Like all amphibians, D. mexicanus develops a cartilaginous skeleton and skull, which then become ossified during development. By the time fetuses are about 30 mm long, they already have a well developed chondrocranium and Meckel’s cartilage, including a pronounced articular facet and a retroarticular process. The articular facet and the quadrate begin ossifying around this time.
By the time fetuses are about 40 mm long, the opposing articular surfaces of their Meckel’s cartilage and palatoquadrate have started ossifying. These processes will form the jaw joint. This is important because the fetuses have access to a limited amount of egg yolk. When the yolk runs out, the fetus begins to feed on the interior lining of the uterus, and need well developed jaws and teeth by this point. By the time the fetuses reach about 50 mm, their jaw joints have nearly finished ossifying.
When the fetuses reach about 60mm in length, most of their chondrocranium has eroded, and Meckel’s cartilage is eroding medially. However, the nasal capsules continue to chondrify. The anterior rims of the nasal capsules will remain permanently cartilaginous. By the time they reach 70 mm, most of the cartilage has ossified and only isolated patches of cartilage remain.
When fetuses are 70 mm long, their nasal bones are beginning to fuse with the premaxillae, the pterygoids begin fusing with the quadrates, and the prefrontal and maxilla begin to fuse.. Additionally, the fetuses have already developed several rows of teeth with well-mineralized pedicels. This allows them to feed on the internal uterine linings.
When the fetuses reach 105 mm in length, their skulls begin to elongate and flatten. In addition, most bones are becoming thicker and more ossified.
The development of the chondrocranium in D. mexicanus is remarkably similar to its development in the genera Ichthyophis, Grandisonia, Hypogeophis, and Gegeneophis. However, the ossification process differs in D. mexicanus. While several of these genera display early ossification of jaw suspension elements. However, the specific bones that ossify first, in particular the Meckel’s cartilage, differ between D. mexicanus and the other genera. In addition, D. mexicanus has lost several primary ossification sites that are present in all other caecilians, including the quadratojugal, postparietal, lacrimal, periorbital, ectopterygoid, and interparietal elements. Instead, the fusion of multiple ossification centers forms the single median os basale.
The skulls of developing D. mexicanus are also highly kinetic. In particular, the quadrate is highly moveable because it ossifies before any of the bones around it. This allows fetal jaw movement, which assists in feeding on the uterine lining. Skull kinesis is highly reduced by adulthood, although the adult skulls are still streptostylic and slightly kinetic.
Metamorphosis from the larval to adult stage in D. mexicanus occurs in the last 4-5 months of gestation. By the time they are born, they have completed metamorphosis and emerge in their adult morphs.
Larval fetuses have long gill filaments that branch into groups of three. They keep these gills for slightly more than half of the 11 month gestation period. After 5-6 months of gestation, the gills are reabsorbed, although the spiracle stays open until about the 8th month of gestation.
Fetal teeth are present at hatching, 2.5-3 months into gestation. The fetus continues to add teeth and rows until birth, at which point they lose their larval teeth and start developing adult teeth. The loss of fetal teeth marks the end of metamorphosis.
- Wake, M. H., and J. Hanken. 1982. The development of the skull of Dermophis mexicanus (Amphibia: Gymnophiona), with comments on skull kinesis and amphibian relationships. J. Morphol. 173: 203–233.
- Wake, Marvalee H. "Fetal Adaptations for Viviparity in Amphibians." Journal of Morphology (2014)