Placochelys placodonta (Jaeckel 1902) Mid to Late Triassic ~215 mya, 90 cm in length, was derived from a sister to Cyamodus and was the last in this lineage.
Distinct from Cyamodus, the skull of Placochelys had a narrow, elongated, toothless premaxilla and a more ornate postorbital region. The maxilla was reduced with only three teeth. The dentary, which was taller than long, had only two teeth. The quadratojugal was retained only over the quadrate. The orbit was reduced to a crescent by an expanded jugal. The quadratojugal fused to the squamosal. The ventral margin of the mandible was flat to concave.
The vertebral columnn is largely unknown, hidden beneath the carapace. The cervical ribs were lost. The tail was much shorter.
The girdles are unknown, hidden beneath the carapace. All four limbs were transformed into paddles with slender digits and digit 3 was the longest. Metacarpal 1 was as long as 2 and 3. Metacarpal 4 was reduced to the size of 5. The metatarsals were all short and subequal. Extra phalanges appear on manual and pedal digit 2.
A single carapace covers the torso and hips convergent on turtles.
The family tree of the Enaliosauria is here. The complete reptile family tree is here. A series on placodont and ichthyosaur palate evolution is here. |
