Taxonomy

Diagnostic description

An adult macroconch has:

  • a body chamber occupying 0.65–0.81 whorls
  • a mouth-border similar in shape to the growth lines
  • a ventral rostrum, and a forwardly-pointing bulge dorsal of the mid-lateral line
  • evolute whorls, which are quadrate or almost square in cross-section
  • tricarinate and bisulcate venter with prominent ventro-lateral keels
  • strong spiral groove on the whorl, just dorsal of the mid-lateral line
  • curved striae ornamentation, projecting forwards, on the dorsal side of the groove
  • moderate to strong arcuate ribs on the ventral side of the groove

An adult microconch has similar characteristics to that of a macroconch but variations include:

  • the body chamber occupies 0.56–0.63 whorls
  • the mouth-border is similar in shape to the growth lines and is slightly flared
  • there is a ventral rostrum, and a prominent lateral lappet at the spiral groove

Evolution

Ammonoids were a successful and diverse marine group that originated from bactritid nautiloids. 

Their closest living relatives are the coleoids (cuttlefish, squids and octopuses), as well as bearing a very close resemblance to the modern nautilus.

The earliest Hildoceratinae evolved from a Tenuicostatum Zone species of Protogrammoceras, and the genera Hildaites followed by Hildoceras became common throughout the Falciferum and Bifrons Zones.

Look-alikes

Hildoceras bifrons shares some similarities with Hildoceras lusitanicum and Hildoceras laticosta but can easily be distinguished by its marked spiral groove.  

Hilderoceras semipolitum is also similar but differs in being more involute, smoother and with reduced ribs.

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Glossary

Nekton is a free-swimming animals

Evolute and involute are geometrical terms relating to the shape of a curve

Lappets are spoon shaped extensions from the margin of the aperture

Arcuate is curved

Striae are fine lines of ornament

Ventro-lateral keels are ridges (keels) located on the sides of the venter

Tricarinate and bisulcate are two grooves separating the has three ridges (keels)of the venter.  

Venter is  the shell edge around the circumference