Reduced, plate-like (and typically unossified) pubic bones are present beneath the ilia and paired, cartilaginous prepubic or epipubic bones are sometimes present too. The anuran pelvis consists of a cylindrical, rod-like central unit (the urostyle) surrounded by two super-long, shaft-like ilia. At bottom, the urostyle and ilium are at an obvious angle when the limbs are at rest. At top, you should be able to see how the urostyle and ilium are almost parallel when the hindlimbs are flexed. As the ilium pivots on the sacral vertebra, it rotates. X-rays of two anuran species: Rana temporaria at top, Discoglossus pictus below. A midline element present in anuran ancestors – the interclavicle – is absent. The omosternum and xiphisternum appear to be new structures that might have a role in shock absorption. Both a cleithrum and a cartilaginous suprascapula are present dorsal to the scapula (Havelková & Roček 2006): the anuran cleithrum is a relict and much less important in size and shape than it is in temnospondyls and other fossil relatives of anurans. In others, the epicoracoid cartilages are fused at the midline, forming the firmisternal condition. In some anurans, the epicoracoid cartilages are obviously paired and both components distinctly overlap on the midline, forming what’s known as the arciferal condition. The omosternum projects anteriorly from the junction of the clavicles, and the procoracoids are attached to both the posterior margins of the clavicles and the epicoracoid. The paired clavicles and coracoids are transverse bars, separated along the midline by the epicoracoid cartilages and sternum/xiphisternum. Meanwhile, the bones of the pectoral girdle are complex and complimented by an assortment of new elements. The radius and ulna are fused together, forming a compound element termed the radioulna. Based on a diagram in Havelková & Roček (2006). Skeletal architecture of the anuran scapulocoracoid, specifically that of Discoglossus. Again, there are times when it has been interpreted as a true digit, a conclusion that would make anurans hexadactyl (Galis et al. A similar structure – the prehallux – is present in the foot. This structure is often large and complex and there are even frogs (like the treefrog Hysiboas andinus) where it is a large, curved spike that protrudes from the skin in others its shape and even its presence is sexually dimorphic. However, developmental data shows that the prepollex is a novelty related to increased ossification in the fingers (Fabrezi 2001). Anurans have four fingers, but are these digits I-IV or II-V? An additional structure termed the prepollex has been interpreted as the true digit I by some authors, in which case the large digits are II-V. Credit: Darren NaishĪnuran limbs and limb girdles are also odd compared to those of other tetrapods. Hands of the Australian myobatrachid Limnodynastes tasmaniensis showing sexual dimorphism in the presence in the prepollex. They instead rely on buccal pumping, where movement of throat musculature controls inhalation and exhalation. Obviously, anurans don’t employ costal ventilation in breathing (that is, use of the ribs in operating the lungs). Ribs are either highly reduced or absent. The sacrum involves just a single vertebra that has a pair of stout transverse processes: their junction with the pelvis (on which read on) is mobile, sometimes highly so (Whiting 1961). The anuran vertebral column is substantially reduced: anurans have (at most) nine presacral vertebrae, and some have as few as five. Excluding the urostyle, there are just 9 vertebrae here. The tremendously abbreviated vertebral column of a modern anuran: in this case that of Leptodactylus. Tooth-like structures – so-called odontoids – have evolved on a few occasions (Fabrezi & Emerson 2003). As mentioned here at Tet Zoo before (see links below), Gastrotheca simply must have re-evolved these teeth from ancestors that lacked them, an unambiguous case of re-evolution of a ‘lost’ structure. Virtually all anurans lack a dentition in the lower jaw, the weird exception being the marsupial treefrog Gastrotheca. The palate is what we term highly fenestrated, meaning that most of its surface is occupied by enormous spaces. Most of the skull roof is formed of the giant frontoparietal complex which typically starts out as a paired structure but ends up as a giant plate forming the region between the large eye sockets. The anuran skull is notable for the relatively few elements it contains: elements typically present in tetrapod skulls have been lost, or fused with adjacent structures. It won't be lost on you how small the skeleton is compared to the size of the whole animal. X-rays of the weird African frogs Hemisus (left) and Breviceps (right).
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