Harpoceratinae

Harpoceratinae
Temporal range: Upper Pliensbachian to lower Bajocian[1]
Fossil shell of Harpoceras subplanatum from Isère (France), on display at Galerie de paléontologie et d'anatomie comparée in Paris
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Suborder: Ammonitina
Superfamily: Hildoceratoidea
Family: Hildoceratidae
Subfamily: Harpoceratinae
Neumayr 1875
Genera

at least 10, see text

Synonyms

Polyplectinae Venturi, 1981

Harpoceratinae is an extinct subfamily of cephalopods belonging to the family Hildoceratidae. Ammonites of this subfamily had involute and compressed shells with strong keels. Keel might be rarely missing, but this is considered to be an abnormality. This has been observed both in Cleviceras and Harpoceras and called as genus Monestieria, which is now known to be invalid (into this invalid genus were added also abnormal unkeeled specimens of genus Protogrammoceras). Another example is Pseudolioceras, whose unkeeled specimen was used as type for description of invalid genus Praehaploceras. Oxyconic forms of this subfamily (Polyplectus and Sphenarpites) does not have any keel. Ribs were single, but in some genera also bifurcating with shapes from sigmoidal to falcate. Sometimes, shell can have only striate ribs or is smooth (Sphenarpites and older specimens of Eleganticeras and Ovaticeras). Tubercules are rare. Dimorphism is known in some genera and it is observable mostly in size. Macrochons can be 4, to 6 times larger than microconchs. As an example, macroconchs of Pseudolioceras are more than 150mm in diameter, while adult microconchs are only 35-50mm. There are big size differences even within dimorphs. Specimens ca be 2, or sometimes even 3 times bigger than other specimens of same dimorph.[1]

Genera

At least 10 genera is considered to be members of this subfamily.[1] This number can be increased, as some authors might consider valid some genera, which are by other authors considered to be synonyms of these 10. For example, Maconiceras is sometimes listed within this subfamily,[2] while by other authors, it is considered to be a synonym of Harpoceras.[1]

Sometimes, more genera is referred to this subfamily, as some authors does not recognize validity of subfamily Protogrammoceratinae.[2] This would include genera:

In 1981 a new subfamily has been erected and named Polyplectinae, which is now considered to be a synonym of Harpoceratinae. Following genera were part of this subfamily:[3][1]


Distribution

Fossils of this subfamily have been found in the layers of upper Pliensbachian - lower Bajocian age. Distribution for subfamily is worldwide. All genera have been found in northern hemisphere, but only fossils of Harpoceras, Osperleioceras and Polyplectus comes also from southern hemisphere.[1]

Timeline and evolution

Approximate timeline of Harpoceratinae genera. Sphenarpites is not shown, as its occurrence in time is not exactly known (it is lower Toarcian). Taffertia might have died out in Exaratum subzone which, if true, would mean it would have only about 1/3 of existence time.

If Protogrammoceratinae shall be considered a valid subfamily, then Harpoceratinae is of polyphyletic origin with at least 2 main lineages, both evolving from Protogrammoceratinae. First one started in upper Pliensbachian (Spinatum zone) when Tiltoniceras evolved from Lioceratoides, or Protogrammoceras. In Toarcian, during lower part of Exaratum subzone of Falciferum zone it evolved into Eleganticeras, which soon gave rise to Cleviceras. Polyplectus evolved from this genera. Second lineage originated from Protogrammoceras, which has been predecessor for Harpoceras. Three genera evolved from this genus - Osperleioceras (from Harpoceras subplanatum), Ovaticeras (from Harpoceras falciferrum) and Pseudolioceras (probably also from Harpoceras falciferrum). Pseudolioceras is the only known member of this subfamily that lived also in middle Jurassic. Last common ancestor of both these lineages might have been Protogrammoceras that lived in Spinatum zone of Pliensbachian or sooner. Origin of the remaining 2 genera is probably unknown. Taffertia lived in Falciferrum zone of early Toarcian (maybe only in Exaratum subzone). Sphenarpites is known from single specimen that comes from early Toarcian.[1][4][5]

References

  1. 1 2 3 4 5 6 7 8 M. K. Howarth 2013. Treatise on Invertebrate Paleontology, Part L, Revised, Volume 3B, Chapter 4: Psiloceratoidea, Eoderoceratoidea, Hildoceratoidea.
  2. 1 2 Jérémie Bardin, Isabelle Rouget, Mohamed Benzaggagh, Franz Theodor Fürsich & Fabrizio Cecca (2015) Lower Toarcian (Jurassic) ammonites of the South Riffian ridges (Morocco): systematics and biostratigraphy, Journal of Systematic Palaeontology, 13:6, 471-501, DOI: 10.1080/14772019.2014.937204
  3. ↑ Venturi F, Bilotta M (2008) New data and hypotheses on early Jurassic ammonite phylogeny. Rev Paléobiol 27:859–901
  4. ↑ M. K. Howarth 1992. HILDOCERATIDAE IN THE LOWER JURASSIC OF BRITAIN
  5. ↑ J. Rai, S. Jain 2013. Pliensbachian nannofossils from Kachchh: Implications on the earliest Jurassic transgressive event on the western Indian margin
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