Euhoploceras is an extinct genus of ammonite (a subclass of fossil cephalopods) that lived during the Middle Jurassic Period. It was first named and described by the paleontologist S. S. Buckman in 1913.
Temporal Range: Upper Aalenian to Lower Bajocian stages of the Jurassic, approximately 174.7 to 168.6 million years ago.
Classification:
- Kingdom: Animalia
- Phylum: Mollusca
- Class: Cephalopoda
- Order: Ammonitida †
- Family: Sonniniidae (or Ammonoidea awaiting allocation)
- Genus: Euhoploceras (sometimes treated as a subgenus of Sonninia)
Taxonomic History: Buckman originally described Euhoploceras as the earliest sonniniid, considered transitional between the Hammatoceratidae and Sonniniidae families. Some later authors have treated it as a subgenus of Sonninia, designated as Sonninia (Euhoploceras). Synonyms include Nannoceras (Buckman, 1923), Sherbornites (Buckman, 1923), and Stiphromorphites (Buckman, 1923).
Morphology: Euhoploceras ammonites range from evolute to moderately involute forms, with rectangular to ovate, slightly compressed whorl sections. The venter is subtabulate, often slightly bisulcate (grooved), with a low hollow keel. Ornamentation varies widely: some species have strong ribs and well-developed tubercles or spines throughout ontogeny, while others become smooth in later stages. Macroconchs (the larger sex, presumed female) were more robust, while microconchs (the smaller sex, presumed male) were much smaller with persistent ribbing and spatulate lateral lappets.
Key Species (after Sandoval & Chandler, 2000):
- Euhoploceras acanthodes — strongly ornamented with tubercles and strong ribs throughout ontogeny.
- Euhoploceras marginatum — tuberculate inner whorls with relatively strong ribs persisting to the outer whorls.
- Euhoploceras modestum — ribbed inner whorls becoming smooth or only slightly ornamented in later stages.
- Euhoploceras adicrum — a younger species with a type horizon in the Laeviuscula Zone.
Paleoecology: Euhoploceras was a marine, fast-moving nektonic (free-swimming) carnivore with well-developed vision. Its shell was composed of aragonite.
Biostratigraphy and Distribution: Euhoploceras ranges from the uppermost Aalenian (Concavum Zone, Limitatum Subzone) to the Lower Bajocian (Laeviuscula Zone). Fossils have been found across a wide geographic area, including: the United Kingdom (southwest England), Spain (Betic Cordillera), France, Germany, Morocco (Atlas Ranges), Italy, Hungary, Saudi Arabia, China (Tibet), Australia (Western Australia), Canada (British Columbia, Yukon), the United States (Alaska, Oregon, California), and Chile. This wide distribution indicates that during the late Aalenian to earliest Bajocian, English and Mediterranean paleobiogeographical regions were connected.
References:
- Buckman, S. S. (1913). Yorkshire Type Ammonites, Volume II.
- Sandoval, J. & Chandler, R. B. (2000). "The sonniniid ammonite Euhoploceras from the Middle Jurassic of south-west England and southern Spain." Palaeontology, 43(3): 495–532.
- Sepkoski, J. J. (2002). "A compendium of fossil marine animal genera (Cephalopoda entry)." Bulletins of American Paleontology, 363: 1–560.
- Howarth, M. K. (2013). Treatise on Invertebrate Paleontology, Part L, Ammonoidea.
