Well, “brood care,” really. The embryos were developing inside their eggs under the carapace of the female Waptia. Instead of being dropped somewhere shortly after fertilization and left to whatever fate …
From Science Daily:
Waptia fieldensis is an early arthropod, belonging to a group of animals that includes lobsters and crayfish. It had a two-part structure covering the front segment of its body near the head, known as a bivalved carapace. Caron and Vannier believe the carapace played a fundamental role in how the creature practised brood care.
“Clusters of egg-shaped objects are evident in five of the many specimens we observed, all located on the underside of the carapace and alongside the anterior third of the body,” said Caron.
The clusters are grouped in a single layer on each side of the body with no or limited overlapping among the eggs. In some specimens, eggs are equidistant from each other, while in others, some are are closer together, probably reflecting variations in the angle of burial and movement during burial. The maximum number of eggs preserved per per individuals probably reached 24. More.
Here’s the earliest from my notes:
190 million years ago: A nest site for dinosaur eggs suggests that dinosaurs kept an eye on their young. The findings “suggest that Massospondylus returned to the site repeatedly, laying their eggs in groups in the earliest-known case of ‘colonial nesting.'” (Middle Jurassic reptiles may have cared for young as well.)
Note: Parental care is hardly unheard of amid reptiles today. The alligator lays eggs whose embryos don’t seem very developed, but anyone who thinks they won’t care for or defend their eggs and hatchlings is probably not the best customer for life insurance.
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Here’s the abstract:
Brood care, including the carrying of eggs or juveniles, is a form of parental care, which, like other parental traits [ 1 ], enhances offspring fitness with variable costs and benefits to the parents [ 2 ]. Attempts to understand why and how parental care evolved independently in numerous animal groups often emphasize the role of environmental pressures such as predation, ephemeral resources, and, more generally, the harshness of environment. The fossil record can, in principle, provide minimum age constraints on the evolution of life-history traits, including brood care and key information on the reproductive strategies of extinct organisms. New, exceptionally preserved specimens of the weakly sclerotized arthropod Waptia fieldensis from the middle Cambrian (ca. 508 million years ago) Burgess Shale, Canada, provide the oldest example of in situ eggs with preserved embryos in the fossil record. The relatively small clutch size, up to 24 eggs, and the relatively large diameter of individual eggs, some over 2 mm, contrast with the high number of small eggs—found without preserved embryos—in the bivalved bradoriid arthropod Kunmingella douvillei from the Chengjiang biota (ca. 515 million years ago). The presence of these two different parental strategies suggests a rapid evolution of a variety of modern-type life-history traits, including extended investment in offspring survivorship, soon after the Cambrian emergence of animals. Together with previously described brooded eggs in ostracods from the Upper Ordovician (ca. 450 million years ago), these new findings suggest that the presence of a bivalved carapace played a key role in the early evolution of parental care in arthropods.
(paywall) – Jean-Bernard Caron, Jean Vannier. Waptia and the Diversification of Brood Care in Early Arthropods. Current Biology, December 2015 DOI: 10.1016/j.cub.2015.11.006