs.

Buchwitz and Voigt (2012) also reported, The hypothesis that integumentary structures of high complexity evolved in the context of display is not unreasonable if the mating systems and display behaviour were likewise highly derived.” That concept is right in line with the hypothesis that wings powered by bird-like flapping anchors developed first as display structures in pterosaur ancestors discussed here, here and in Peters (2002).

Details
Buchwitz and Voigt (2012, fig. 3) includes a patch of tissue in the upper right hand corner with parallel striations. This is where I illustrated a torn uropatagium, as in Sharovipteryx, which also reinforced its uropatagium with embedded fibers that look like parallel striations. Mike Buchwitz (pers. comm.) considered that patch a displaced cockroach wing. We’ll see…

Summary
Buchwitz and Voigt (2012) provided a much needed and appreciated look at the dorsal scales of Longisquama. Unfortunately their phylogenetic base mistakenly nested Longisquama (and pterosaurs) with unspecified archosaurs leading toward a plume/filament origin rather than an elongated scale origin, as in Iguana.

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
Buchwitz M and Voigt S 2012. The dorsal appendages of the Triassic reptile Longisquama insignis: reconsideration of a controversial integument type. Paläontologische Zeitschrift (advance online publication) DOI: 10.1007/s12542-012-0135-3
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Sharov AG 1970
. A peculiar reptile from the lower Triassic of Fergana. Paleontologiceskij Zurnal (1): 127–130.

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