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Description
Marine reptiles were a defining feature of the Mesozoic from the Triassic all the way to the end of the Cretaceous. While most were piscivorous or hunted other marine reptiles, some- particularly plesiosaurs -did engage in filter feeding. One extreme example is the pliosaurid Xenodonoides lethargicus ("lethargic strange toothed one"), with unsual dentition among pliosaurids shows it was specialized for filter feeding unlike its more predatory relatives.
The teeth of xenodonoides may seem totally alien to those unfamiliar with zoology, but their structure actually bares a strong resemblance to those of a modern day animal of totally different descent: the crabeater seal. These phocids are specialized to feed on Antarctic krill and it is therefore likely that xenodonoides had a similar lifestyle, although as it is unclear of when krill evolved it is unclear what kind of crustaceans it was feeding on. As a pliosaurid, their large jaws would allow it to take in large mouthfuls of these crustaceans, although it is not certain how similar it was to baleen whales in its feeding.
As it was a dedicated filter feeder, xenodonoides would have been preyed upon by the largest of its relatives, although adults would be immune to predation from most other threats. It is not clear exactly what its social structure was like as well, or if it was even social at all. Given that parental care is known in plesiosaurs, it is likely that some pliosaurs and even xenodonoides did engage in parental care but exactly how social it was as an adult is unclear at best.
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Two for one day!

Description
Marine reptiles were a defining feature of the Mesozoic from the Triassic all the way to the end of the Cretaceous. While most were piscivorous or hunted other marine reptiles, some- particularly plesiosaurs -did engage in filter feeding. One extreme example is the pliosaurid Xenodonoides lethargicus ("lethargic strange toothed one"), with unsual dentition among pliosaurids shows it was specialized for filter feeding unlike its more predatory relatives.
The teeth of xenodonoides may seem totally alien to those unfamiliar with zoology, but their structure actually bares a strong resemblance to those of a modern day animal of totally different descent: the crabeater seal. These phocids are specialized to feed on Antarctic krill and it is therefore likely that xenodonoides had a similar lifestyle, although as it is unclear of when krill evolved it is unclear what kind of crustaceans it was feeding on. As a pliosaurid, their large jaws would allow it to take in large mouthfuls of these crustaceans, although it is not certain how similar it was to baleen whales in its feeding.
As it was a dedicated filter feeder, xenodonoides would have been preyed upon by the largest of its relatives, although adults would be immune to predation from most other threats. It is not clear exactly what its social structure was like as well, or if it was even social at all. Given that parental care is known in plesiosaurs, it is likely that some pliosaurs and even xenodonoides did engage in parental care but exactly how social it was as an adult is unclear at best.
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Two for one day!