Golden Thorius
(Thorius aureus)
The golden thorius is the most colourful and sizeable member of the Thorius salamanders, although it measures just 57 mm in total length. Very little is known about this species, and it is found living at very high altitudes in terrestrial habitats within montane pine-oak-fir forests and upper cloud forests, hiding under stones and logs. This species does not adapt to degraded or secondary habitats, and is thought to be threatened by factors such as habitat destruction, climate change and possibly a virulent disease, such as chytridiomycosis.
Urgent Conservation Actions
Surveys to determine where this species survives in the wild; disease screening; development of a Conservation Action Plan; disease screening.
North-central Oaxaca, Mexico
The scientific species name for the golden thorius, Thorius aureus, is derived from the Latin word “aureus”, meaning golden or splendid, in recognition of the distinctive golden dorsal stripe that is characteristic of this species.

The golden thorius is the largest member of the Thorius salamanders.
Evolutionary Distinctiveness
Order: Caudata
Family: Plethodontidae
The Plethodontidae is by far the largest family of salamanders, comprising nearly 70% of all living species. In total there are 378 known plethodontids divided between four subfamilies and 24 genera. The plethodontids are united by the fact that they do not possess lungs and breathe entirely through their skin and mouth lining. They are often referred to as the lungless salamanders, although they are thought to have evolved from highly aquatic, lunged ancestors in the streams of the Appalachian Mountains in eastern North America. The earliest plethodontids were hypothesised to have lost their lungs because individuals with reduced, or absent, lungs were less likely to float away in the swift mountain streams where they lived. The vast majority of other salamanders possess lungs, so this makes the lungless salamanders an unusual and fascinating group of animals.

They are thought to have diverged from all other amphibian species 145 million years ago at the boundary between the Jurassic and Cretaceous periods. They are as different from all other amphibian lineages as wombats are from whales, evolving at a time when dinosaurs were still dominant. Overall, plethodontids are the most evolutionary advanced salamanders, so it may at first appear odd that they should have lost lungs, which are one of the most basic features of all vertebrates living on land. Lacking lungs and being dependent on their skin for respiration places a size restriction on these salamanders because large animals have a relatively small surface area of skin compared to their body’s volume, and have greater difficulty in supplying their body tissues with oxygen compared to smaller animals (which have a large surface area to volume ratio). The long, slender form of the lungless salamanders maximises the surface area available for gas exchange, and some species grow to lengths of over 300 mm.

Plethodontid salamanders occupy a great diversity of habitats, ranging from strictly aquatic to strictly terrestrial, exploring niches as diverse as caves, trees, mountain streams, and they are also found burrowing through the earth. Dependence on their skin for breathing places limitations on where and how lungless salamanders can live. Their skin must be kept moist at all times in order for oxygen to be taken up by the blood in capillaries beneath the skin. This means plethodontids are either confined to humid areas, or must find damp hiding places and only emerge in wet weather, typically at night. The life of a lungless salamander in less humid areas, like Europe and temperate North America, therefore comprises brief periods of activity interspersed with inactive phases that are often very long. They are able to survive the periods of inactivity because they have a very low metabolic rate and low energy requirements. Able to store much of what they eat as fat, they do no need to feed very often.

A further adaptation, present among many species of the lungless salamander subfamilies named “Plethodontinae” (from East and West North America) and “Bolitoglossinae” (from tropical Central and South America), is “direct development”. This is a method of amphibian development where the larval stage (e.g. the tadpole stage in a frog’s life history) has been eliminated. Early development takes place in eggs, which may be laid in moist places away from water, and the young hatch out as miniature adults. The well known amphibian metamorphosis, most commonly appreciated in the transition from tadpole to adult frog, does not occur outside of the egg. This mean that certain lungless salamanders in these two subfamilies may live away from water bodies, allowing them to expand their ranges to new areas.

The history and characteristics of the lungless salamanders go some way to explaining their range. They are mostly found in the New World, where they are widely distributed in eastern and western North America, as well as Central and South America. However, continental drift over millions of years has also brought them to the Old World, where they are found in parts Europe (e.g. Sardinia) and Korea. The existence of the Korean crevice salamander was unknown until 2005, when its discovery was a shock to science, indicating a long history of lungless salamanders in Asia. This is the only known species is Asia, suggesting that the rate of species generation in this part of the world is very low, especially compared the the huge radiation of lungless salamander species in the New World.

There are 23 known species in the genus Thorius (commonly known as the “Mexican pigmy salamanders”) which represent one of ten genera present within a lungless salamander subfamily called the “Bolitoglossinae”, including all the plethodontids from Central and South America. The Mexican pigmy salamanders include the smallest salamanders on earth – the most diminutive of which measure just 26.9 mm in total length as sexually mature adults. Thorius salamanders occur only in Mexico, where they are restricted to the southern states of Veracruz, Puebla, Oaxaca and Guerrero. In general, Mexican pygmy salamanders live at high elevations, ranging from about 1,500-3,000 metres above sea level or higher, but some species descend to 800 metres above sea level.

Most Mexican pigmy salamanders lack teeth and their skulls are extraordinary because of the poor state of development, the thinness of the bones, and the weak articulation of the elements. Miniaturisation has been achieved by the reduction or loss of some of the cranial (or skull) elements, accompanied by a relative increase in the size of the sense organs. Another interesting feature of these salamanders is that they possess male heterogamy reproduction – the presence of an X or Y-type sex chromosomes in the eggs and sperm, as is the case in humans. This is known as chromosomal sex determination, where females have two X sex chromosomes (XX) in their cells and males have one X chromosome and one “male” Y chromosomes (XY). In non-chromosomal sex determination, being male or female can occur as a result of environmental conditions, such as temperature, whereas with the X and Y-chromosome system, sex is determined from the outset.

The genus Thorius is thought to have originated in the Early to Mid Miocene period, between 23 and ~12 million years ago. This makes Mexican pigmy salamanders as dissimilar to their closest relative as humans are to gibbons. Specifically, within the Thorius salamanders the golden thorius diverged 4 million years ago. It is one of the largest members of its genus and, although most Mexican pigmy salamander are toothless, the golden thorius actually has teeth along its upper jaw.
The golden thorius, like all lungless salamanders in the Bolitoglossinae subfamily, possesses a slender body, long tail and prominent eyes. A distinctive feature of the plethodontid family is a narrow groove (the nasolabial groove) running from each nostril to the upper lip: its function is to carry waterborne odours from the ground into the nasal cavity. Another curious trait of the lungless salamanders are mental (from the Latin “mentum”, meaning chin) glands. These are modified mucus glands and release pheromones, which are chemicals produced by an animal to influence the behaviour of other members if its species, often with regard to breeding receptivity. Mental glands are sometimes visible in males as raised bumps below their lower lip.

Lungless salamanders are very small to medium in size, usually measuring between 25 to 250 mm from the tip of the nose to the end of the tail, which salamanders retain throughout their life. They are unusual among the salamanders in that some species can detach from their tail as a predator-defence mechanism (also known as tail or caudal autotomy). It is therefore not unusual to see individuls missing part or all of their tail, which they may regererate later. Lungless salamanders may have bold patterns on their skin as adults, or they may have a colouration more similar to their environment to aid camouflage. They have well-developed “costal” grooves (successive vertical grooves in the skin along the sides of the body), generally numbering between 12-15. Their limbs are slender and often have largely or completely webbed digits. Species, like the golden thorius, in the Thorius genus (the “Mexican pigmy salamander”) are very similar in form to those in the genus Bolitoglossa (the “mushroom-tongue salamanders”), although they are generally much smaller, the largest representatives seldom exceeding 70 mm in total length.

Despite only reaching a total length of around 57 mm (the tail accounting for half of this measurement), the golden thorius is one of the largest and most robust members of the Mexican pigmy salamander genus. The head is narrow with well-developed eyes and prominent “nasolabial” grooves, the snout being more rounded in females and more pointed in males. This species has teeth on its upper jaw, but these are missing in most other members of the Thorius genus. Nostrils of many species of Mexican pigmy salamander are very large, but in this species they are relatively small and oval in shape. The limbs are short with very small, narrow hands and feet, and the digits are partially joined to neighboring digits but are free at the tips. This species is light to yellowish brown with a prominent golden stripe present along the back and tail, sharply edged along the sides by a dark brown colouration. This stripe is broadest over the head and becomes pinched over the shoulders, forming an hourglass shape. The strip is also marked by a series of brown chevrons, pointing forwards, which fade over the tail. The ventral surface (or underside) is pale and unmarked. This an unusual coloration in the group of generally dull, dark brown to gray-black Mexican pigmy salamanders.
Most Mexican pigmy salamanders are usually terrestrial (or ground-dwelling), living mainly under surface cover, inside logs, or especially beneath the bark of fallen and rotting logs. In 1908, the German naturalist Hans Friedrich Gadow in his publication “Through southern Mexico, being an account of the travels of a naturalist” remarked after observing some members of this family:

These little things showed a predilection for living in a proverbially precarious position, namely, “between the bark and the wood” of decaying pine-trees, amongst the boring-dust of beetles and maggots.

The golden thorius is restricted to a terrestrial (or ground dwelling) life in montane pine-oak-fir forests and upper cloud forest habitat, living under stones and logs, and this species does not adapt to degraded or secondary habitats. Very little is known about the golden thorius but it is presumed that direct development of the young occurs within the eggs and they hatch as miniature adults, although the eggs of this species have never been observed. The whole reproductive process is independent of a water body, making the golden thorius a truly land-dwelling species. Some species in the Thorius genus are known to display courtship rituals. The pheromone releasing mental gland on the chin of male Mexican pigmy salamanders plays an important role in mating to influence the receptivity of females. During amplexus (the mating embrace), the male clasps the female with both his arms and legs, and rubs pheromones across the female’s snout. Female Mexican pigmy salamanders have been found to guard the eggs throughout their development in many species, often in special hides, until hatching occurs.

Mexican pigmy salamanders may at first appear very vulnerable to predators but a number of defense mechanisms have been found among the members of this genus. These include caudal autotomy (tail detachment) and behavioural defensive methods, including immobile posture, coiling and flipping of the body, and displays where the stomach is exposed and the tail is held up or undulated. When uncovered, golden thorius salamanders make a tight coil that hides the head. They have a constriction at the base of the tail and are capable of caudal autotomy at this site, or at any point along the length of the tail.
The golden thorius is known only from the north slopes of Cerro Pelón, a prominent massif of the Sierra de Juárez. This species is found in high cloud forest under rocks, logs and surface debris, and can be found under the bark of fallen logs and in moss on road banks. The recorded precise elevational range is 2,475-2,930 metres above sea level.
This species is known from the highest peak of Cerro Pelon and surrounding areas on the northern slopes of the Sierra de Juarez, north-central Oaxaca, Mexico. It occurs at an altitude of 2,600-3,000 metres above sea level.
Population Estimate
The golden thorius was once common, but now seems to have largely disappeared. Despite several attempts to locate a population, there has been just one observation in the last few years.
Population Trend
The IUCN Red List of Threatened Species indicates that the total population size of the golden thorius is in decline.
The golden Thorius is listed as Critically Endangered in the IUCN Red List of Threatened Species because its extent of occurrence is less than 100 km sq., all individuals are in a single location, and there is continuing decline in the extent and quality of its habitat, and in the number of mature individuals, in north-central Oaxaca, Mexico.
The main threats to the golden thorius are habitat destruction and degradation caused by human land uses and possible climate change. Although logging is taking place within the species' range, this is not sufficient to explain the extent of the decline. It is possible that, being a mountain top species, it might have been adversely affected by climate change. It may also have been affected by recent volcanic activity and disease (such as chytridiomycosis, although this normally impacts species that are associated with water).
Conservation Underway
The golden thorius does not occur in any protected areas, and there are currently no conservation measures underway for this species.

  • Establish spatial knowledge of the distribution and occurrence of some of the threatened species in Mexico.
  • Screen individuals and other amphibians in these areas for the presence of chytrid fungus.
  • Increase the capacity in Mexico for the long term conservation of these species

Conservation Proposed
The protection of remaining primary habitat for this species is an urgent priority. There is also a need for research into the reasons for its recent dramatic decline, including disease screening and population surveys. All information collected could then be used to inform a Conservation Action Plan for the protection of this species in the wild.

In addition to conserving wild habitat for this species, the IUCN Technical Guidelines for the Management of Ex situ Populations, part of the IUCN Red List of Threatened Species, recommend that all Critically Endangered species should have an ex situ population managed to guard against the extinction of the species. An ex situ population is ideally a breeding colony of a species maintained outside of its natural habitat, giving rise to individuals from that species that are sheltered from problems associated with their situation in the wild. This can be located within the species’ range or in a foreign country that has the facilities to support a captive breeding programme for that species. Further investigation is therefore required into the possibilities of establishing a captive breeding programme for any surviving golden thorius salamanders. Captive animals could then be a source of new individuals to repopulate any restored habitat.
Associated EDGE Community members

James is an expert in evolutionary morphology, developmental biology, and systematics.

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  1. wildam

    Golden Thorius


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