The origin of giant birds: Struthio, the ostrich (a giant tinamou)

Earlier we looked at other giant birds and their progenitors. The day before yesterday we looked at the embryo of a giant ostrich.

Today
we look at Struthio, the ostrich, and Rhynchotus, the tinamou (Fig. 1). And let’s start noticing some of the differences flightless giants have in common, distinct from their volant ancestors.

  1. smaller skull
  2. more cervicals and a longer neck
  3. ribs tilt anteriorly at the bottom where they meet the smaller sternum
  4. center of balance (foot placement) beneath the hips, not the pectorals
  5. smaller pectoral girdle
  6. smaller forelimbs, especially distally
  7. longer and more robust posterior pelvis
  8. hind limbs more robust, including large patella and cnemial crest (proximal tibia)

Despite these similar traits,
the large reptile tree (LRT, 1119 taxa; Fig. 2) still splits the giants apart and lumps them with their diverse ancestors.

Figure 1. Skeletons of Struthio, the ostrich, and Rhynchotus, the tinamou.

Figure 1. Skeletons of Struthio, the ostrich, and Rhynchotus, the tinamou for comparison and to scale.

Struthio camelus (Linneaus 1758) The extant ostrich is the largest and fastest living bird. Weight: up to 320 lbs (115 kg). Height: 9 feet (2.8m). Speed:43 mph (70 km/h). It is most closely related to other flightless extant birds. Here it nests as a sister to Gallus. This is the only living bird with two toes. Skull courtesy of Digimorph.org and used with permission.

Rhynchotus maculicollis (Temminck 1815, Gray 1867) is a genus from the Tinamidae, or tinamous. Here it nests basal to the much larger Struthio and Casuarius as one of the most primitive of all living birds, transitional to all other extant birds. So here Rhynchotus is a basal ratite, not a tinamou. It prefers to run, but can still fly. Note the long legs and small feet.

Figure x. Bird giants in the bird subset of the LRT.

Figure 2. Bird giants in the bird subset of the LRT.

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.
Temminck 1815. Histoire naturelle generale des pigeons et des gallinaces.
Accompagne de planches anatomiques. 3: 552–747

wiki/Ostrich
 wiki/Rhynchotus

 

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