P. dmanisensis

The femur of various specimens from P. dmanisensis measure from 380 – 405 mm in length and a have minimum shaft circumference of 220 – 240 mm, notably more robust than those of extant ostriches and other species within Struthio. The latter value can be used to calculate body weight, yielding values of 382 – 472 kg depending on the specimen.^[4][3] Other weight estimates for this species from the femur have been 453 and 462 kg, using different formulas.^[1]

Pachystruthio indet.

An incomplete femur from the Nihewan Formation (China) early Pleistocene in age has been assigned to Pachystruthio indet. and has smaller measurements than P. dmanisensis, with a length of 340 mm and minimum shaft circumference of 199 mm, which gives it a weight estimate of 300 kg. However, even this relatively smaller femur is more robust than those of modern ostriches and even those of larger extinct species such as Struthio anderssoni.^[4]

Eggs

Pachystruthio laid eggs much larger than those of the extant ostriches, with estimated egg size from shells assigned to P. pannonicus measuring an 22 x 18 cm in length and width and an estimated weight of 4.25 kg.^[8] Eggshells associated with P. pannonicus are also much thicker than those of modern ostriches (2.6 – 3.4 mm).^[1] P. transcaucasicus has also been associated with very thick eggshell fragments (2.9–3.3 mm)^[8]

Mikhailov and Zelenkov et al note eggshells known from the late Miocene to early Pleistocene in the eastern part of Kazakhstan, being 2.5–3.0 mm or even up to 3.5 mm may also belong to P. transcaucasicus primarily due to its age, thickness, and geographical occurrence. Before the description of the Nihewan femur, they considered this the most eastern known range of Pachystruthio.^[2]

Comparisons

Species of Pachystruthio are among the largest of birds within their family.^[2]

P. transcaucasicus has been described as larger than Struthio wimani, Struthio brachydactylus, Struthio karatheodoris, and Struthio camelus (the extant common ostrich).^[9]

Weight estimates for P. dmanisensis are the highest of any species within their order (Struthioniformes)^[3][4]

The femur assigned to Pachystruthio indet. is slightly larger than that of Struthio anderssoni.^[4]

The egg of P. pannonicus is one of the largest eggs from its order, although surpassed in thickness by those of some possibly related oogenera, such as Diamatornis, Namornis, and Psammornis.^[8] Additionally, an estimated egg size for Psammornis rothschildi is significantly larger (28 x 21 cm) in dimensions than that of P. pannonicus.^[10][2]

Size

Some authors have proposed that larger body weight may have been an adaptation for tougher, low-nutrition food of arid environments, noting increased body weight in Dromornithidae has been related to increased aridity of Australia. This connection is also present in Eogruidae and has been proposed for Struthio anderssoni, whose large body weight may be explained by an increase in aridity in Inner Asia during cold climatic phases.^[1] This hypothesis is contested, as Buffetaut and Angst note that in China where Pachystruthio lived was not especially arid. Additionally, Struthio anderssoni, although large, was smaller than Pachystruthio despite that it apparently lived under a more arid climate, and living ostriches that are also smaller lived and used to live in arid environments.^[4]

Locomotion

The robust femur of P. dmanisensis, compared to those more slender of other Pleistocene ostriches such as Struthio oldawayi, may indicate it was less adapted for fast running.^[3] Similarly, Zelenkov et al. proposed Pachystruthio may not be as good a runner as modern ostriches due to its weight.^[4] However, it has also been suggested that P. dmanisensis was a better runner than the giant species of Aepyornithidae which possessed an even more robust femur.^[1]

Interactions with humans

Buffetaut and Angst note whether early hominins of the Nihewan Basin hunted the giant ostrich is uncertain, but a bird twice the weight of the living ostrich (Pachystruthio indet.) could not have been an easy prey, although egg collecting may have been less hazardous.^[4]

Other authors note that these large birds might have been a source of meat, bones, feathers, and eggshell for early hominin populations.^[1]