Eozygodactylus: a basal roadrunner, not a ‘songbird’

Today: a little, long-legged bird from the Green River formation
Eozygodactylus americanus (Weidig 2010; Eocene; 53mya; FMNH PA726) Originally considered a perching songbird related to Passer (the sparrow), this long-legged, lakeshore wading bird (think: sandpiper), nests with Geococcyx (the roadrunner) in the large reptile tree (LRT, 1224 taxa). Like Geococcyx (the roadrunner), pedal digit 4 is also retroverted (= zygodactyl). The right (lower) femur appears to be broken in two.

Figure 1. Eozygodactylus in situ with two lighting schemes and bones colorized.

Figure 1. Eozygodactylus in situ with two lighting schemes and bones colorized. If pterosaurs had survived the K-T event, this is where we would find them.

As a basal taxon,
Eozygodactylus was derived from the predatory bird clade, the toothed bird clade and, more distantly, Megapodius (the mound builder). Eozygodactylus has a relatively large skull and short rostrum, which might suggest a possible juvenile stage. No related adult taxa (Figs. 4, 5) have similar proportions.

Figure 2. Eozygodactylus reconstructed from figure 1.

Figure 2. Eozygodactylus reconstructed from figure 1. Not sure how deep the sternum is. Sister taxa can fly, but prefer to walk.

Closely related basal neognath birds include
long-legged screamers (genus: Chauna) at the base of the sparrow/chicken/parrot clade and long-legged stone curlews (genus: Burhinus) at the base of crows + jays + woodpeckers + swallows. By convergence, woodpeckers also have a zygodactylus pes. Apparently this trait comes and goes, without a single long zygodactyl lineage.

Figure 3. Psophia the trumpeter in vivo and skeleton.

Figure 3. Psophia the trumpeter in vivo and skeleton, a close relative of Eozygodactylus, larger and without a zygodactylus foot.

The nesting of Eozygodactylus
at the base of a basal bird clade supports the antiquity of long-legged shore birds (Figs. 4, 5) in the evolution of birds, distinct from traditional and DNA tree topologies.

Figure 1. Click to enlarge. Duck origins recovered by the LRT. Duck descendants were long-legged walkers and later waders.

Figure 4. Click to enlarge. Duck origins recovered by the LRT. Duck descendants were long-legged walkers and later waders.

Figure 2. Basal bird phylogeny based on the LRT (morphology)

Figure 5. Basal bird phylogeny based on the LRT (morphology)

References
Weidig I 2010. New Birds from the Lower Eocene Green River Formation, North America. Records of the Australian Museum 62:29-44.

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 )

Google photo

You are commenting using your Google 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.