Protogrammoceras or A rare immigrant from the Tethys

Protogrammoceras (M.) geometricum and Amaltheus bifurcus, both 6 cm

Protogrammoceras (M.) geometricum and Amaltheus bifurcus, both 6 cm

In the stokesi subzone of the Yorkshire coast, there are very few cephalopods other than Amaltheus. There is the very occasional Lytoceras, maybe even a very rare nautilus and a few immigrant ammonites from the Tethys, a new ocean that had begun to form during the triassic (for a visualization of the movement of continents during the ages see e.g. http://www.scotese.com/earth.htm)

One of these immigrant genera from the Tethys is Protogrammoceras, an early member of the Hildoceratidae family of ammonites, that is going to be one of the dominant ammonite families in the Toarcian, with genera like Hildoceras, Harpoceras, Eleganticeras, Hildaites, Pseudolioceras etc.
Much of the evolution leading to Protogrammoceras is assumed to have occurred in the Tethys, so appearance of Protogrammoceras in Yorkshire seems rather “sudden”.
Protogrammoceras is also rather a rare ammonite in Yorkshire, finding good specimen requires intense searching of the hard flat limestone nodules of the stokesi subzone.
There are 2 species that I´ve found so far :
Protogrammoceras (Matteiceras) geometricum (PHILLIPS, 1829)
Protogrammoceras (Matteiceras) nitescens (YOUNG & BIRD, 1828)
There are 2 further species known from Yorkshire (HOWARTH 1992) :

Protogrammoceras (Protogrammoceras) paltum (BUCKMAN, 1922) from the paltum subzone
Protogrammoceras (Protogrammoceras) turgidulum (FUCINI, 1904) from the hawskerense subzone

As usual, if you do have a specimen of these species in your collection, and would allow me to photograph it for the book, please let me know…
Again, M.K. HOWARTH has provided the perfect reference for these ammonites in the form of a monograph of the Palaeontographical Society :”The Ammonite Family Hildoceratidae in the Lower Jurassic of Britain, London, 1992″

The two species mentioned above can be separated by their rib densities (measurements were taken from the specimen shown here):
Diagram of rib density differences between P. (M.) nitescens and P. (M.) geometricum

Diagram of rib density differences between P. (M.) nitescens and P. (M.) geometricum

Protogrammoceras (Matteiceras) nitescens (YOUNG & BIRD, 1828)
P. (M.) nitescens has lower rib densities on the outer whorls, ribs are more angled backwards (rursiradiate) on the outer half of the whorl than
P. (M.) geometricum.
Protogrammoceras (M.) nitescens, 7 cm, "bowl" preparation

Protogrammoceras (M.) nitescens, 7 cm, “bowl” preparation

Protogrammoceras (M.) nitescens, 8 cm, only aperture was visible before prep

Protogrammoceras (M.) nitescens, 8 cm, only aperture was visible before prep

Protogrammoceras (Matteiceras) geometricum (PHILLIPS, 1829)
 
P. (M.) geometricum has higher rib densities throughout, the ribs do not angle backwards as much as P. (M.) nitescens
Protogrammoceras (M.) nitescens (left, 7 cm) and Protogrammoceras (M.) geometricum (right, 6 cm)

Protogrammoceras (M.) nitescens (left, 7 cm) and Protogrammoceras (M.) geometricum (right, 6 cm)

Possible Intermediates
This interesting cluster of 8 Protogrammoceras, 1 Amaltheus stokesi and various bivalves was found in July 2009.
By their rib densities , of the 8 Protogrammoceras 2-3 are P. (M.) geometricum or intermediates, while the other 5 are P.(M.) nitescens.
Cluster of 8 Protogrammoceras, between 6 and 8 cm

Cluster of 8 Protogrammoceras, between 6 and 8 cm

The following 2 large specimen don´t really fall into the rib density range given by HOWARTH with ribs/ whorl over 40, but they are also
larger than the specimen HOWARTH used for his rib density observations.
Large 10.5 cm Protogrammoceras, possible intermediate from P. occidentale ?

Large 10.5 cm Protogrammoceras, possible intermediate from P. occidentale ?

Large 9 cm Protogrammoceras, possible intermediate from P. occidentale ?

Large 9 cm Protogrammoceras, possible intermediate from P. occidentale ?

Identification of species by rib densities is certainly no exact science, but they could also be intermediates from P. (P.) occidentale ?
AndyS
Leave a comment

1 Comment

  1. Steve

     /  December 1, 2012

    Andy… Your doing some great prepwork and some extraordinary finds… Very nice to see….

    Steve

    Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: