Huehuecuetzpalli mixtecus (Reynoso 1998, Early Cretaceous, Middle to Late Albian, Fig. 1) is very primitive lepidosaur known from two closely associated specimens, one a juvenile, the other an adult. Huehuecuetzpalli has taken center stage here (at PterosaurHeresies) and at ReptileEvolution.com as a basal member of the Tritosauria. This third clade of squamates includes an odd assortment of drepanosaurs, tanystropheids and fenestrasaurs including pterosaurs that have been completely ignored and overlooked in all professional lizard studies.
A recent paper by Gauthier et al. (2012) considered Huehuecuetzpalli among the many lizards in its tree. A few quotes from that paper are worthy of attention. They nested Huehuecuetzpalli alone between sphenodontids, like Gephyrosaurus and Sphenodon, and the Squamata (all other known lizards, divided by the Iguania and Scleroglossa (including snakes)).
Unfortunately Gauthier et al. (2012) did not reference the large reptile tree (or create their own more expansive tree of lizards) so they completely overlooked the tritosaurs as descendants of a sister to Huehuecuetzpalli. It should not have nested all alone. There was an opportunity missed that became yet another case of taxon exclusion. Below I make comments (in hot pink) on the small section of Gauthier (2012) regarding Huehuecuetzpalli.
Annotated Notes from Gauthier et al. (2012):
“Stem squamata – Given the antiquity of the squamate stem—which must extend deep into the Triassic (1)—surprisingly few stem fossils can be referred with any confidence to that great branch of the lepidosaur tree (2). Huehuecuetzpalli mixtecus, from the Early Cretaceous of Mexico, seems to be one of these (Reynoso 1998). This species is reasonably well known by the standards of Mesozoic lizard paleontology, as it is represented by two fairly complete skeletons, with some patches of skin impressions, of juvenile and nearly adult individuals. H. mixtecus apparently represents an entirely extinct side branch off the squamate stem (3). All major living clades of lizards— Iguania, Gekkota, Scincomorpha and Anguimorpha—diverged by the Late Jurassic (Estes 1983; Conrad 2008 (4)). Albian-age H. mixtecus must therefore have been separated from the surviving branch of the lizard tree by anywhere from 25 to 50 million years (5). Unsurprisingly, it displays several distinctive autapomorphies (see Appendix 4).
“Huehuecuetzpalli mixtecus is joined to the lizard crown by 20 unambiguous squamate synapomorphies (100% BP, 100% PP, 16 BS; see Appendix 4). Three of those are unique and unreversed on our tree: 177(1), 181(1) and 295(1). Among these diagnostic characters are many of those involved in the kinetic masticatory system unique to lizards (6). H. mixtecus is, however, also quite primitive in many ways; for example, skin impressions indicate that it retained a mid-dorsal row of spiny scales (7), a feature diagnostic of lepidosaurs that is retained today only among iguanian lizards and Sphenodon punctatus (scleroglossans generally lack the mid-dorsal scale row originally present in Reptilia; Gauthier, Kluge and Rowe 1988). The upper temporal arch of H. mixtecus displays a mixture of ancestral and derived traits; the postorbital, for example, still fits into a V-shaped recess on the lateral face of the squamosal as in diapsids ancestrally (8); Its squamosal is nevertheless distinctly lizard-like in having a peg at its posterior tip, on which pivots the mobile (streptostylic) quadrate uniquely diagnostic of crown lizards (Robinson 1967).(9)
“Crown Squamata – Huehuecuetzpalli mixtecus shares a few apomorphies characteristic of (at least some) iguanians, such as fused hourglass-shaped frontal bones and a small subtriangular postfrontal bone confined to the orbital rim (Reynoso 1998) (10). Nonetheless, it seems to lie well outside the lizard crown, because it lacks—so far as it is preserved—the 13 unambiguous synapomorphies that diagnose Squamata. … In any case, this morphological “long branch” simultaneously underscores our confidence in squamate monophyly while highlighting just how little we know about their evolutionary origins.(11)“
Notes: (1) Actually the antiquity of the non-sphenodontid lepidosaurs extends back at least to Lacertulus of the Early Permian with more primitive taxa, like Homoeosaurus, and Dalinghosaurus (both ignored by Gauthier et al. 2012) surviving into later ages (Late Jurassic and Early Cretaceous respectively).”
(2) Listed above and look for others here.
(4) We can be confident of a much earlier Late Permian date for the tritosaur/squamate split due to the preponderance of fenestrasaur tracks in the Early Triassic (Peabody 1948).
(5) Add at least the entire Triassic to this number.
(6) This is reversed in some, but not all tritosaurs, principally by the shortening of the lateral temporal fenestra and the redevelopment of a quadratojugal with a loose connection to the quadrate.
(7) This dorsal series finds the acme of its expression in Longisquama.
(8) The Diapsida that Gauthier et al. 2012 is thinking of is diphyletic.
(9) See (6).
(10) As in tanystropheids and fenestrasaurs including pterosaurs.
(11) With the large reptile tree we actually know a very good set of sample ancestral taxa back to Ichthyostega (and we know its ancestors, so…, the list really extends as far back as you care to look.)
Besides the Tritosauria, Gauthier et al. (2012) excluded several fossil lepidosaurs that were key to understanding relationships in the large reptile tree. Without these their tree suffers by comparison despite its size. Taxon exclusion needs to become a thing of the past. Professional studies have suffered long enough.
Gauthier JA, Kearney M, Maisano JA, Rieppel O and Behlke ADB 2012. Assembling the Squamate Tree of Life: Perspectives from the Phenotype and the Fossil Record. Bulletin of the Peabody Museum of Natural History 53(1):3–308.
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 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.