An especially bizarre member of the already strange graptolites, Attollograptus aeronauta is a late Silurian relative of the spiral-shaped cyrtograptids.
During their reproductive season these helical colonial animals develop a highly elastic membranous flotation bladder, expanding up to 1m in diameter (~3’3″), which fills with hydrogen gas produced by symbiotic bioluminescent bacteria in their body tissues.
The “balloons” originated as smaller buoy-like structures used to suspend the colony at the surface — as a semelparous species they die shortly after reproducing, so a passive means of holding the dying colony in place while releasing their larvae into the water column was advantageous. Over time the bladders grew gradually larger, and now they’re big enough to actually lift Attollograptus up into the air, safely out of reach of aquatic predators and into the otherwise unoccupied Silurian skies.
These “death flights” are spectacular but brief, usually lasting less than a day before too much of the hydrogen diffuses out of the membrane to maintain lift. (Or air pressure changes at higher altitudes make the “balloon” pop!) But it’s enough time for the winds to send them drifting long distances across the ocean, dispersing their offspring far and wide.