Early Triassic turtle tracks and the Permian pareiasaur origin of turtles

From the Lichtiga et al. 2017 abstract:
“Turtle (Testudines) tracks, Chelonipus torquatus, reported from the early Middle Triassic (Anisian) of Germany, and Chelonipus isp. from the late Early Triassic (Spathian) of Wyoming and Utah, are the oldest fossil evidence of turtles, but have been omitted in recent discussions of turtle origins. Recent literature on turtle origins has focused entirely on the body fossil record to the exclusion of the track record.”

Turtle tracks are distinct 
because they appear to walk on their lateral four unguals with little to no heel impression. Images here.

Figure 1. Chronology of Triassic turtle tracks and trackmakers.

Figure 1. Chronology of Triassic turtle tracks and trackmakers from Lichtiga et al 2017. Blue taxa are added here from the LRT. Yellow taxa are ‘turtle’ tracks. The post-crania of Elginia is the big question. Pappochelys is not related to turtles, but Lichtiga et al. included it.

Unfortunately
Lichtiga et al. did not reference the large reptile tree (LRT, 2027 taxa) which nests Pappochelys with placodonts, apart from turtles arising from Sclerosaurus, Elginia, Bunostegos and other pareiasaurs, all descending from Stephanospondylus in the Early Permian.

Even so,
the turtle tracks in the Lower and Lower Middle Triassic indicated to Lichtiga et al. that turtles arose from pareiasaurs based on the similarity of their tracks. They wrote,  Chelonipus also resembles the Permian track Pachypes dolomiticus, generally assigned to a pareiasaur trackmaker.”

So that takes us back
to the odd pareiasaur Bunostegos, the mini pareiasaur/basal turtle Elginia and the not widely recognized basal turtle, Meiolania at the transition to dome-shelled turtles (Fig. 1).

Figure 1. Hard shell turtle evolution with Bunostegos, Elginia, Meiolania and Proganochelys.

Figure 1. Hard shell turtle evolution with Bunostegos, Elginia, Meiolania and Proganochelys.

You might remember
that not only does Meiolania (Fig. 2) most closely resemble and nests with toothy Elginia (Fig. 2), but Meiolania is also the only dome-shelled turtle that can extend its forelimbs laterally. All others, including sea turtles, extend the humerus anteriorly.

Among so-called soft-shelled turtles
and their ancestors, Sclerosaurus, Odontochelys and to a lesser extent, Trionyx can/could also extend the humerus laterally.

Figure 1. The basal turtle, Niolamia, compared to the toothed pareiasaur/turtle?, Elginia. We have no post-crania for Elginia. Figure 1. The basal turtle, Niolamia, compared to the toothed pareiasaur/turtle?, Elginia. We have no post-crania for Elginia.

Figure 2. The basal turtle, Niolamia, compared to the toothed pareiasaur/turtle?, Elginia. We have no post-crania for Elginia.

The Lichtiga et al. paper confirms
all earlier studies that link pareiasaurs and turtles, including the LRT at ReptileEvolution.com —and it helps invalidate all other bogus turtle origin hypotheses.

References
Lichtiga AJ, Lucas AJ, Klein H and Lovelace DM 2017. Triassic turtle tracks and the origin of turtles.Historical Biology, 2017 online

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