A pre-Cosesaurus: the BES SC111 specimen

Earlier
here and here we looked at the pterosaur traits found in the lepidosaur tritosaur fenestrasaur, Cosesauru aviceps (Fig. 1).

Today
let’s look at Cosesaurus traits found in the more primitive BES SC 111 specimen (Fig. 1) traditionally assigned to the larger set of specimens traditionally attributed to Macrocnemus (Fig. 2).

Traditional paleontologists
consider the BES SC111 specimen a juvenile based on its size and short rostrum relative to other Macrocnemus specimens. The large reptile tree (LRT, 1412 taxa) nests the BES SC111 specimen apart from Macrocnemus, basal to Langobardisaurus + Fenestrasauria. Since tritosaurs mature isometrically, juvenile Macrocnemus specimens should have adult proportions, but none are known at present.

Phylogenetic miniaturization
produce smaller tritosaur specimens with a shorter rostrum via neotony. Rather than juvenile traits, late stage embryo (= pre-hatchling) traits are retained into adulthood. Phylogenetic bracketing indicates the BES SC111 specimen was close to adult size.

Figure 1. The BES SC111 specimen attributed to Macrocnemus compared to Cosesaurus, the taxon transitional to pterosaurs. See text for detais.

Figure 1. The BES SC111 specimen attributed to Macrocnemus compared to Cosesaurus, the taxon transitional to pterosaurs. See text for detais.

Traits shared in the BES SC111 specimen and Cosesaurus:

  1. The skulls are virtually identical, including orbit size, antorbital fenestra, tooth size
  2. Torsos quite similar, both with many more gastralia than in ancestors
  3. Tail attenuated
  4. Interclavicle cruciform
  5. Sternum present
  6. Clavicles short, relatively straight and robust
  7. Scapula with longer posterior process (even longer in Cosesaurus)
  8. Metacarpal 4 is the longest, so is manual digit 4
  9. Ilium anterior process present (longer in Cosesaurus)
  10. Prepubis present (larger in Cosesaurus)
  11. Metatarsal 4 is the longest, so is pedal digit 4
  12. Metatarsal 5 is short
  13. Pedal 1.1 is elongate (longer in Cosesaurus)

Derived traits in Cosesaurus relative to BES SC111

  1. Overall smaller in Cosesaurus (neotony)
  2. Epipterygoid absent in Cosesaurus (neotony)
  3. Shorter neck in Cosesaurus (neotony)
  4. 5 sacrals in Cosesaurus (3 in BES SC111)
  5. Sternal complex in Cosesaurus with shifted elements
  6. Coracoid reduced to a curved stem in Cosesaurus (neotony, less ossification)
  7. Hand much larger in Cosesaurus (slightly longer than antebrachium)
  8. Centrale bones migrate to become preaxial carpal and pteroid in Cosesaurus
  9. Thyroid fenestra absent in Cosesaurus
  10. Pedal unguals rounded in BES SC111 
Tanystropheus and kin going back to Huehuecuetzpalli.

Figure 2. Tanystropheus and kin going back to Huehuecuetzpalli. Cosesaurus is not shown here (see figure 1).

Due to convergence,
adding taxa is, perhaps, the only way to split protorosaurs (= prolacertiformes) from tritiosaurs. Make sure you add Huehuecuetzpalli (Fig. 2) to any such analysis.

Figure 3. BES SC111 pectoral region. Colors correspond to figure 1.

Figure 3. BES SC111 pectoral region. Colors correspond to figure 1. The left scapula(?) is incomplete. The interclavicle and sternum are largely hidden beneath the vertebrae. Not sure what that elliptical bone is at upper left and blue. It may be two.

The shifting of pectoral elements
from Huehuecuetzpalli to pterosaurs was detailed earlier here and here.

Figure 4. BES SC111 pelvic region. Colors correspond to those in figure 1. Note the tiny blue prepubes.

Figure 4. BES SC111 pelvic region. Colors correspond to those in figure 1. Note the tiny blue prepubes.

Several indicators of bipedal ability
are present in the BES SC111 specimen, as in the extant Chlamydosaurus kingii.

  1. Elongate ilium anterior process
  2. More than two sacral vertebrae
  3. Prepubes + stiff belly (more gastralia)
  4. Attenuated tail
  5. Elongate cervicals
Figure 6. Green iguana demonstrating the curling of pedal digit 5 in tendril-toed arboreal lepidosaurs, as hypothesized in the BES SC111 specimen and pterosaurs.

Figure 5. Green iguana demonstrating the curling of pedal digit 5 in tendril-toed arboreal lepidosaurs, as hypothesized in the BES SC111 specimen and pterosaurs.

Cosesaurus and Rotodactylus, a perfect match.

Figure 5. Cosesaurus and Rotodactylus, a perfect match. Elevate the proximal phalanges along with the metatarsus, bend back digit 5 and Cosesaurus (left) fits perfectly into Rotodactylus (right).

The curling of pedal digit 5
in the Rotodactylus trackmakers (Fig. 6) is a lepidosaur trait (Fig. 5) carried to extremes in basal pterosaurs, like ‘Sauria aberrante’ and Dimorphodon.


References
Ellenberger P and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier 12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Peabody FE 1948.  Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah.  University of California Publications, Bulletin of the  Department of Geological Sciences 27: 295-468.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Renesto S and Avanzini M 2002. Skin remains in a juvenile Macrocnemus bassanii Nopsca (Reptilia, Prolacertiformes) from the Middle Triassic of Northern Italy. Jahrbuch Geologie und Paläontologie, Abhandlung 224(1):31-48.
Sanz JL and López-Martinez N 1984. The prolacertid lepidosaurian Cosesaurus aviceps Ellenberger & Villalta, a claimed ‘protoavian’ from the Middle Triassic of Spain. Géobios 17: 747-753. 

wiki/Cosesaurus
wiki/Macrocnemus

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