Mesomallien

Mesomallien is a term used in speculative biology and science fiction to denote a hypothetical order or clade of mid-sized, semi-aquatic vertebrates. Descriptions place mesomallien individuals at roughly 0.8 to 1.8 meters in length, with a streamlined body, flexible finned limbs, and a broad, flat tail. They are commonly depicted as having smooth, mucus-coated skin and double respiration, combining lungs with gill-like structures or cutaneous gas exchange to thrive in varying aquatic environments. Many accounts describe a sensory system combining a lateral line with acute vision and olfactory cues, enabling effective navigation in murky waters.

Mesomallien are typically shown inhabiting estuaries, riverine deltas, mangrove systems, or shallow coastal zones, on Earth-like worlds or in exoplanetary settings with temperate to subtropical climates. They are often portrayed as omnivorous, exploiting aquatic plants, invertebrates, and small vertebrates, with a flexible dentition suitable for varied diets. Social behavior ranges from solitary to cooperative foraging, with pack- or clan-based structures in several depictions; some authors emphasize parental care and cooperative rearing of offspring.

In fictional taxonomies, mesomallien serve as examples of convergent evolution toward a middle-sized, semi-aquatic niche, illustrating how similar ecological pressures can yield comparable morphologies across different lineages. They are frequently used in world-building to explore ecosystem dynamics, predator–prey interactions, and the impacts of climate change on coastal habitats. The concept appears in field guides, field notes in speculative biology, and various sci-fi settings, with variations in anatomy and life history across works.