The Hildoceratidae is a family of ammonoid cephalopods from the Lower Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 (U Pliensbachian- Toarcian stages), generally with strongly ribbed, involute shells. They are combined with the Hammatoceratidae (= Phymatoceratidae), Graphoceratidae, and Sonniniidae to make up the Hildocerataceae

Hildocerataceae

Hildocerataceae is a superfamily of compressed or planulate ammonites, some tending to develop acute outer rims; generally with arcuate or sigmoidal ribs. Aptichus were found in place are double-valved....

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As many as seven subfamilies have been included, principle being the Arieticeratinae, Harpocertinae, Hildoceratinae, to which the Treatise, Part L, adds the Bouleiceratinae, Grammoceratinae, and Tmetoceratinae. Donovan et al (1981) includes the same and adds the Leioceratinae which the older Treatise includes in the Graphoceratidae.

The Hildoceratidae are derived from the Polymorphitidae, a family in the Eoderocerataceae

Eoderocerataceae

Eoderocerataceae is a superfamily of true ammonites from the Lower Jurassic, comprising seven phylogenetically related families, characterized in general by having ribbed evolute shells that commonly bear spines or tubercles...

, and gave rise to the Hammatoceratidae and Graphoceratidae.

References

• Arkell, et al,1957. Mesozoic Ammonoidea; Treatise on Invertebrate Paleontology, Part L. Geological Society of America and Univ. Kansas Press.
• Donovan, D.T., Callomon, and Howarth, 1981. Classification of the Jurassic Ammonitina, in The Ammonoidea, Systematics Association special volume 18.
• Hildoceratidae-Paleodb