Description
Thorius longicaudus is a large minute salamander with a snout-vent length range of 23.6 - 28.3 mm in males and 24.4 - 27.7 mm in females. The head is narrow compared to the body, only about 1/8 the snout-vent length. The head is slightly longer than wide. The snout is bluntly pointed or pointed. Its nostrils are relatively large and elliptical. The eyes are moderately large and in the dorsal view, slightly protruding over the margin of the jaw. Grooves below the eye reach the lips on both sides of the head. The parotoid gland is present, but may be indistinct. Males possess mental glands. The limbs are moderately long relative to the body size and the hands and feet are well developed. The outer digits are fused, central digits are long with rounded ends. The relative finger lengths are 3 > 2 > 4 > 1 and the relative toe lengths are 3 > 4 > 1 > 5 > 1. There is an indistinct postiliac gland. The tapered tail is relatively long compared to the body, with the snout-vent length being 63 - 73% of the tail length in males and 62 - 91% of the tail length in females (Parra-Olea et al. 2016).

DIAGNOSIS:

Although similar to other species in the genus Thorius, particularly T. pinicola, T. longicaudus can be distinguished based on a combination of seven characteristics: its large size (greater than 23.5 mm), relatively short limbs, relatively long tail, stretched ovate nostrils, lack of maxillary teeth, presence of 5 - 10 vomerine teeth, and considerable sexual dimorphism of the skull. More specifically, T. longicaudus can be distinguished from most other Thorius species, including T. arboreus, T. insperatus, T. minutissimus and T. papaloae, by its larger snout-vent length. Thorius longicaudus has shorter limbs than Thorius adelos, T. arboreus, T. insperatus, T. macdougalli and T. smithi. The relatively long tail of T. longicaudus is longer than T. boreas and T. tlaxiacus. The highly elliptical nostrils are larger than T. narisovalis and less distorted than in T. pulmonaris and T. tlaxiacus. The lack of maxillary teeth in T. longicaudus set them apart from T. adelos, T. aureus and T. smithi. And lastly, there are more vomerine teeth in T. longicaudus than in both T. pinicola and T. tlaxiacus (Parra-Olea et al. 2016).

COLORATION:

In life, the dorsal background color of the head, body, and tail is blackish-brown. There is a lighter tan-reddish to brown dorsal stripe in an indistinct chevron pattern with coppery-brassy highlights that begins at the head and extends along the entire length of the head, body, and tail. The stripe has sharp, distinct edges in the anterior region but is less distinct at the tail. The parotoid gland is not distinctly colored. There is a slight reddish brown coloration at the insertion of the limbs. The dorsal surface of the limbs is dark brown but the ventral surface is paler. There is a lack of pigmentation on the costal grooves, but a white wash of densely packed whitish markings in the lower, lateral flanks that become less dense ventrally. There is also a lack of pigmentation on the gular fold. In general, the ventral surface is pale brown and flecked with white up onto the lateral surfaces of the head, body, and tail. The postiliac gland is pale. The irises are reddish-brown (Parra-Olea et al. 2016). In preservative, the species is relatively dark with a distinctly paler dorsal stripe. The stripe extends from the head to the tail tip and may have a vague herringbone pattern or a dark, thin, medial line. The ventral surface is paler than the flanks and is flecked with white spots on the belly and gular region. A pale nuchal spot is also typically present (Parra-Olea et al. 2016).

VARIATION:

There is variation in coloration (see above) and morphology between individuals and sexes. Individually, snouts can vary from sharply pointed to bluntly pointed. Parotoid glands range from inconspicuous to distinct. Number of teeth varies from 0 - 4 pre-maxillary teeth and 5-10 vomerine teeth. Females can have longer relative tails than males. Males have mental glands. There are also significant sexual dimorphism with respect to cranial morphology. Males have poorly ossified skulls, while females have a highly developed structure (Parra-Olea et al. 2016).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Mexico

View distribution map in BerkeleyMapper.

Thorius longicaudus is known from two small, isolated areas in Oaxaca, Mexico that are 40 km apart from each other. The first is along the northern ridges of the Sierra Madre del Sur, about 19 km south of the village of Sola de Vega. The second is roughly 40 km northwest of the first area, near San Vicente Lachixio. They were found at elevations between 2,085 and 2,615 meters asl. Originally these areas were pine-oak forests, although deforestation has left the region quite bare (Parra-Olea et al. 2016, IUCN 2020).

Life History, Abundance, Activity, and Special Behaviors
Thorius longicaudus is a terrestrial species that was originally found in pine forests under logs and other natural cover objects or clustered together in deep cracks under crusted dirt. In recent years, specimens were found in smaller numbers near scattered trees, due to deforestation. Specimens have also previously been found near dirt roads leading from the main highway to a ridge near a microwave station (Parra-Olea et al. 2016). However, as of 2020, the species has not been seen since 1998 (Parra-Olea et al. 2016, IUCN 2020).

The species breeds by direct development and breeding is independent of water (Parra Olea et al. 2016, IUCN 2020).

Sympatric species include Pseudoeurycea conanti and P. cochranae at the Sola de Vega site and T. narisovalis, T. tlaxiacus, P. cochranae, and P. anitae at the San Vicente Lachixio site. Bolitoglossa oaxacensis can also be found nearby (Parra Olea et al. 2016).

Larva
The species breeds by direct development (Parra Olea et al. 2016, IUCN 2020).

Trends and Threats
Thorius longicaudus was very abundant in the 1970’s but was impossible to find by the time of the species description due to deforestation and possibly disease (chytridiomycosis). The last recorded sighting of a live individual in the wild was in March 1998, despite extensive surveys. There is still ongoing habitat destruction and degradation due to agriculture and wood extraction; and the species is not found in any protected area. Based on this information, the species has a “Critically Endangered (Possibly Extinct)” threat status on the IUCN Red List. It is estimated that if a population does exist, it may have fewer than 50 mature individuals (Parra-Olea et al. 2016, IUCN 2020).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Intensified agriculture or grazing
Disease

Comments

PHYLOGENETIC RELATIONSHIPS:

References
Hanken, J. (1983) Genetic variation in a dwarfed lineage, the Mexican salamander genus Thorius (Amphibia: Plethodontidae): taxonomic, ecologic and evolutionary implications. Copeia, 1983(4), 1051-1073. [link]

IUCN SSC Amphibian Specialist Group. 2020. Thorius longicaudus. The IUCN Red List of Threatened Species 2020: e.T119243838A119243842. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T119243838A119243842.en. Accessed on 24 October 2023.

Parra-Olea, G. Rovito, S.M., García-Paris, M., Maisano, J.A., Wake, D.B., and Hanken, J. (2016). Biology of tiny animals: three new species of minute salamanders (Plethodontidae: Thorius) from Oaxaca, Mexico. PeerJ 4, e2694. [link]

Rovito, S.M., Parra-Olea, G., Hanken, J., Bonett, R.M., and Wake, D.B. (2013). Adaptive radiation in miniature: the minute salamanders of the Mexican highlands (Amphibia: Plethodontidae: Thorius). Biological Journal of the Linnean Society, 109(3), 622–643. [link]

Species Account Citation: AmphibiaWeb 2024 Thorius longicaudus: Long-tailed Minute Salamander <https://amphibiaweb.org/species/8530> University of California, Berkeley, CA, USA. Accessed Nov 21, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 21 Nov 2024.

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