Fossils have been in the news this month, e.g., stories about a Paleozoic sea scorpion: "Claws for Thought: The Fossilised Pincer of the Largest Arthropod Ever Found" and "Fossil Shows Sea Scorpions Stretched to 8 Feet" (original article "Giant Claw Reveals the Largest Ever Arthropod" in Biology Letters). The abstract of the original article describes the fossil scorpion as "pterygotid eurypterid ('sea scorpion') Jaekelopterus rhenaniae." The article has both "Arthropoda" and "Eurypterida" as keywords.
Works about this Paleozoic sea scorpion are classed in 565.493 Eurypterida. The Relative Index entry "Scorpions—paleozoology" points to the broader number 565.4 Chelicerata, which is a subdivision of 565 Fossil Arthropoda; however, this particular scorpion belongs in the narrower number for eurypterids. Neither this particular species nor its genus (Jaekelopterus) approximates the whole of the subclass Eurypterida; hence there is no possibility to add further.
There have also been news stories about Nigersaurus, e.g., "'Mesozoic Cow' Rises from the Sahara Desert" and "Dino's look is hard to swallow" plus a web site and an exhibit "Extreme Dinosaur—Nigersaurus: Africa's Long-Necked Fern Mower" (original article "Structural Extremes in a Cretaceous Dinosaur" in Public Library of Science ONE). (Incidentally, the Cretaceous Period is part of the Mesozoic Era.) The abstract of the original article speaks of "dinosaur, Nigersaurus taqueti. . . rebbachisaurid sauropod . . . diplodocoids, the larger clade of sauropods that includes Nigersaurus." The article itself uses the term "sauropodomorph" several times and has a diagram that gives a clear picture of the relationships among related taxa (Figure 4. Calibrated phylogeny of diplodocoid sauropods).
Works about Nigersaurus are classed in 567.913 Sauropodomorpha (Herbivorous Saurischia), which has the note "Including Diplodocus." The number appears under the centered entry 567.912-567.915 Specific dinosaurs. The one subdivision of 567.913 (567.9138 Apatosaurus [Brontosaurus]) is for a kind of dinosaur closely enough related to Nigersaurus to appear in figure 4 in the article, but at the opposite side of the diagram.
The work in hand is always the first source of taxonomic information needed to classify the work, but sometimes additional help is welcome. The Manual note 579–590 Taxonomic nomenclature and sources of information does not explicitly give advice for 560 Paleontology Paleozoology; however, one of the sources mentioned there—Encyclopaedia Britannica (print or online version)—is often helpful to classifiers navigating the taxonomic nomenclature in the Dewey 560s.