Four Bipedal Lineages in the Early to Mid Triassic

There must have been something in the water. Four bipedal lines from distinctly different heritages appeared in the Early to Mid Triassic.

Bipeds of the Triassic

Figure 1. Bipeds of the Triassic. Top to bottom: Cosesaurus, Scleromochlus, Marasuchus and Tropidosuchus. Each represents a distinct lineage of bipeds with bipedal sister taxa.

Cosesaurus is a bipedal lizard, close to other bipeds Langobardisaurus, Sharovipteryx and Longisquama. Sister left occassionally bipedal tracks, known as Rotodactylus, distinctive for their asymmetrical digitigrade impressions that elevate the proximal phalanges and imprint just a hint of digit 5 posterior to the others. The femur does not have a strongly inturned head until you get to basal pterosaurs.

Scleromochlus is a bipedal early croc, close to Terrestrisuchus and Saltopus. No tracks so far. There’s no digit 5 here.

Marasuchus, a basal theropod dinosaur (all of which are bipeds) may have inherited its bipedal abilities from a common ancestor with Scleromochlus. Distinctly digit 3 was likely longer than 4 (incomplete preservation makes this a good guess at this point). Again, no digit 5.

Tropidosuchus is closely related to another likely biped, Lagerpeton, which is often wrongly associated with basal dinosaurs. Both have a strongly asymmetric pes (unlike basal dinos but like Rotodactylus trackmakers), and little to no digit 5 (unlike Rotodactylus trackmakers).

Recent work by Brusatte et al. (2012) and Lovelace and Lovelace (2012) considered Rotodactylus tracks to be made by basal dinosauromorphs, but Rotodactylus includes a digit 5 impression, something basal dinosauromorphs would find it impossible to make.

Long hind limbs, an anteriorly extended ilium (with added sacrals) and a relatively short torso are bipedal landmarks. A slender tail is common to all. Note that only Cosesaurus has a distinct pectoral girdle shared with flapping taxa (birds, bats and pterosaurs).

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Brusatte SL Niedźwiedzki G and Butler RJ 2011. Footprints pull origin and diversification of dinosaur stem-lineage deep into Early Triassic. Proceedings of the Royal Society of London, Series B, 278, 1107-1113.
Lovelace DM and Lovelace SD 2012. Paleoenvironments and paleoecology of a Lower Triassic invertebrate andvertebrate ichnoassemblage from the Red Peak Formation (Chugwater Group). Palaios 27:636-657. doi: 10.2110/​palo.2012.p12-011r

2 thoughts on “Four Bipedal Lineages in the Early to Mid Triassic

  1. Something was certainly going on in the Mid-to-Late Triassic that suddenly favored bipedality in multiple lineages, but I doubt that it had anything to do with the water.

    The title of your post suggests that some of these bipedal lineages were from the Early Triassic, but none is older than the Middle Triassic:

    Cosesaurus — Middle Triassic lizard
    Scleromochlus — Late Triassic (Carnian) croc
    Marasuchus — Middle Triassic (Ladinian) theropod
    Tropidosuchus — Middle Triassic (Ladinian) chanaresuchid.

    Are there any obligatory bipeds from the Early Triassic?

  2. Early Triassic bipedal footprints matched to Cosesaurus were reported by the referenced authors. So at least Cosesaurus and kin were present. It should also be remembered that when you find a fossil, you’re probably finding one of a million or more specimens with origins much earlier. With Scleromochlus nesting near the base of the Archosauria (crocs + dinos) that puts it or its sisters in the Early Triassic. Thanks. I should have mentioned this in the text.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.