A shrimp-like critter that lived 508 million years ago carried its eggs around under its hard upper shell. According to a new Current Biology study, their fossils provide the oldest evidence for a form of parental care called brood care – like kangaroo moms carrying their joeys in a pouch or termites taking care of larvae in mounds.
Called Waptia fieldensis, this early arthropod was first discovered in the Canadian Burgess Shale fossil deposit a century ago. It can reach 7.5 centimeters (2.9 inches) in length, and the front part of its body, near the head, is capped with a two-part shell called the “bivalved” carapace. In females, this structure can be between 1.75 centimeters (0.68 inches) and 2.06 centimeters (0.81 inches) long.
After examining 1,845 Waptia specimens collected from Yoho National Park in British Columbia, Royal Ontario Museum’s Jean-Bernard Caron and colleagues discovered clusters of eggs with embryos preserved within the body of five specimens. The eggs were all located on the underside of the carapace, along the anterior (or front) third of the animal. They were grouped into two clusters (one on each side of the body), and each cluster consisted of a single layer of eggs with little or no overlap among them. Until now, fossil embryos have always been separated from the adults, Caron explains to IFLScience.
Waptia fieldensis with eggs brooded between the inner surface of its carapace and its body. Danielle Dufault/Royal Ontario Museum
The diameter of the individual eggs ranged between 0.7 and 2.4 millimeters (0.002 inches and 0.09 inches). The smaller ones were round, while the largest ones were elliptical. One female Waptia could carry as many as 24 eggs, which is relatively few. The clutches of some lobsters, for example, can consist of as many as 100,000 eggs (though egg mortality is high as well.)
The small clutch size and relatively large eggs contrast greatly with the high number of small eggs of another bivalved arthropod called Kunmingella douvillei from the Chengjiang biota. This 515 million-year-old early Cambrian creature also carried its young differently; the eggs were attached lower on the body of the female, along three pairs of appendage (while Kunmingella predates Waptia by a few million years, none of the eggs found contained embryos). Additionally, tiny crustaceans called ostracods from the Upper Ordovician around 450 million years ago also brooded eggs, but posteriorly within the carapace chamber.
The presence of these different parental strategies, the authors write, suggests the rapid evolution of a variety of life-history traits – such as extending investment in offspring survivorship – pretty soon after the Cambrian emergence of animals.
Furthermore, these fossils show that the presence of a bivalved carapace played an important part in the early evolution of parental care in arthropods. Not only does it protect the animal and its eggs from damage and predators, the carapace also offers a substrate for the eggs to attach to. And that space in-between the carapace and the body is the best ventilated area, offering a richly oxygenated environment for developing embryos. It’s possible that the carapace originally served a different function, but was then co-opted for brooding.
Waptia fieldensis and preserved eggs with embryos. Royal Ontario Museum