Milleropsis – another biped at the base of the Diapsida

Milleropsis, a largely forgotten taxon that displays possible bipedal traits at the base of the Diapsida.

Figure 1. Milleropsis, a largely forgotten taxon that displays possible bipedal traits at the base of the Diapsida.

Milleropsis pricei (Gow 1972) Early Permian ~290 mya, ~20 cm in length, was originally considered a milleretid close to Milleretta and a captorhinid (close to Captorhinus). Here Milleropsis was derived from a sister taxon to Heleosaurus and it phylogenetically preceded Eudibamus and Petrolacosaurus at the base of the Diapsida, despite a lack of upper temporal fenestrae. The fossil is poorly known, but the skull, manus and pes provide many of the diagnostic characters.

Heleosaurus, an ancestral taxon to Milleropsis.

Figure 2. The protodiapsid, Heleosaurus, an ancestral taxon to Milleropsis originally considered a possible biped.

Distinct from Heleosaurus, the skull of Milleropsis had a wider set of parietals and a smaller parietal opening. The lower temporal arch was missing by reduction of the quadratojugal. The maxilla does not appear to disconnect the lacrimal and naris, but that part of the fossil is damaged or the interpretation of Heleosaurus may be mistaken, especially since Eudibamus is similar to Milleropsis in this regard. The naris was probably closer to the jaw tips. The mandible had a slightly higher coronoid process. The transverse process of the pterygoid leaned anteriorly. The anterior pterygoids were separated along with the posterior vomers.

The vertebral column is not well known, but the caudal series is very long attenuated with little to no trace of any chevrons.

The scapula was distinct from the coracoid and dorsally was reduced to a thin strap. Metacarpal 3 was longer than mc4. The manus was larger with digits 3 and 4 subequal.

The ilium has a small anterior process and a reduced posterior process. The pubis and ischium were not separated. The pubis had a dorsal process.

The calcaneum was elongated, producing a pseudo tuber. Metatarsals 3 and 4 were subequal. The penultimate phalanges were short. Metatarsal V was elongated.

Bipedal lizard video marker

Figure 3. Click to play video. You’ve seen this bipedal lizard video before. Note the morphological similarities, completely by convergence!

Synder 1954 on traits shared by bipedal lizards
Indicators of bipedality in living lizards according to Snyder (1954) (1) shortening of the trunk and the entire presacral region; (2) an increase in tail length; (3) stronger and more massive transverse processes of the two sacral vertebrae; (4) a narrower interacetabular width of the pelvis; (5) a longer ilium; (6) more extensive development of the preacetabular process; and (7) a longer femur and tibia. In terrestrial bipedal lizards, as in prolacertiforms, a shortening of the manus is indicated, but this is not the case with arboreal forms.

Convergence galore~
Lots of morphological convergence here, with great phylogenetic distance between basal diapsids and lizards.

How does Milleropsis stack up?
1. Slightly shorter trunk. 2. Increase in tail length. 3, 4. not sure about the sacrum or interacetabular width. 5. Longer ilium. 6. Longer preacetabular process. 7. not longer hind limbs compared to Heleosaurus, but it was promoted as a biped also.

Figure 1. Click to enlarge. Eudibamus in situ (above), traced (middle) and reconstructed (below). The revised skull retains a large orbit and has a shorter rostrum

Figure 1. Click to enlarge. Eudibamus in situ (above), traced (middle) and reconstructed (below). The revised skull retains a large orbit and has a shorter rostrum

Any bipedal sister taxa?
Yes, Eudibamus and maybe Heleosaurus. Milleropsis was overlooked previously as a biped candidate, but this reconstruction and analysis admit the possibility on several levels (morphology, phylogeny, etc.)

References
Gow CE. 1972. The osteology and relationships of the Millerettidae (Reptilia: Cotylosauria). Journal of Zoology, London 167:219-264.

wiki/Milleropsis

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