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Marine organisms

Marine invertebrates suffered the greatest losses during the P–Tr extinction. Evidence of this was found in samples from south China sections at the P–Tr boundary. Here, 286 out of 329 marine invertebrate genera disappear within the final two sedimentary zones containing conodonts from the Permian. The decrease in diversity was probably caused by a sharp increase in extinctions, rather than a decrease in speciation.

The extinction primarily affected organisms with calcium carbonate skeletons, especially those reliant on stable CO levels to produce their skeletons. These organisms were susceptible to the effects of the ocean acidification that resulted from increased atmospheric CO. Organisms that relied on haemocyanin or haemoglobin for transporting oxygen were more resistant to extinction than those utilising haemerythrin or oxygen diffusion. There is also evidence that endemism was a strong risk factor influencing a taxon's likelihood of extinction. Bivalve taxa that were endemic and localised to a specific region were more likely to go extinct than cosmopolitan taxa. There was little latitudinal difference in the survival rates of taxa.

Among benthic organisms the extinction event multiplied background extinction rates, and therefore caused maximum species loss to taxa that had a high background extinction rate (by implication, taxa with a high turnover). The extinction rate of marine organisms was catastrophic. Bioturbators were extremely severely affected, as evidenced by the loss of the sedimentary mixed layer in many marine facies during the end-Permian extinction.

Surviving marine invertebrate groups included articulate brachiopods (those with a hinge), which had undergone a slow decline in numbers since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which very nearly became extinct but later became abundant and diverse. The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification; more heavily calcified organisms with simpler breathing apparatuses suffered the greatest loss of species diversity. In the case of the brachiopods, at least, surviving taxa were generally small, rare members of a formerly diverse community.

Conodonts were severely affected both in terms of taxonomic and morphological diversity, although not as severely as during the Capitanian mass extinction.

The ammonoids, which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective extinction pulse 10 million years before the main event, at the end of the Capitanian stage. In this preliminary extinction, which greatly reduced disparity, or the range of different ecological guilds, environmental factors were apparently responsible. Diversity and disparity fell further until the P–Tr boundary; the extinction here (P–Tr) was non-selective, consistent with a catastrophic initiator. During the Triassic, diversity rose rapidly, but disparity remained low. The range of morphospace occupied by the ammonoids, that is, their range of possible forms, shapes or structures, became more restricted as the Permian progressed. A few million years into the Triassic, the original range of ammonoid structures was once again reoccupied, but the parameters were now shared differently among clades.

Ostracods experienced prolonged diversity perturbations during the Changhsingian before the PTME proper, when immense proportions of them abruptly vanished. At least 74% of ostracods died out during the PTME itself.

Bryozoans had been on a long-term decline throughout the Late Permian epoch before they suffered even more catastrophic losses during the PTME.

Deep water sponges suffered a significant diversity loss and exhibited a decrease in spicule size over the course of the PTME. Shallow water sponges were affected much less strongly; they experienced an increase in spicule size and much lower loss of morphological diversity compared to their deep water counterparts.

Foraminifera suffered a severe bottleneck in diversity. Approximately 93% of latest Permian foraminifera became extinct, with 50% of the clade Textulariina, 92% of Lagenida, 96% of Fusulinida, and 100% of Miliolida disappearing. The reason why lagenides survived while fusulinoidean fusulinides went completely extinct may have been due to the greater range of environmental tolerance and greater geographic distribution of the former compared to the latter.

Cladodontomorph sharks likely survived the extinction because of their ability to survive in refugia in the deep oceans. This hypothesis is based on the discovery of Early Cretaceous cladodontomorphs in deep, outer shelf environments. Ichthyosaurs, which are believed to have evolved immediately before the PTME, were also PTME survivors.

The Lilliput effect, the phenomenon of dwarfing of species during and immediately following a mass extinction event, has been observed across the Permian-Triassic boundary, notably occurring in foraminifera, brachiopods, bivalves, and ostracods. Though gastropods that survived the cataclysm were smaller in size than those that did not, it remains debated whether the Lilliput effect truly took hold among gastropods. Some gastropod taxa, termed "Gulliver gastropods", ballooned in size during and immediately following the mass extinction, exemplifying the Lilliput effect's opposite, which has been dubbed the Brobdingnag effect.