It is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes.
The only vertebrate which does that is the Hagfish, which is a craniate, but is not universally accepted to be a vertebrate.
The only other vertebrates capable of detecting infrared radiation are boas, pythons and pit vipers, all of which have pit organs.
The only vertebrates lacking a spleen are the lampreys and hagfishes.
A recent study disclosed that Dolichopteryx longipes is the only vertebrate known to use a mirror (as well as a lens) in its eyes for focusing images.
Another barreleye species, the brownsnout spookfish, is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes.
Lampreys and hagfishes are the only vertebrates without jaws.
Amphibians are the only vertebrates to go through metamorphosis.
It is the only vertebrate known to be capable of this.
Chimaera are the only vertebrates to retain traces of a third pair of limbs.