syn·​ap·​sid sə-ˈnap-səd How to pronounce synapsid (audio)
: any of a subclass (Synapsida) of terrestrial vertebrates (such as the pelycosaurs and therapsids) having a single pair of lateral temporal skull openings

Note: Traditional classifications typically consider synapsids to be mammal-like, extinct reptiles ancestral to mammals. Cladistic classifications also consider these extinct forms ancestral to mammals but do not categorize them as reptiles.

synapsid adjective

Examples of synapsid in a Sentence

Recent Examples on the Web Researchers unearthed a large cache at the site, including the remains of ancient reptiles, amphibians and synapsids, the precursors to mammals. Taylor Nicioli, CNN, 21 Mar. 2024 Life on land throughout the Permian Period, which lasted from about 298 million to 252 million years ago, was dominated by synapsids, the evolutionary precursors to mammals or protomammals. Jeanne Timmons, New York Times, 22 May 2023 Dimetrodon was a synapsid, sometimes called a proto-mammal or a mammal-like reptile. Gemma Tarlach, Discover Magazine, 31 May 2016 At the end of the Permian, there was a sort of bottleneck that caused only smaller synapsids to survive into the Triassic. John Wenz, Popular Mechanics, 22 Nov. 2018 Four-limbed vertebrates were becoming more diverse and among them were primitive amphibians, early reptiles and synapsids, the group that would one day include mammals. Fox News, 23 Jan. 2011

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'synapsid.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


New Latin Synapsida, from Greek syn- + apsid-, apsis arch, vault — more at apsis

First Known Use

1940, in the meaning defined above

Time Traveler
The first known use of synapsid was in 1940

Dictionary Entries Near synapsid

Cite this Entry

“Synapsid.” Dictionary, Merriam-Webster, Accessed 20 Jun. 2024.

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