Researchers studying the 430-million-year-old fossil of a new early arthropod species have discovered 10 tiny arthropods tethered to it with long, individual threads. The findings, described in Proceedings of the National Academy of Sciences this week, reveal a unique mode of brood care that reflects the variety of strategies we see in today’s arthropods, the diverse group that includes insects, spiders, and crabs.
Yale’s Derek Briggs and colleagues analyzed a single, three-dimensional arthropod fossil uncovered from the Silurian Herefordshire Lagerstätte deposit in the U.K. The specimen’s head is covered by a shield with a snout-like projection, and it has four pairs of head appendages: a long, stout antenna (h1), a limb for manipulating food (h2), and two branched appendages (h3, h4) on either side. Its elongated trunk has 11 segments that are covered by tergites (or dorsal plates) with long, slender lateral spines, and there are limbs coming off each of the trunk segments. A short tail-like projection called the telson bears long, parallel paired structures called cerci. The body is 9.5 millimeters long, the antennae are about as long as the body, and the cerci are 7.3 millimeters long.
Ten tiny arthropods – shaped like flattened lemons between 0.5 and 2 millimeters long – are attached to the tergites with threads up 3.3 millimeters long. These likely represent a brooding strategy where juveniles are latched onto an adult, probably the female. Each of the juveniles has about six pairs of appendages – in contrast to the 15 pairs of the adult. They must gain segments as they transition to their adult form.
This combination of characters is different than any known arthropod (living or fossil), and the team named it Aquilonifer spinosus. The name refers to the resemblance between the tethered juveniles and kites, and it echoes the title of Khaled Hosseini’s 2003 novel "The Kite Runner." The genus combines "aquila," for eagle or kite, with the suffix "fer," meaning carry, to create "aquilonifer," or kite bearer. The species name comes from "spinosus" for spiny, referring to the long spines on the tergites.
There’s no exact parallel of this brooding strategy among living arthropods, and evidence of brooding in general in fossil arthropods is rare. This form of brood care kept the offspring safe from predators by keeping them close to the parent. Aquilonifer likely lived on the seafloor, where it used its antennae to sweep the sediment for food, which it then manipulated with its second head appendages.