​The anurans (order Anura, class Amphibia) comprise the frogs and toads. The order comprises more than 7,000 species distributed across more than 50 families.

The crown-Anura are characterized by the following synapomorphies (Merck, 2021):

1. Frontal and parietal bones fused into paired frontoparietal bones
2. Extremely large otic notch (behind the eye) accommodates tympanum
3. Jaw muscles reorganized to accommodate large middle-ear cavity
4. Extremely wide interpterygoid vacuities (openings in the palate)
5. Lower jaw without teeth
6. Hyoid skeleton (at the root of the tongue) specialized to allow prey capture by protrusion of the tongue.
7. No more than eight presacral vertebrae (headward of the sacrum, or base of the spine)
8. Caudal vertebrae (tailward of the sacrum) ossified as a urostyle - a single long rod
9. The ilium (part of the sacrum) very long, articulating with the sacral rib anteriorly and the femur posteriorly
10. Tibia and fibula (lower bones of hind legs) fused into a single unit
11. Proximal tarsal bones (i.e. ankle and heel) elongate
12. Scales completely lost, facilitating cutaneous breathing.

(Note that Synapomorphies 1 through 6 pertain to the cranial skeleton   (the head), while those numbered 7 through 12 describe the post-cranial part of the body.) Many of these evolutionary novelties, which developed along the anuran stem line, should be clearly visible in well-preserved specimens.

Only a few stem-group anurans have been recognized, and few phylogenetic studies have been performed in recent years. The analysis by Chen et al (2016), used in the time tree below, appears not to have been superseded.

Figure 1.  ​Phylogenetic time tree of the  stem-Anura

​There are two possible candidates for the oldest-known fossil of the anuran stem group: 

1. Triadobatrachus massinoti, found in the Early Triassic (Late Induan - Early Olenekian) Sakamena Group of the Ambilobe district of the Diana Region, northwest Madagascar (Rage and Rocek, 1989; Sigurdsen et al, 2012);
2. Czatkobatrachus polonicus, from an Early Triassic (Olenekian) fissure-fill in Carboniferous limestone in the Czatkowice 1 quarry, Malopolskie, Poland (Borsuk-Bialynicka and Evans, 2002).

Triadobatrachus massinoti is illustrated below, together with other stem-group fossils for which images are available in the public domain (click on image for larger version):

​Figure 2. Images of stem-group  anurans

The above images are placed in order of their progression from most basal to those closest to the crown group. They all look very similar, apart from a possible trend of increasing protuberance of the eyes.
​
The earliest-known fossil representative of the crown group anurans is Eodiscoglossus oxoniensis, a member of the Alytidae family found in the Middle Jurassic (Middle-Late Bathonian) Forest Marble Formation at Old Cement Works Quarry, near Kirtlington, Oxfordshire, England (Evans et al, 1990; Benton et al, 2015). No images of this fossil are available in the public domain, but a reconstruction of another, younger, Eodiscoglossus species is shown below:

Figure 3.  Images of crown-group frog  Eodiscoglossus santonjae

​The above time tree (Figure 1) indicates that the anuran stem group developed from Early Triassic to Middle Jurassic time, representing a stem-to-crown transition of at least 82 million years.

References

Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). Constraints on the timescale of animal evolutionary history. Palaeontologia Electronica, 18(1), 1-106.

Borsuk-Bialynicka, M., & Evans, S. E. (2002). The scapulocoracoid of an Early Triassic stem-frog from Poland. Acta Palaeontologica Polonica, 47(1).

Chen, J., Bever, G. S., Yi, H. Y., & Norell, M. A. (2016). A burrowing frog from the late Paleocene of Mongolia uncovers a deep history of spadefoot toads (Pelobatoidea) in East Asia. Nature Scientific Reports, 6(1), 1-7.

Evans, S. E., Milner, A. R., & Mussett, F. (1990). A discoglossid frog from the Middle Jurassic of England. Palaeontology, 33(2), 299-311.

Merck, J. (2021). Tetrapoda. Course notes for GEOL 431 Vertebrate Paleobiology, Spring Semester 2021, University of Maryland.

Rage, J. C., & Rocek, Z. (1989). Redescription of Triadobatrachus massinoti (Piveteau, 1936) an anuran amphibian from the early Triassic. Palaeontographica A, 206(1-3), 1-16.

Sigurdsen, T., Green, D. M., & Bishop, P. J. (2012). Did Triadobatrachus jump? Morphology and evolution of the anuran forelimb in relation to locomotion in early salientians. Fieldiana Life and Earth Sciences, 77-89.

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Image credits – Stem-Anura

• Header (Green and Black Dart-poison Frog, Dendrobates auratus):  Geoff Gallice from Gainesville, FL, USA, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
• Figure 2 (Triadobatrachus massinoti, fossil):  Eduardo Ascarrunz; Jean-Claude Rage; Pierre Legreneur; Michel Laurin:, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
• Figure 2 (Triadobatrachus massinoti, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
• Figure 2 (Prosalirus bitis):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
• Figure 2 (Vieraella herbstii):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
• Figure 2 (Notobatrachus degiustoi):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
• Figure 3 (Eodiscoglossus santonjae, fossil):  Báez, Ana Maria & Gómez, Raúl O., CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
• Figure 3 (Eodiscoglossus santonjae, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license