The Back Half of Longisquama Recovered by Phylogenetic Bracketing

The back half of Longsiquama is so difficult to see that all prior experts took Sharov’s (1970) word for it, accepting his observation that the back half was missing. I’ll admit (and you’ll see) that even when the parts are delineated and labeled (Fig. 1), it’s hard for the eye to “see” what’s going on here. Skin, hair, membranes, plumes and scales are everywhere and just make the job more difficult. Only DGS (digital graphic segregation) enables one to pick out the crushed (and I do mean CRUSHED) outlines of the formerly missing parts.

New tracings of Longsiquama

Figure 1. Click to enlarge. New tracings of Longsiquama soft tissues and bones digitally segregated. Black squares are for alignment.

Be that as it may…
Let’s assume the back half isn’t there… for argument’s sake. No problem. Believe it or not, there is another way to figure out what the back half of Longisquama looks like without resorting to actually tracing the parts. This method gives strong clues, but without providing 100% assurance. And it’s called…

Phylogenetic Bracketing
Take the more primitive related taxon (Sharovipteryx) and the more derived related taxon (MPUM6009) and see what they do and don’t have in common. The taxon in the middle, Longisquama, should share most of the traits that both of these taxa share. Phylogenetic bracketing can be used to imagine a transitional taxon that we haven’t discovered yet. However, we’re at an advantage with Longisquama. We already know what the skull, forelimbs and upper torso look like. So, here’s where we’ll start (Fig. 2):

 

Figure 2. Origin of pterosaurs and figuring out the back half of Longisquama through phylogenetic bracketing. Gray areas are in question because others have not yet seen them on the slab.

Figure 2. Origin of pterosaurs and figuring out the back half of Longisquama through phylogenetic bracketing. Gray areas are in question because others have not yet seen them on the slab.

Traits shared by Sharovipteryx and MPUM6009 (and therefore likely present in the purportedly missing back half of Longisquama) include the following:

  1. short torso, shorter than or subequal to femur – this trait is actually not present in  Longisquama (Fig. 1), which has a uniquely longer, lemur-like torso, better for leaping than gliding
  2. bipedal configuration (relatively shorter fore limbs than hind limbs)
  3. four or more sacrals
  4. long, attenuated tail with chevrons parallel to centra
  5. elongated anterior ilium
  6. prepubis
  7. femur at least twice as deep as torso
  8. tibia longer than femur
  9. tibia more than twice the length of the pes
  10. simple hinge ankle joint between proximal tarsals (astragalus and calcaneum) and distal tarsals (principally the centralia and distal tarsal 4, but distal tarsal 3 is sometimes present and smaller)
  11. metatarsals increase in length from 1 to 4, but mt5 is less than a quarter of mt4
  12. feet relatively small – this trait is not present in the actual Longisquama, which has larger feet than either sister
  13. pedal digits increase in length from 1 to 5, but p.5.1 is subequal to mt4 in length and and p5.2 is hyperflexed.
  14. uropatagia present

So, add it all up and you get pretty much what I found in Longisquama, sans the torso length, which is uniquely long and lemur-like in Longisquama, and sans the foot size, which is uniquely larger in Longisquama. All prior reconstructions of Longisquama missed the long torso, long hind limbs, pterosaur like pedal digit 5 and attenuated tail. Instead authors imagined a shorter legged, shorter torso, more typically basilisk-like back half, but then none of them imagined that Longisquama was phylogenetically bracketed by Sharovipteryx and pterosaurs with a common ancestor in Cosesaurus.

Then we take the opposite tack: 
Traits that are distinctly different between the bracketing taxa, Sharovipteryx and MPUM6009, include the following:

  1. relative skull size – skull is small in both Sharovipteryx and Longisquama
  2. relative neck length – neck is short in both MPUM6009 and Longisquama
  3. pectoral girdle size – girdle is small in both Sharovipteryx and Longisquama
  4. forelimb size – midway between the vestigial fore limbs of Sharovipteryx and hyper elongated fore limbs of MPUM6009.
  5. The pelvis – was deep with ilial processes diverging at right angles in Longisquama and MPU6009

Dorsal plumes are distinct only on Longisquama
Dorsal plumes, which are secondary sexual traits (not gliding tools), along with the other extradermal membranes (originating with Cosesaurus), were but part of the package when wings developed in Longisquama and pterosaurs to enhance whatever mating ritual was going on with leaps, flapping and extravagant showing off. Matrix impressions indicate that smaller dorsal plumes were also present on MPUM 6009 (fig. 1), the basalmost pterosaur and the one with the longest hind limbs and shortest forelimbs.

Missing from this discussion is Cosesaurus
…which nests as a common ancestor for Sharovipteryx, Longisquama and pterosaurs in that phylogenetic order. The skull, neck and torso proportions of Cosesaurus are closer to pterosaurs than to either Longisquama or Sharovipteryx that diverged along their own morphological directions.

Current interpretation of Cosesaurus.

Figure 2. Current interpretation of Cosesaurus. Note the proportions of the skull, neck and torso are closer to pterosaurs than to either Sharovipteryx or Longisquama.

Inherited traits
Given that Sharovipteryx, Longisquama and pterosaurs (Fig. 2) shared a common ancestor, like Cosesaurus (sharing a quadrant-shaped coracoid, pteroid, sternal complex, antorbital fenestra (without fossa), prepubis, anteriorly extended ilium, 4 or more sacrals, an attenuated tail, elongated p5.1, etc. etc,)… we cam expect than Longisquama shared these traits by inheritance, giving further clues to the back half of Longisquama.

So, even if the back half of Longisquama were NOT present on the slab, the reconstruction  would look much the same, but with a smaller foot and shorter torso than presented here (fig. 1) based on tracing these elements. The manual fingers were also larger on Longisquama. So, phylogenetic bracketing validates the DGS tracings, but the tracings also revealed certain traits not revealed by phylogenetic bracketing. So, there was no “cheating” here, deciding what to trace prior to tracing it on the specimen.

Tomorrow I’ll show you a close-up of four Longisquama metatarsals that were mistaken for plume bases, which they align with.

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.

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