Anura: Characteristics, Order & Classification of Frogs (Explained)

Anura is a scientific animal Order that contains two main types of amphibians: frogs, and toads.

Despite the existence of several other amphibious species, these two species share some specific characteristics unique to Anura as an Order and warrant them being separated from the other two amphibian Orders, Caudata and Gymnophiona.

All frogs and toads are classified in the Anura Order, one of the three Orders that categorize amphibious species. General Anura characteristics include a squat, tailless adult body with long hind limbs, large eyes, and an external tympanum.

Each animal classification level has characteristics that warrant placement in that particular grouping, and Anura is no different. Not only are we going to discuss the unique characteristics of this Order, but we will also detail their evolutionary purposes and compare them to the two other Orders under the Amphibia class.

Anura is A Scientific Animal Order Containing Frogs

Anura is one of the three Orders classified under the Class Amphibia containing over 7,500 frog species. This number increases every month as new discoveries under this order are made.

Every living creature currently known to humankind is categorized under a system of animal classification where they are total of seven levels, including:

Kingdom
Phylum
Class
Order
Family
Genus
Species

People are most familiar with the final level of classification, “species,” where a living being is provided its scientific and common name. For example, a Canis lupus familiaris is a domesticated dog. As we stated previously, there are a total of three Orders under the Class Amphibia, which include:

Anura (Salientia): Comprised of frog and toad species
Caudata (Urodela): Comprised of salamander, newt, and siren species
Gymnophiona (Apoda): Comprised of caecilian species

Therefore, throughout this article, we will pay particular attention to the characteristics of frogs and toads in accordance with their Anura Order and describe what characteristics generally grant them being classified separately from the Caudata and Gymnophiona Orders (CTNF).

What Are the Core Characteristics of Anura?

Several characteristics set Anura apart from other species and amphibians, such as their squat, tailless body and long hind legs. Anura also have larger, more developed eyes and an external tympanum that is not present in the two other Orders.

As an Order listed underneath the Amphibia class, there are several characteristics Anura shares with the other two Orders, such as their smooth, slimy skin they can breathe through, they’re cold-blooded, and have complex life cycles, to name a few.

However, we’d like to delve more deeply into what makes Anura unique characteristically as an Order and how we can see this clearly in nearly every frog and toad species currently known to science. To better understand each characteristic and its evolutionary purpose, let’s discuss them separately below.

Anura Have A Tailless Adult Body

Despite the fact that both toads and frogs have tails while they are in their tadpole forms, their tails absorb into their bodies at the froglet and toadlet stage, and disappear by the time they are adults. Therefore, adult frogs and toads spend the majority of their lives without tails.

This is such a unique characteristic under the Amphibia Class that it is the inspiration for the Anura name. Anura is a Greek word that roughly translates to “without a tail” because frogs and toads took a distinctive evolutionary turn away from other amphibians when tails were no longer beneficial to their survival.

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Considering Anura are vertebrates, meaning they have skeletal systems and backbones, it is particularly odd that they do not have tails. However, science believes their absorption of their tail as adults comes down to the way frogs and toads move on land and the importance of their life saving and sustaining ability to jump.

Only two Anura species are an exception to this characteristic: the coastal tailed frog (Ascaphus truei Stejneger) and the Rocky Mountain tailed frog (Ascaphus montanus). However, an argument could be made that this feature isn’t a tail since it is only present in males and is part of the cloaca to insert sperm while mating to improve reproductive productivity.

Apart from this rare exception, no Anura have tails once they reach adulthood, a characteristic not found in the other two Orders.

Anura Have a Squat Posture and Long Hindlimbs

Another distinctive characteristic of the Anura is their skeletons, which result in a hunched, squat posture and their exceptionally powerful and muscular hindlimbs. This is especially true for aquatic frogs that have the longest most powerful legs of all types of frogs.

Despite the fact that all Amphibia are vertebrates, Anura is the only ones that boast this broad stature since all Caudata and Gymnophiona either slither or crawl directly on the ground or, in the case of salamanders and newts, which are part of the Caudata Order, walk much lower to the ground than most Anura.

The purposes behind these unique characteristics are simple: they promote the Anura’s uncommon mode of transportation, which is jumping or hopping. Aquatic frogs and tree frogs are the best at jumping. Although toads are not as good at jumping high, they can hop very fast.

There are no Amphibia with legs as long as the Anura, and this is because they have more complex pectoral and pelvic girdles designed to absorb the shock of landing away from its weaker forelimbs as well as having the ability to put immense amounts of power behind each jump, resulting in impressive distances.

In fact, many frogs can jump up to 20 times their body length, which is about the human equivalent of jumping 100 feet. Not only is this vastly impressive coming from relatively small animals, but it is something only the Anura amphibians can do.

Of course, the benefits of these extraordinary legs and skeletal formation aren’t exclusive to jumping. They also help Anura climb trees and other obstacles, resulting in countless arboreal species with long limbs and digits built with adhesive pads for exceptional grip.

Toads, which are also classified under Anura, have limbs that are slightly different to aquatic or tree frogs which are made for burrowing. Toads mostly live on land and burrowing allows them to hide from predators and hibernate safely.

Anura Have Well-Developed Bulging Eyes

Anura have excellent closeup 360° vision and unique bulging eyes that sit on top of their heads, generally allowing them to submerge their bodies in water to breathe and drink, while keeping their eyes above water-level.

The eyes of Anura have two types of rod-like photoreceptors: blue-sensitive rods and green-sensitive rods. The pigments within these rods allow Anura to detect signals even in the lowest lighting conditions, making them optimal for nighttime.

One reason behind their large, well-developed eyes is that their survival relies on them for hunting/catching food and navigation while they move. Additionally, their high motion sensitivity and night vision make hunting easier (particularly since a decent number of Anura species are nocturnal), and it also assists in their pursuit of mates.

Where this characteristic is unique to Anura is that it is present in every Anura species, whereas many Caudata and Gymnophiona species have extremely poor vision due to small, under-developed eyes. This is especially the case with cave-dwelling species that rely on other senses for locomotion and feeding.

Anura Have External Tympanum (Eardrum)

The final characteristic unique to Anura is its development of an external tympanum, more commonly known as an eardrum.

The external tympanum separates an animal’s outer ear from its middle ear and is present in Anura because of its reliance on vocalization when attracting mates and performing territorial demonstrations.

Anyone who has heard a frog or toad make their classic croaks knows that these animals can be quite loud and vocal, and the presence of an external tympanum makes their vocalizations more effective.

The Anura’s advanced hearing is one of the key components to their survival, as it also allows them to hear distress signals, mating calls, and territorial warnings.

Species within the other two Amphibia Orders don’t generally rely on vocalizations as much, so they don’t have an external tympanum. In fact, most have very minimal auditory abilities and can only hear low-frequency sounds.

More About Anura

As an individual Order under the Amphibia class, Anura has several unique characteristics that warrant their separate classification. These predominantly include their tailless adult bodies, squat posture, and long hind legs that all allow them to be exceptional jumpers. Their large, well-developed eyes and external tympanum make them exceptional hunters and provide lifesaving skills.

This entire blog is dedicated to Anura so be sure to check out our other guides below for more:

Now, whenever you see various examples of amphibians together, you’ll know exactly what sets the Anura apart from Caudata and Gymnophiona.

Sources

Amphibia.org

Laurie J. Vitt, Janalee P. Caldwell, Chapter 17 – Frogs, Editor(s): Laurie J. Vitt, Janalee P. Caldwell, Herpetology (Fourth Edition), Academic Press, 2014, Pages 471-522, ISBN 9780123869197, https://doi.org/10.1016/B978-0-12-386919-7.00017-4.

Helmer Peter J., Whiteside Douglas P., Amphibian anatomy and physiology, Science Direct, 2005.