Dibamidae


Dibamidae or blind skinks is a family of lizards characterized by their elongated cylindrical body and an apparent lack of limbs. Female dibamids are entirely limbless and the males retain small flap-like hind limbs, which they use to grip their partner during mating. They have a rigidly fused skull, lack pterygoid teeth and external ears. Their eyes are greatly reduced, and covered with a scale.
Blind skinks are native to Mexico, Southeast Asia, Indonesia, the Philippine Islands, and western New Guinea. They are small insectivorous lizards, with long, slender bodies, adapted for burrowing into the soil. They usually lay one egg with a hard, calcified shell, rather than the leathery shells typical of many other reptile groups.
The family Dibamidae has two genera, Dibamus with 23 species and the monotypic Anelytropsis. Recent phylogenetic analysis places the dibamids as the sister clade to all the other lizards and snakes. Other recent analysis has classified dibamids as sharing a common ancestor with the infraorder Gekkota, and that Dibamidae and Gekkota form the sister clade to all other squamates.

Characteristics

General appearance

Dibamids are burrower lizards characterized by their elongated bodies with blunt head and tail, and an apparent lack of limbs. Relatively small, blind skinks can reach a maximum length of 250 mm from head to tail and the snout vent length is variable between both genus Anelytropsis and Dibamus. In Anelytropsis, the tail is longer than in Dibamus and represents between 34 and the 38% of the snout vent length which can range from 77 to 180 mm. In Dibamus, the tail corresponds to 9 to 25% of the SVL that varies from 52 to 203 mm.
Usually dibamids are dark colored, from brown to dark purple, with little to no variation along their body and frequently lack elaborate patterns. It is common to find a color gradation from the darker back towards a lighter ventral side. Scales are shiny and smooth and very similar and overlapping along with some variation in number and shape in the head and anal regions where males usually have additional scales to cover anal pores. Scale row counts varies between both genera; Anelytropsis has 19 to 25 rows whereas Dibamus has 18 to 33. In both groups osteoderms are absent.
General characteristics of the soft tissue includes a tongue that is covered in lamellae except in the tip, heavily modified ears without external openings or middle ear cavity or eustachian tubes, and highly reduced eyes that lack internal structure and are covered by a scale and lack internal structure, particularly in Dibamus.

Limbs

Dibamids are lizards with highly reduced limbs but they are not completely limbless. Males and females have rudimentary poorly developed hind limbs containing a femur, tibia and fibula in males, and distal cartilage cap, this elements are more developed on Dibamus than in Anelytropsis. Female dibamus lack the tibia and the fibula.

Skull

The skull is approximately 5 – 7 mm in length with reduced kynesis and a more rigid skull for burrowing. The combination of fossorial habits and small size, contributes to the development of a skull configuration that is frequently found in other groups of burrowers and miniaturized species. Among those characteristics are the closure of the supratemporal fenestra and the post-temporal fenestra, the relative large braincase, tubular or scroll-like palatines and modified jaw suspension mechanism with the quadrate articulating with the lateral wall of the braincase.
Other characteristics of the skull of blind skinks include the absence of a parietal foramen, a well developed secondary palate formed by three different bones, the maxillae, vomers and palatines which are expanded ventromedially to form a scroll, and the lack of palatal teeth. Nasal and frontal bones are paired and contact each other in a W-shape suture with no overlap between the two bones, and several bones are lost or highly reduced. The main cranial differences, besides sizes, between Anelytropsis and Dibamus is the presence of epipterygoid and postfrontal in the Central American genus.
The mandible of Dibamidae bears less than 10 teeth and is composed of only three bones, the dentary, the coronoid and the compound bone. A remnant of the splenial bone is only present in one species of Dibamus, Dibamus novaeguineae.

Classification

The family Dibamidae contains two genera, Anelytropsis and Dibamus, and the close relationship of the genera was based on two morphological characteristics that are unique to these groups, the secondary palate and the lamellae covering the tongue, and additional cranial characteristics that can be shared with other groups of lizards.
The anatomical characteristics that dibamids share with other squamates contributed to the formulation of different taxonomic hypothesis. Dibamids, and particularly Dibamus was considered to be part of geckos and precisely the family of legless geckos; snakes, considering the organization of the skull and jaw muscles; or was proposed to be closely related to a group fossorial skinks with elongated bodies and reduced limbs.

Phylogeny

Relationships among Dibamidae

The relationships within Dibamidae have only be assessed until recently in a phylogenetic analysis that included DNA sequences from seven nuclear genes and one mitochondrial gene for 8 species, seven species of Dibamus and the one species of Anelytropsis. This analysis shows that there are two major clades within Dibamidae, one that includes the one species form the genus Anelytropsis, Analytropsis papillous, and the species of Dibamus that are distributed along continental Southeast Asia. The other clade includes species that are currently distributed in the peninsular Southeast Asia and Islands. These clades diverged 72 million years ago. Anelytropsis diverged from all mainland Dibamus at approximately 69.2 million years ago.

Dibamidae and its relationship with Squamata

The relationship of Dibamidae with other Squamata has a long history of phylogenetic studies in which the morphological characteristics are used to determine those relationships. Those analysis found close relationships between Dibamidae and all other lizards with elongated bodies, limb reduction and usually, a fossorial habit like amphisbaenians, snakes or fossorial skinks. In morphology based phylogenies, dibamids are sister taxa to amphisbaenians and the clade that includes amphisbaeninas and dibamids is sister to all snakes. The close relationships of this groups are the result of convergent evolution among this groups since some of the morphological traits have evolved independently in different groups.
More recent phylogenies using DNA sequences of nuclear and mitochondrial genes include a large taxonomic sample of squamates and place dibamids as the sister group to all other lizards and snakes, or with Gekkota as the sister group to all other squamates.

Biodiversity

There are two recognized genera within the family, Anelytropsis and Dibamus. According to The Reptile Database, Anelytropsis is monotypic and Dibamus includes 23 species:
Anelytropsis
Dibamus
For additional details, see
An extinct monotypic genus, Hoeckosaurus was recently proposed from the description of fossil material from the early Oligocene of the Valley of Lakes in Central Mongolia.
Dibamids have a disjunct distribution with one genus living in Northern Mexico, Anelytropsis, and the other one, Dibamus, living in South East Asia. Biogeographical studies suggest that the separation between Anelytropsis and Dibamus, specifically the clade with species that are distributed in continental South East Asia, occurred approximately 69 million years ago during the late Cretaceous and the migration from Asia to North America took place during the Late Paleocene or Eocene through Beringia.

Biology

Blind skinks are insectivorous and feed on arthropods and earthworms. Blind skinks are characterized by their fossorial or burrowing habits. They can dig their own burrows, use old burrows or other openings in the ground, or dwell under the leaf litter or logs.
Species of the genus Dibamus are frequently found in primary and secondary forests in a wide range of altitudinal variation. Anelytropsis is found in drier environments and is adapted to xeric conditions of different environments in northern Mexico.
Little is known about the reproduction of this group of lizards, but the inspection of female specimens from herpetological collections indicate that dibamids lays single egg with hardened shell, and eggs are laid frequently, at least in Dibamus.

Conservation

None of the species of Dibamidae are listed as endangered species in the Convention on International Trade in Endangered Species of Wild Fauna and Flora CITES.
The International Union for Conservation of Nature include some of the species of the genus Dibamus and the single species of Anelytropsis in the red list of endangered species, most are in the category of least concern, and two species, Dibamus kondaoensis and Dibamus tiomanensis are listed as nearly threatened and endangered respectively.