Rhea: Looks like an ostrich. Nests with a cassowary in the LRT.

Nesting Rhea with Casuarius agrees with prior analyses.

Figure 1. Rhea skull shares more traits with Casuarius, the cassowary.

Figure 1. Rhea skull shares more traits with Casuarius, the cassowary. Note the confluent naris, antorbital fenestra, orbit and temporal fenestrae.

 

Rhea americana (extant) is the largest rhea of South America. The juvenile has a large thumb claw and unfused fingers that fuse with maturity. Essentially this is a giant tinamou.

Figure 2. The cassowary skull shows the mesethmoid (yellow green) is greatly expanded from its original flat appearance in Rhea.

Figure 2. The cassowary skull shows the mesethmoid (yellow green) is greatly expanded from its original flat appearance in Rhea (Fig. 1).

Casuarius casuarius (Brisson 1760; up to 2m tall) The extant cassowary is one of the few birds with a bony crest or casque on its head. Derived from a tinamou like Rhynchotus, this flightless omnivore feeds mainly on fruit, shoots and small seeds. The crest shape variws with the individual. Females are larger and more brightly colored. The manus is a fused (undifferentiated) vestige.

Figure 2. Rhea skeleton with enlarged manus and pes.

Figure 2. Rhea skeleton with enlarged manus and pes. The inset shows an unfused manus, fused in the adult.

 

References
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

wiki/Rhynchotus
wiki/Cassowary
wiki/Elegant_crested_tinamou