Embryogenesis of the Manus of the Laysan Albatross (Phoebastria Immutabilis)

Birds are established descendants of therapod dinosaurs and, like them, contain hands with 3 fingers. The structure of their hand consists of three phalanges, supported by a series of metacarpals and 2 carpals, which differs from the 6 carpals in their dinosaur ancestors. The homologies of the phalanges and carpals have been an area of debate amongst therapod paleontologists, who believe they correlate with the first three fingers of dinosaurs, and a radiale and ulnare, and bird embryologists, who believe it is instead the middle three fingers, and a pisiform and scapholunare. The Laysan Albatross (Phoebastria immutabilis) and its long developmental period displays temporary primitive characteristics which can provide an insight into adaptations made throughout evolution. In this study, histologically sectioned forelimbs of Laysan albatrosses that were stained with Masson' trichrome throughout stages of ontogeny ranging from embryonic stages 29 to 37 were analyzed using 3D reconstructive methods. In stages 31 to 37, The presence of a 5th metacarpal supports claims that dinosaurs' middle three fingers evolutionarily developed into the fingers of birds, and the reduction of a cartilage condensation medially to the outside carpal closest to the ulna upholds evidence that the ulnare is actually a pisiform and that the ulnare degenerates in development just as bird embryologists believe. The radiale developed normally as believed by paleontologists in stages 31 to 37. 2 cartilage condensations in stage 32 formed a semilunate carpal (a feature in dinosaurs) by stage 36, which fuses with two metacarpals before hatching, a feature developed in birds. While primitive features in the developing albatross exist, the hand isn't quite as primitive as was thought. Further developmental evidence could be collected via fluorescent immunohistochemistry for developmental signals to search for additional primitive characteristics in early embryogenesis.