What exactly IS a pterosaur? – part 2 of 3

In part 1 we looked at two competing hypotheses of pterosaur origins: the more popular and traditional archosaur hypothesis and the typically avoided and heretical fenestrasaur hypothesis.

In part 2 we take a closer look at the character traits used by Senter (2003) and Nesbitt (2011) to connect pterosaurs to archosaurs. Reconstructions of the featured taxa can be seen by clicking on these links: Scleromochlus, Eudimorphdon, Austriadactylus, Cosesaurus, Sharovipteryx and Longisquama.

In part 3 we will review a series of taxa that actually document a gradual increase in pterosaurian traits: the basal squamates, MeyasaurusLacertulus, Huehuecuetzpalli and the fenestrasaurs, Cosesaurus, Sharovipteryx and Longisquama.

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Senter (2003) employed the Triassic pterosaurs Eudimorphodon and Austriadactylus and found the following 21 synapomorphies for the Pterosauromorpha (Scleromochlus + pterosaurs):

1. Angle between scapular blade and acromion process > 160 degrees.
True for Scleromochlus.
True for Pterosaurs.
True for Cosesaurus through Longisquama. BTW, these taxa all have a strap-like scapula.

2. Sharp tapering of snout to an anterior point in lateral view.
True for Sclermomochlus, but only in dorsal view.
True for pterosaurs.
True for Cosesaurus through Longisquama.

3. Maxilla contacts external naris.
False for Scleromochlus, as in sister taxa Gracilisuchus and Terrestrisuchus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

4. Anterior extremity of maxilla anterior to external naris.
False for Scleromochlus as in sister taxa Gracilisuchus and Terrestrisuchus.
True for pterosaurs.
True for Cosesaurus through Sharovipteryx (unknown in Longisquama).

5. Antorbital fossa absent anterior to internal antorbital fenestra.
False for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

6. Jugal extends anterior to contact with lacrimal.
True for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

7. Ventral margin of squamosal (anteriorly) below mid-height of orbit.
False for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

8. Interdental plates absent.
This has not yet been determined for these crushed taxa.

9. Number of sacral vertebrae increased to four.
True for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

10. Anterior hemal arches < 1.5x length of associated vertebrae.
True for Scleromochlus for first arch, but subsequent arches are > 2x. Hemal arches are oriented ventrally.
True for Austriadactylus. Unknown in Eudimorphodon.
True for Cosesaurus through Longisquama. In fenestrasaurs, including pterosaurs, hemal arches are oriented posteriorly.

11. Anteriormost T-shaped hemal arch proximal to ninth caudal vertebra.
No hemal arches are T-shaped in these taxa.

12. Sternum > 2x length of sternal end of coracoid.
False for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

13. Metacarpal 1 > 0.6x length of metacarpal 2.
True for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

14. Metacarpal IV subequal in length to or longer than metacarpal III.
False for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

15. Manual phalanx 2.2 > 1.3x length of phalanx 2.1.
False for Scleromochlus.
True for pterosaurs, but only for Eudimorphodon and Austriadactylus among basal pterosaurs.
False for Cosesaurus through Longisquama.

16. Manual digit V demonstrably absent, including metacarpal.
Probably false for Scleromochlus. Digits 4 and 5 are not visible, but they would be extremely tiny. In the sister taxon, Terrestrisuchus digit 5 is present.
False for pterosaurs. Widely considered absent in pterosaurs, manual digit 5 is actually present as a vestige on all pterosaurs.
False for Cosesaurus through Longisquama.

17. Dorsoventrally shallow extension of iliac blade anterior to pubic peduncle.
False for Scleromochlus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

18. Fibular midshaft diameter < 0.5x tibial midshaft diameter.
True for Scleromochlus. But also true for sister taxa, Gracilisuchus and Terrestrisuchus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

19. Pedal phalanx p5.1 elongated.
False for Scleromochlus. Also absent in sister taxa, Gracilisuchus and Terrestrisuchus.
True for pterosaurs.
True for Cosesaurus through Longisquama.

20. Height of maxilla ventral to internal antorbital fenestra, < 1/3 total maxillary height.
True for Scleromochlus.
False for Eudimorphodon. True for Austriadactylus.
False for Cosesaurus and Longisquama.

21. Mandibular fenestra absent.
False for Scleromochlus and sister taxa including Terrestrisuchus and Gracilisuchus.
True for pterosaurs. Nesbitt (2011) reported a mandibular fenestra in a specimen of Dimorphodon, but that is a misidentification detailed here.
True for Cosesaurus and Longisquama.

In summary, according to these characters, pterosaurs were closer to Cosesaurus, Sharovipteryx and Longisquama when traits are properly scored.

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Nesbitt (2011) listed the following characters to support a relationship of pterosaurs with dinosaurs. My comments follow in blue.

Archosauria/Avesuchia – crocodilians, pterosaurs and dinosaurs – SUPPORT:

(1) Palatal processes of the maxilla meet at the midline. Not in basal pterosaurs.

(2) Lagenar/cochlea recess present and elongated and tubular. Not in pterosaurs.

(3) External foramen for abducens nerves within prootic only. I can’t comment on this.

(4) Antorbital fossa present on the lacrimal, dorsal process of the maxilla and the dorsolateral margin of the posterior process of the maxilla (the ventral border of the antorbital fenestra). Not in pterosaurs.

(5) Posteroventral portion of the coracoid possesses a ‘‘swollen’’ tuber. Not in pterosaurs. The stem of the coracoid is the remainder after expanded fenestration.

(6) Lateral tuber (= radius tuber) on the proximal portion of the ulna present. Not in pterosaurs.

(7) Longest metacarpal: Longest metatarsal < 0.5. Not in pterosaurs.

(8) Anteromedial tuber of the proximal portion of the femur present. Not in pterosaurs.

(9) Tibial facet of the astragalus divided into posteromedial and anterolateral basins. Not in pterosaurs.

(10) Calcaneal tuber orientation, relative to the transverse plane, between 50 and 90 degrees posteriorly. Not in pterosaurs.

Ornithodira/Avemetatarsalia – pterosaurs and dinosaurs – SUPPORT:

(1) Distal end of neural spines of the cervical vertebrae unexpanded. Also in Huehuecuetzpalli and fenestrasaurs.

(2) Distal expansion neural spines of the dorsal vertebrae absent. Also in Huehuecuetzpalli and fenestrasaurs.

(3) Second phalanx (2.2) of manual digit II longer than first phalanx. Also in Huehuecuetzpalli, but not in Cosesaurus through Longisquama.

(4) Trenchant unguals on manual digits I–III. Also in Huehuecuetzpalli, Sharovipteryx and Longisquama.

(5) Tibia longer than the femur. Also in Sharovipteryx and Longisquama, but not in Herrerasaurus or Saturnalia.

(6) Distal tarsal 4 transverse width subequal to distal tarsal 3. In pterosaurs the distal tarsal that is subequal to distal tarsal 4 is the centrale, a trait shared with fenestrasaurs. Whenever present in pterosaurs, distal tarsal 3 is tiny.

(7) Size of articular facet for metatarsal V less than half of lateral surface of distal tarsal 4. In pterosaurs and fenestrasaurs the articulation is always more than half.

(8) Anterior hollow of the astragalus reduced to a foramen or absent. Also in Huehuecuetzpalli and fenestrasaurs.

(9) Anteromedial corner of the astragalus acute. Also in Huehuecuetzpalli and fenestrasaurs.

(10) Compact metatarsus, metatarsals II–IV tightly bunched (at least half of the length). Also in Huehuecuetzpalli and fenestrasaurs except Sharovipteryx.

(11) Osteoderms absent. Also in Huehuecuetzpalli and fenestrasaurs.

(12) Gastralia well separated. Also in Huehuecuetzpalli and fenestrasaurs.

Pterosauromorpha – closer to pterosaurs than to dinosaurs – SUPPORT:

(1) Anterodorsal process (= nasal process) of the premaxilla greater than the anteroposterior length of the premaxilla. Also in Huehuecuetzpalli and fenestrasaurs.

(2) Anterodorsal margin of the maxilla borders the external naris. Also in Huehuecuetzpalli and fenestrasaurs.

(3) Concave anterodorsal margin at the base of the dorsal process of the maxilla. Also in Huehuecuetzpalli and fenestrasaurs.

(4) Skull length more than 50% of length of the presacral vertebral column. Also in Huehuecuetzpalli, but not fenestrasaurs.

(5) Dentition markedly heterodont. Also in Sharovipteryx and Longisquama.

(6) Cervical centra 3–5 longer than a middorsal vertebra. Also in fenestrasaurs, except Longisquama.

(7) Distal caudal vertebrae prezygapophyses elongated more than a quarter of the adjacent centrum. Not true in the basalmost pterosaur MPUM 6009, but certainly true in many other pterosaurs.

(8) Postglenoid process of the coracoid elongate and expanded posteriorly. Also in fenestrasaurs.

(9) Pteroid bone present. Also in fenestrasaurs.

(10) Manual digit IV length more than or equal to 50% of total forelimb length. Also in Sharovipteryx and Longisquama.

(11) Anterior (= preacetabular, = cranial) process of the ilium long and extends anterior to the acetabulum but shorter than the posterior process of the ilium. Not present in any pterosaur. The anterior process is always longer.

(12) Metatarsal I length 85% or more [typo: of metarsal IV?]. Also in fenestrasaurs.

(13) Metatarsal V dorsal prominence separated from the proximal surface by a concave gap. Also in fenestrasaurs.

The big problem with Nesbitt (2011) was the refusal to test fenestrasaurs with pterosaurs after acknowledging the literature (Peters 2000). A more comprehensive analysis of the Reptilia recovers fenestrasaurs, including pterosaurs, in a new third clade of lizards, the Tritosauria, now all extinct. When pterosaurs are recognized as lizards and the new reptile tree is widely accepted, definitions for the Ornithodira, Pterosauromorpha, Avesuchia, Panaves and Avemetatarsalia will become redundant with the Reptilia.

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Overlooked by all prior workers, the lizard Huehuecuetzpalli and related fenestrasaurs had an unfused mesotarsal tarsus without a calcaneal heel, convergent with dinosaurs and homologous with pterosaurs. Fenestrasaurs had an antorbital fenestra. These are the two basic characters that have traditionally nested pterosaurs with archosaurs. All archosaurs have a reduced manual digit 4 and pedal digit 5, but Huehuecuetzpalli and fenestrasaurs did not. The origin of the pterosaur wing is documented here and will be discussed in future blogs.

References:
Nesbitt SJ 2011. 
The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Senter P 2003. Taxon Sampling Artifacts and the Phylogenetic Position of Aves. unpublished Ph. D. dissertation. Northern Illinois University, 1-279.

 

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