Q86. From the perspective of developmental origin, which of the following structures is homologous to a tortoise shell?
- Exoskeleton of a lobster
- Bones of a fish
- Skull of humans
- Feathers of birds
The tortoise shell, or turtle carapace and plastron, originates from modified endoskeletal elements like ribs and vertebrae during embryonic development. This makes it a bony structure internal in origin, distinct from true exoskeletons. Understanding homology here focuses on shared embryonic tissues and developmental pathways among vertebrates.
Correct Answer
Bones of a fish are homologous to the tortoise shell from a developmental perspective.
Both derive from the endoskeleton, specifically modified ribs and neural arches that ossify into protective bony plates.
In tortoises, “axial arrest” halts rib elongation, allowing ribs to broaden and fuse with dermal ossifications for the shell.Option Analysis
Exoskeleton of a Lobster
Lobster exoskeleton is ectodermal, secreted externally as chitinous armor.
Tortoise shell is endoskeletal, from mesodermal ribs and vertebrae, not homologous.
Arthropod exoskeletons evolved separately from vertebrate endoskeletons.Bones of a Fish
Fish bones, including ribs and dermal plates, share mesodermal origins with tortoise shell components.
Embryonic ribs in both undergo similar ossification for skeletal support.
This homology reflects shared vertebrate axial skeleton development.Skull of Humans
Human skull includes dermal (membrane) and endochondral bones from mesoderm.
While both are endoskeletal, skull bones (neurocranium and dermatocranium) differ from rib-derived shell elements.
No direct developmental equivalence to modified thoracic ribs.Feathers of Birds
Bird feathers are ectodermal derivatives from skin follicles, beta-keratin based.
Tortoise shell scutes (keratin overlays) are epidermal but overlie bony endoskeleton, unlike feather homology.
Feathers evolved for insulation/flight, unrelated to skeletal ribs.Option Developmental Origin Homologous to Tortoise Shell? Key Reason Lobster Exoskeleton Ectodermal (chitin) No External secretion vs. internal ribs Fish Bones Mesodermal (endoskeleton) Yes Shared rib/vertebral ossification Human Skull Mesodermal (dermal + cartilage) No Cranial vs. thoracic specialization Bird Feathers Ectodermal (keratin) No Skin appendage, not skeletal This comparison highlights endoskeletal homology in vertebrates, key for evolutionary biology exams.
