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Gastrotheca trachyceps is a marsupial frog with snout-vent lengths (SVL) up to 50.0mm in males and 67.9 mm in females. Their heads are characterized as slightly wider than long with a bluntly round snout from the dorsal view and truncate snout from the profile. This species has an interorbital distance that is twice the width of the eyelid, and an eye diameter that is approximately equal to the distance between the eye and the nostril. Gastrotheca trachyceps have long tibias that measure approximately half the SVL and long feet that are just slightly shorter than the tibias. The skin of G. trachyceps is smooth on the dorsum but co-ossified with the head. Relative finger lengths are 1 = 2 < 4 <3, with all fingers displaying large discs. Relative toe lengths are 1 < 2 < 3 < 5 < 4, with webbing that extends from the fourth toes penultimate subarticular tubercle to the fifth toes distal tubercle and sometimes the fifth toes disc. Like adults, tadpoles of G. trachyceps have bluntly round snouts in the dorsal view and abruptly inclined profiles. Also from the profile, tadpole throats are concave and they have small eyes that point dorsolaterally and are positioned greater than a third of the width of the head apart from each other. Gastrotheca trachyceps tadpoles have gradually rising dorsal fins that starts at the posterior edge of the body. Their cloaca is located medially, and their mouthparts consist of alternating ventral and ventrolateral rows with a single anterolateral row (Duellman 1987).
Gastrotheca trachyceps is one of only two known Gastrotheca species to have co-ossifed skin on the head. The other species, G. nicefori can be differentiated from G. trachyceps by having a bony transverse occipital crest, dark uniform flanks, hidden limb surfaces, and a longer first than second finger (Duellman 1987).
In life, the dorsum of G. trachyceps is green or brown with dark brown strips and/or dark spots paravertebrally while the ventral surface is a pale creamy grey with a darker vocal sac and dark flecks along the belly. The species has pale strips along the lip, dorsoventrally, and on the supra-cloacal. Like the dorsum, G. trachyceps flanks are green or brown, while the groin and thighs, both anterior and posterior, are speckled with black dots on a blue background. The tympanum is brown in life. When preserved, the dorsum of the head, forelimbs, shanks, and loreal region become a uniform bluish grey. There is a large cream spot located on the upper base of the forearm. The postorbital region becomes brown and the flanks and dorsal surfaces of the thigh a lighter brown. The full-length narrow dorsolateral stripe becomes creamy white and is connected to the labial stripe by a pale rostral stripe that is located above the insertion of the forelimbs and continues for two-thirds the length of the flank. The supra-cloacal and heel strip, which runs along the outer side of the food to the disc, are cream colored. The axilla is blue as are the groin, thighs, ventral surface of the shanks, and inner surfaces of the feet; the latter surfaces also have dark grayish brown or black spots or mottling. The belly, throat, and ventral surfaces of the thighs become creamy grey with black speckles (Duellman 1987).
The species epithat trachyceps roughly translates to "rough head" in Greek and is in reference to G. trachyceps co-ossified head that binds the skin to the skull (Duellman 1987).
Phylogeny and Evolution: In 2005, the entire genus was moved from the family Hylidae to Amphignathodontidae (Falvovich et al. 2005). In 2007, Wiens et al. examined alternative hypotheses for the evolution of genus Gastrotheca. In that analysis a phylogeny for marsupial frogs (Hemiphractidae) was reconstructed using nuclear and mitochondrial DNA sequences, with resultant estimation of patterns of life-history evolution across the ensuing tree. Wiens et al. then reasoned that all genus Gastrotheca species with complex life cycles (egg, tadpole and adult stages) are phylogenetically nested among species and genera with direct development (only egg and adult stages). As a consequence Wiens et al. further argued that the most likely explanation for tadpole evolution in this genus is that the tadpole stage evolved from direct development within Gastrotheca, "the only known case of such a reversal in frogs."
Generally all the ancestral development of genus Gastrotheca occurred approximately sixty million years before present in the lowlands of northwestern South America. As Andean uplift occurred, corresponding speciation resulted for the higher altitude niche species members including G. trachyceps; this higher altitude adaptation likely reflected the floral palette and microclimate more than the air pressure of the altitude itself, but in any case such altitude speciation occurred around twenty million years before present (Hertwig and Sinsch 1995).
Associates: Biodiversity within the range of G. trachyceps is very high, with an exceptional level of endemism as well. Notable mammals present here are the Spectacled Bear (Tremarctos ornatus), jaguar (Panthera onca) and Andean Tapir (Tapirus pinchaque). Example avian species occurring in this eco-region are the endemic Booted Racket-tail (Ocreatus underwoodii), Empress Brilliant (Heliodoxa imperatrix), the Fulvus Treerunner (Margaromis stellatus), the Black Solitaire (Entomodestes corocinus) and the Borgeted Sunangel (Heliangelus strophianus).
There are an extraordinary number of amphibian taxa within the same eco-region inhabited by G. trachyceps. Example associate endemic amphibians are the Burrowes Robber Frog (Pristimantis laticlavius), Duellman's Robber Frog (Pristimantis duellmani) and the Pirri Range Stubfoot Toad (Atelopus glyphus). Example reptilian endemics that overlap (or nearly overlap) the range of G. trachyceps are Antioquia Anole (Anolis Antioquia) and the Saphenophis Snake (Saphenophis sneiderni).