Nature has imbued its mothers with many surprisingly strange ways to reproduce and care for their babies.
Some moms, like crocodiles and sharks, can skip the whole sex thing, using an adaptation that can be traced back to dinosaurs to clone themselves and deliver a 'virgin birth.'
Scorpion mothers carry still-soft-shelled babes on their backs, reminding us that even arachnids can be adorable – never mind that a peckish mama might decide to devour a scorpling or two.
But exploring the evolutionary roots of these diverse maternal methods is tough, because reproductive tissues decay quickly after an animal's death.
Now, an incredible discovery published in Scientific Reports offers some rare insight: an international research team has described a 125-million-year-old 'pregnant' shellfish with preserved soft tissues, including its itty-bitty babies.
Scientists have searched for a specimen like this for decades.
"This is the earliest known fossil evidence that these shellfish cared for and protected their developing young. Until now, this reproductive strategy was known only from living species," says Martin Munt, a curator at the Dinosaur Isle Museum in the UK and a visiting researcher at the University of Portsmouth.
The ancient shellfish, a creature not generally imagined as motherly, is a bivalve. It belongs to a huge group of double-shelled mollusks, comprising more than 40,000 fossil and 50,000 extant species, including a who's who of seafood-dish toppings, such as clams, oysters, scallops, and mussels.
The fossilized shellfish specimens were found on the Isle of Wight, an island off the southern coast of England and a site famous for its plentiful Cretaceous fossil finds, including what may have been one of Europe's largest terrestrial terrors.
"Not only does this discovery provide a rare glimpse into how ancient freshwater shellfish reproduced," Munt says, "but it also helps explain how these animals successfully adapted to life in rivers and lakes millions of years ago."
The creatures studied herein, Margaritifera valdensis, are distantly related to today's freshwater pearl mussels, which encompass up to 1,000 living species.
They also have one of the most unique reproductive strategies among all invertebrates, worthy of a xenomorph.
First, a basic birds-and-bees scenario occurs: Males release sperm into the water, which the females siphon and use to fertilize eggs inside a brood chamber that sits within a specialized portion of their gills.
In addition to shelter, the mother mollusks provide their growing young with calcium, a mineral that may have helped preserve these specimens.
The young then develop into larvae which, like parasites, must infect fish to mature. The larvae attach to gills and fins and grow under a fish's skin, eventually dropping away to form new mussel beds.
(Comparatively, the mammalian dinner-and-movie reproductive technique seems much more straightforward.)
"These new fossils demonstrate that this complex reproductive strategy had already evolved by the Early Cretaceous," says Aleksandra Skawina, an expert in fossil bivalves at the University of Warsaw and study co-author.
This research also elucidates the origin of a dark, mysterious 'molluskite' material first described nearly 200 years ago.
"We found that this material is actually made up of fossilised soft tissues and reproductive structures that have been exceptionally preserved by minerals," explains study co-author Rafael P. Lozano, a geochemist at the Geological and Mining Institute of Spain.
Unfortunately, these amazing animals are currently among the most threatened creatures. Pollution, construction projects, exploration, climate change, and other factors are destroying freshwater systems.
While we can applaud their sexual adaptability, mollusks are often overlooked as drab little shells – even though they represent the second-largest phylum of invertebrates after arthropods.
Related: Condors Had 'Virgin Births' Despite Having Access to Fertile Mates, a First For Birds
And they are a key component of modern freshwater ecosystems, so we would do well to watch them closely: they might already be ringing alarm bells of Earth's sixth mass extinction.
This research was published in Scientific Reports.
This article was fact-checked by Rachel Garner and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.