Should “megafauna” take a plural verb or a singular one?
No, they are not wrong. They are simply using collective nouns in a way that you are unfamiliar with. This is called notional concord, or sometimes notional agreement. This happens all the time in English. That makes this question a stealth duplicate of “Are collective nouns always plural, or are certain ones singular?”
The OED says of megafauna that it can take plural concord as a collective noun:
Chiefly Palaeontology. With plural concord: large vertebrates, esp. the larger mammals; spec. those of a particular epoch or region. Also (with singular concord): a group, class, community, or list of such animals. Cf. microfauna n. 1.
Ecology. = macrofauna n.
This is no surprise.
The word fauna is a collective noun, which means it can take plural verbs but does not require them. Here are examples from Google Books:
-
Preliminary results suggest that the endemic spider fauna are confined to the intact natural forests in the south-west and the central highland region.
—The Fauna of Sri Lanka: Status of Taxonomy, Research, and Conservation -
The law further defines protected, controlled and common aquatic fauna in terms of the three-category classification system established in the law on Aquatic Animals and Wildlife: protected aquatic fauna are classified as Category I; controlled ...
—Review of statutory and customary law in the Xe Champhone Ramsar site -
Sand fauna, silty-sand fauna, and silty-clay fauna are common to this zone.
—Ocean Dumping of Sludge in New York Bight: Environmental Impact Statement
This works just like uncountably many other collective nouns that are singular in form yet often plural in use: group, team, community, couple, pair, council, club, company, troop, crowd, gang, et cetera ad infinitum.
Evidence from Google Books suggests that the term “megafauna” has been used both both in the singular and the plural forms at least since the ‘60s
Megafauna
n. pl. megafauna or megafaunas:
Large or relatively large animals of a particular region, period, or habitat: Pleistocene megafauna; crabs and other aquatic megafauna.
Megafauna:
(mass noun) The large mammals of a particular region, habitat, or geological period.
From Mass Extinction :
Some Australian researches maintain that megafauna are absent in Australian fossil sites dated younger than ~46 ka, the suggested date of colonization by humans.
From Biodiversity:
The megafauna is still well represented, even if 50 varieties disappeared around 40 000 years ago. Africa is the continent that currently has the most diversified fauna of large herbivores, including the elephant, hippopotamus, rhinoceros, etc. – groups that were plentiful on other continents before the Pleistocene extinctions.