Embryology
The embryonic development of both emus and cassowaries occur in eggs that have to be incubated by a parent. This exhibits relatedness because it shows that both species have the same behavioral reproductive activities and similar reproductive systems derived from a common ancestor. These organ systems function the same way with little to no change or differentiation. During the same incubation period of approximately seven and a half weeks, the embryos of both species start out looking identical but eventually develop slightly different characteristics that are unique to their own species. This demonstrates relatedness by proving that if the ratite embryos require the same amount of time to develop and have many resemblances in structure and appearance, then they must have similar stages of development and thus evolutionary phases.
Fossil Record
The fossil record provides a “snapshot” of the past and the types of organisms that were alive during that time. It can show evolution through the evidence of gradual change in the specimens. Fossils that share characteristics of different species hints at them descending from a common ancestor and being related. Fossils of ancestral struthioniformes share the similar characteristic of a long neck with current-day struthioniformes.
Pseudocrypturus cercanaxius
This specimen is found to be from 56 million years ago during the Paleocene Epoch and early Eocene Epoch. Pseudocrypturus cercanaxius has a long neck that is bending down and around its wing.
Paleotis weigelti
This fossil is from 40 million years ago during the middle of the Eocene Epoch. Its necks is long and curving around and downwards.
Struthio asiaticus
This specimen is though to come from 5.333 to 0.0117 million years ago during the early Pliocene Epoch to early Holocene Epoch. This creatures’s tall neck is extended at a considerable length.
Ornithomimus edmontonicus
Ornithomimus edmontonicus arose from 76.5 to 66.5 million years ago during the late Cretaceous period. The neck is slightly shorter than the other specimens and appears to have a dinosaur-like tail. It may have been part of a species that was in between a dinosaur and a bird.
Dromornis stirtoni
This specimen is from 11.6 to 3.6 million years ago during the late Miocene Epoch and early Pliocene Epoch. The neck of the Dromornis stirtoni is stretched out and holds many similarities in length to modern struthioniforme.
Gastornis gigantea Paleocene – Eocene, 56–45 Ma
Gastornis gigantea is from 45 to 56 million years ago during the Paleocene Epoch and Eocene Epoch. Like the specimen before this, Gastornis gigantea has a long neck that is comparable to ostriches and emus.
Anatomy and Physiology
All of the organisms within the Order Struthioniformes have wings that cannot be used to fly. They vary in size and shape depending on the species. After millions of years of evolution, the wings of ratites became vestigial structures. These wings are remnants of the wings from their once-flying ancestors. Over the years, ratites no longer needed to fly due to a change in their environment. As a result, their bodies adapted to the change, and their wings are now only used 
for maneuvering at high speeds or camouflage. Another structure, or lack of it, that prevents all struthioniformes from flying is the keel. The keel is a ridge on the sternum where the wing muscles are attached. This trait is one of the main reasons why struthioniformes are classified as a separate order from other birds that are able to fly.
The wings are also a homologous structure found within the Order Struthioniformes. The bone structure of ostrich wings is the same as those of a emu, kiwi, cassowary, and rhea. All of the appendages of these organisms exhibits the trend of have the same major bone components that constitutes the whole structure. These anatomical similarities demonstrate the evidence of their descendant from a common ancestor. The original use for wings in the ancestor bird was to fly, but evolution and speciation resulted in ratites having a different use for their wings. All species of Struthioniformes use their wings as rudders to help them change directions at high speeds, to help stabilize themselves and maintain balance, insulation and temperature control, mating rituals, scaring predators, etc.
Ostrich Skeletal Structure
Chromosomal Analysis
Every living thing consists of chromosomes that are composed of tightly coiled strands of proteins and DNA. That DNA contained within the chromosomes holds the specific instructions that make each type of living creature unique. Similarities and differences in chromosomes are the reason why organisms have similar or different characteristics than other organisms within the same taxonomic group. Ostriches and rheas are found to have 78 autosomes and two sex chromosomes (ZZ or ZW), and emus have 80 autosomes and two sex chromosomes (ZZ or ZW). These numbers are very close in range to one another and illustrates that the species are closely related.
In relation to humans, who have 44 autosomes and two sex chromosomes (XX or XY), struthioniformes are very different in many aspects. They both have some of their own distinct features that the other does not have, such as hands, beaks, feathers,
Human Karyotype
hair, wings, etc. This is a result of the difference in chromosome number, the sizes of the chromosomes, the location of the gene on the chromosome, the traits that is expressed by those genes, and the dissemblance of sex chromosomes. Humans have about half the number of chromosomes as ostriches, emus, and rheas. In these struthioniformes, the sex chromosome pattern of ZZ produces a male and the pattern ZW produces a female. However, in humans the sex chromosome pattern of XY produces and male and the pattern XX produces a
Ostrich Karyotype
female. The sex determination system of humans and struthioniformes are reversed for one another, and the X and Y chromosomes show no signs of shared genes with the Z and W chromosomes .
Order Struthioniformes 