Consider the following characteristic of an amphibian :
(A) Reduced lungs
(B) Copulatory organ present and fertilization is internal
(C) Tail muscles are absent
(D) Vertebrae are amphicoelous and ribs are free The name of this amphibian is:

Question

Consider the following characteristic of an amphibian : 
(A) Reduced lungs 
(B) Copulatory organ present and fertilization is internal 
(C) Tail muscles are absent 
(D) Vertebrae are amphicoelous and ribs are free The name of this amphibian is:

Accepted Answer

The correct option is 2: Ascaphus

The correct answer is Ascaphus, commonly known as the Tailed Frog, because it is the only genus among the options that satisfies the specific and highly primitive combination of all four characteristics listed. The most distinctive feature is (B), the presence of a copulatory organ and internal fertilization, which is exceptionally rare in the order Anura (frogs and toads). Ascaphus possesses an intromittent organ, which is a cloacal extension often referred to as a "tail," to transfer sperm. This adaptation is crucial because Ascaphus lives in fast-flowing, cold mountain streams where external fertilization would be washed away. Furthermore, the genus represents the most basal lineage of extant frogs, supported by characteristic (D): having amphicoelous vertebrae (concave at both ends) and free ribs, both of which are considered primitive skeletal traits in amphibians and are lost or modified in nearly all other modern anurans. Condition (A), reduced lungs, is an expected adaptation for this cold-water aquatic frog, allowing for greater reliance on cutaneous (skin) respiration. Finally, as an adult frog, it is tailless (Anura), making the tail *muscles* absent (C), even though the cloacal extension is present.

The incorrect option is 1: Xenopus

Xenopus, the African Clawed Frog, is a highly aquatic anuran, but it exhibits classic anuran reproductive behavior, which includes external fertilization. Unlike Ascaphus, Xenopus adults do not possess a copulatory organ or any form of "tail" extension for internal sperm transfer. Additionally, the vertebral structure of Xenopus is typically opisthocoelous (concave posteriorly), not amphicoelous, which contradicts the specified primitive trait (D). While Xenopus also has reduced lungs (A), and its adult form lacks a true tail (C), the key contradiction lies in its mode of reproduction and vertebral morphology.

The incorrect option is 3: Alytes

Alytes, the Midwife Toads, are famous for their unique parental care where the male carries the string of eggs. However, like the vast majority of frogs, Alytes employs external fertilization, where the male releases sperm onto the eggs outside of the female's body; thus, a copulatory organ (B) is absent. Alytes is considered a more derived frog, and its vertebrae are procoelous (concave anteriorly), which is the most common vertebral type in modern anurans and directly contradicts the requirement for amphicoelous vertebrae (D). Therefore, this genus only shares the general amphibian traits of having reduced lungs and being tailless as an adult.

The incorrect option is 4: Rhacophorus

Rhacophorus, commonly known as Flying Frogs due to their webbed feet used for gliding, are a highly derived and specialized group of anurans. Their reproductive strategy involves building foam nests, a form of external fertilization, which means they do not possess the required copulatory organ for internal fertilization (B). Furthermore, Rhacophorus species have procoelous vertebrae, which is an advanced feature and therefore not consistent with the primitive amphicoelous type specified in characteristic (D). The characteristics that define this unique genus are not aligned with the key primitive and reproductive traits listed in the question.