The squamates (Subclass Squamata in Class Reptilia) comprise the lizards and snakes. The clade comprises six orders, as illustrated in the phylogenetic tree below:
Figure 1. Phylogenetic tree of living squamates
According to Benton (2015), the following synapomorphies characterize the crown-Squamata:
As is the case for the rhynchocephalians, much research is currently underway on the squamate stem line, and there are significant differences between the recently published phylogenetic trees (compare Simões et al, 2018; Sobral et al, 2020; Bittencourt et al, 2020; Simões et al, 2020; Martínez et al, 2021; Simões et al, 2021; Griffiths et al, 2021; Simões et al, 2022; and Bolet et al, 2022). Some studies (e.g. Simões et al, 2018) place the Middle Triassic Megachirella wachtleri in the stem-Squamata, but the only species that appears in all trees as a stem squamate is Huehuecuetzpalli mixtecus, described from the Early Cretaceous (Albian) Middle Member of the Tlayúa Formation in the Tlayúa Quarry near Tepexi de Rodríguez, Puebla, Mexico (Reynoso, 1998). No useful images are available in the public domain.
The earliest known crown-group squamate is Ardeosaurus brevipes, described from the Late Jurassic (Late Kimmeridgian) Plattenkalk limestone at Eichstätt in southern Germany (Mateer, 1982; Talanda, 2018). A fossil and a life restoration are shown below:
- High degree of skull kinesis (significant movement of skull bones relative to each other)
- Premaxillae (small cranial bones at the very tip of the upper jaw) fused
- Nasal bones reduced
- Squamosal bone (in the skull) reduced or absent
- Parietals (bones that form the rear edge of the skull roof) fused
- Quadratojugal (small bone on each side of the skull) absent
- Posterior conch-like notch on quadrate (bone to which the lower jaw is articulated)
- Proatlas (rudimentary vertebra in some reptiles) absent
- Ribs single-headed
- Gastralia (dermal bones found in the ventral body wall of many fossil tetrapods) absent.
As is the case for the rhynchocephalians, much research is currently underway on the squamate stem line, and there are significant differences between the recently published phylogenetic trees (compare Simões et al, 2018; Sobral et al, 2020; Bittencourt et al, 2020; Simões et al, 2020; Martínez et al, 2021; Simões et al, 2021; Griffiths et al, 2021; Simões et al, 2022; and Bolet et al, 2022). Some studies (e.g. Simões et al, 2018) place the Middle Triassic Megachirella wachtleri in the stem-Squamata, but the only species that appears in all trees as a stem squamate is Huehuecuetzpalli mixtecus, described from the Early Cretaceous (Albian) Middle Member of the Tlayúa Formation in the Tlayúa Quarry near Tepexi de Rodríguez, Puebla, Mexico (Reynoso, 1998). No useful images are available in the public domain.
The earliest known crown-group squamate is Ardeosaurus brevipes, described from the Late Jurassic (Late Kimmeridgian) Plattenkalk limestone at Eichstätt in southern Germany (Mateer, 1982; Talanda, 2018). A fossil and a life restoration are shown below:
Figure 2. Images of oldest crown squamate
No conclusions can be drawn about the duration of the squamate stem-to-crown transition as the only known member of the stem group is younger than the oldest known crown-group squamate.
References
Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.
Bittencourt, J. S., Simões, T. R., Caldwell, M. W., & Langer, M. C. (2020). Discovery of the oldest South American fossil lizard illustrates the cosmopolitanism of early South American squamates. Nature communications biology, 3(1), 1-11.
Bolet, A., Stubbs, T. L., Herrera-Flores, J. A., & Benton, M. J. (2022). The Jurassic rise of squamates as supported by lepidosaur disparity and evolutionary rates. Elife, 11.
Griffiths, E. F., Ford, D. P., Benson, R. B., & Evans, S. E. (2021). New information on the Jurassic lepidosauromorph Marmoretta oxoniensis. Papers in Palaeontology, 7(4), 2255-2278.
Martínez, R. N., Simões, T. R., Sobral, G., & Apesteguía, S. (2021). A Triassic stem lepidosaur illuminates the origin of lizard-like reptiles. Nature, 597(7875), 235-238.
Mateer, N.J. (1982). Osteology of the Jurassic lizard Ardeosaurus brevipes (Meyer). Palaeontology, 25(3), 461-469.
Simões, T. R., Caldwell, M. W., Tałanda, M., Bernardi, M., Palci, A., Vernygora, O., ... & Nydam, R. L. (2018). The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps. Nature, 557(7707), 706-709.
Simões, T. R., Vernygora, O., Caldwell, M. W., & Pierce, S. E. (2020). Megaevolutionary dynamics and the timing of evolutionary innovation in reptiles. Nature Communications, 11(1), 1-14.
Simões, T. R., & Pyron, R. A. (2021). The squamate tree of life. Bulletin of the Museum of Comparative Zoology, 163(2), 47-95.
Simões, T. R., Kinney-Broderick, G., & Pierce, S. E. (2022). An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny. Nature communications biology, 5(1), 1-19.
Sobral, G., Simões, T. R., & Schoch, R. R. (2020). A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications for the early evolution of lepidosauromorphs and the Vellberg fauna. Nature scientific reports, 10(1), 1-9.
Tałanda, M. (2018). An exceptionally preserved Jurassic skink suggests lizard diversification preceded fragmentation of Pangaea. Palaeontology, 61(5), 659-677.
Watanabe, A., Fabre, A. C., Felice, R. N., Maisano, J. A., Müller, J., Herrel, A., & Goswami, A. (2019). Ecomorphological diversification in squamates from conserved pattern of cranial integration. Proceedings of the National Academy of Sciences, 116(29), 14688-14697.
Bittencourt, J. S., Simões, T. R., Caldwell, M. W., & Langer, M. C. (2020). Discovery of the oldest South American fossil lizard illustrates the cosmopolitanism of early South American squamates. Nature communications biology, 3(1), 1-11.
Bolet, A., Stubbs, T. L., Herrera-Flores, J. A., & Benton, M. J. (2022). The Jurassic rise of squamates as supported by lepidosaur disparity and evolutionary rates. Elife, 11.
Griffiths, E. F., Ford, D. P., Benson, R. B., & Evans, S. E. (2021). New information on the Jurassic lepidosauromorph Marmoretta oxoniensis. Papers in Palaeontology, 7(4), 2255-2278.
Martínez, R. N., Simões, T. R., Sobral, G., & Apesteguía, S. (2021). A Triassic stem lepidosaur illuminates the origin of lizard-like reptiles. Nature, 597(7875), 235-238.
Mateer, N.J. (1982). Osteology of the Jurassic lizard Ardeosaurus brevipes (Meyer). Palaeontology, 25(3), 461-469.
Simões, T. R., Caldwell, M. W., Tałanda, M., Bernardi, M., Palci, A., Vernygora, O., ... & Nydam, R. L. (2018). The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps. Nature, 557(7707), 706-709.
Simões, T. R., Vernygora, O., Caldwell, M. W., & Pierce, S. E. (2020). Megaevolutionary dynamics and the timing of evolutionary innovation in reptiles. Nature Communications, 11(1), 1-14.
Simões, T. R., & Pyron, R. A. (2021). The squamate tree of life. Bulletin of the Museum of Comparative Zoology, 163(2), 47-95.
Simões, T. R., Kinney-Broderick, G., & Pierce, S. E. (2022). An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny. Nature communications biology, 5(1), 1-19.
Sobral, G., Simões, T. R., & Schoch, R. R. (2020). A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications for the early evolution of lepidosauromorphs and the Vellberg fauna. Nature scientific reports, 10(1), 1-9.
Tałanda, M. (2018). An exceptionally preserved Jurassic skink suggests lizard diversification preceded fragmentation of Pangaea. Palaeontology, 61(5), 659-677.
Watanabe, A., Fabre, A. C., Felice, R. N., Maisano, J. A., Müller, J., Herrel, A., & Goswami, A. (2019). Ecomorphological diversification in squamates from conserved pattern of cranial integration. Proceedings of the National Academy of Sciences, 116(29), 14688-14697.
Image credits - Stem-Squamata
- Header (Aquatic Coral Snake, Micrurus surinamensis): Bernard DUPONT from FRANCE, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
- Figure 2 (Ardeosaurus brevipes, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Ardeosaurus brevipes, life restoration): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons