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AndyS

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Amaltheus bifurcus HOWARTH 1958




Size : 2.5" / 6.5 cm
Location : Hawsker
Age : Middle Lias, margaritatus zone, stokesi subzone 

Diagnostics : Usually relatively small ammonite with stong ribs that form chevrons on the keel, discus shaped whorl section in larger specimens. Several transition stages exist between Oistoceras figulinum and Amaltheus bifurcus, varying in whorl section.

Differential diagnostics : A. bifurcus can be distinguished from other Amaltheus species, especially A. margaritatus and A. stokesi by it's stronger ribbing.

Distribution : In Yorkshire the ammonite can be found at Robin Hoods Bay / Hawsker, Staithes 
Relatively rare, especially in sizes above 1".

References : 
Schlegelmilch : Die Ammoniten des sƒÆ’‚¼ddeutschen Lias, Stuttgart 1992
M.K. Howarth : A monograph of the Ammonites of the Liassic Family Amaltheidae in Britain, Palaeontographical Society 1958




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Amaltheus margaritatus DE MONTFORT 1808 




Size : 4.5" / 11.5 cm
Location : Staithes
Age : Middle Lias, margaritatus zone, subnodosus subzone 

Diagnostics : Discus shaped ammonite that can get quite large, forward curving sinuos ribs, strong keel which looks like it's braided. Sometimes spiral striation on whorls.

Differential diagnostics : A. margaritatus can be distinguished from A. stokesi (which range does not overlap) by it's stronger ribbing and it's well differentated keel. A. subnodosus is smaller and differs in showing stronger, tuberculated ribs.
Note : The small ammonites on top of the A. margaritatus pictured above are A. subnodosus.

Distribution : In Yorkshire the ammonite can be found at Robin Hoods Bay / Hawsker, Staithes. Relatively rare.

Synomyms :   Ammonites amaltheus SCHLOTHEIM
                     Ammonites margaritatus MONTFORT
                     

References : 
Schlegelmilch : Die Ammoniten des sƒÆ’‚¼ddeutschen Lias, Stuttgart 1992
M.K. Howarth : A monograph of the Ammonites of the Liassic Family Amaltheidae in Britain, Palaeontographical Society 1958




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Amaltheus stokesi (SOWERBY, 1818)




Size : 8" / 20 cm
Location : Hawsker
Age : Middle Lias, margaritatus zone, stokesi subzone 

Diagnostics : Discus shaped ammonite that can get quite large, keel with strong chevrons but only slightly diffentiated. Sinuos, forward curving ribs. Large specimen show no keel on outer whorl.

Differential diagnostics : A. stokesi can be distinguished from A. margaritatus by it's less differentiated keel and by their different ranges. A. bifurcus has stronger ribbing even at small sizes.

Distribution : In Yorkshire the ammonite can be found at Robin Hoods Bay / Hawsker, Staithes 

Synonyms : Ammonites stokesi SOWERBY
                   Ammonites clevelandicus YOUNG & BIRD
                   

References : 
Schlegelmilch : Die Ammoniten des sƒÆ’‚¼ddeutschen Lias, Stuttgart 1992
M.K. Howarth : A monograph of the Ammonites of the Liassic Family Amaltheidae in Britain, Palaeontographical Society 1958


Smaller (2.5" / 6.5 cm) complete Amaltheus stokesi
Hawsker





Edited by AndyS 2009-08-03 13:33:09

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Amaltheus wertheri (LANGE, 1932)




Size : 1 1/4" / 3 cm
Location : Hawsker
Age : Middle Lias, margaritatus zone, stokesi subzone 

Diagnostics : Small ammonite with flat ribs that fade towards the keel and form tiny crenulations there. Elliptical whorl section, characteristical constriction at the mouth border.

Differential diagnostics : A. wertheri remains small and has reduced ornamentation. The ribs of A. stokesi in small specimen are finer.

Distribution : In Yorkshire the ammonite can be found at Robin Hoods Bay / Hawsker, Staithes 

Synonyms : Proamaltheus wertheri LANGE
                   Amaltheus depressus (QUENSTEDT)
                   Onychoceras wertheri (LANGE)
                   Amaltheus stokesi var. wertheri (LANGE) HOWARTH

References : 
Schlegelmilch : Die Ammoniten des sƒÆ’‚¼ddeutschen Lias, Stuttgart 1992
M.K. Howarth : A monograph of the Ammonites of the Liassic Family Amaltheidae in Britain, Palaeontographical Society 1958

Small crenulations on the keel of A. wertheri




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Amauroceras ferrugineum (SIMPSON 1855)




Size : 3/4 " / 2 cm
Location : Hawsker
Age : Middle Lias, margaritatus zone, subnodosus subzone 

Diagnostics : Small, involute ammonite usually no larger than about 4 cm, no ribbing, just growth lines.
Elliptical whorl section, no keel. Black preservation is very characteristic for Yorkshire.

Differential diagnostics : Amauroceras ferugineum differs from it's potential ancestor Amaltheus wertheri by the loss of ribbing.

Distribution : In Yorkshire the ammonite can be found at Robin Hoods Bay / Hawsker, Staithes 
Relatively common in the subnodosus subzone, but often overlooked.

References : 
Schlegelmilch : Die Ammoniten des sƒÆ’‚¼ddeutschen Lias, Stuttgart 1992
M.K. Howarth : A monograph of the Ammonites of the Liassic Family Amaltheidae in Britain, Palaeontographical Society 1958

3 cm Amauroceras ferrugineum with suture lines




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Amaltheus striatus HOWARTH 1955 

20032007184954.jpg 


Size : 1.5" / 4 cm
Location : Hawsker
Age : Middle Lias, margaritatus zone, subnodosus subzone 

Diagnostics : Small ammonite starting with medium stromg ribbing and small tubercles, ribs fade away at around 3 cm to form fine striations. Well defined but fine braided keel.

Differential diagnostics : A. striatus can be confused with A. subnodosus at small sizes. A. subnodosus has stronger ribs and tubercles also after 3 cm size.

Distribution : In Yorkshire the ammonite can be found at Robin Hoods Bay / Hawsker, Staithes 

Synonyms : Ammonites depressus SIMPSON 1843

References : 
Schlegelmilch : Die Ammoniten des sƒÆ’‚¼ddeutschen Lias, Stuttgart 1992
M.K. Howarth : A monograph of the Ammonites of the Liassic Family Amaltheidae in Britain, Palaeontographical Society 1958




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Amaltheus subnodosus (YOUNG & BIRD, 1828)

20032007190543.jpg 


Size : 2" / 5 cm
Location : Hawsker
Age : Middle Lias, margaritatus zone, subnodosus subzone 

Diagnostics : Small ammonite starting with strong ribbing and small tubercles, strong sinuous ribbing continues after 3 cm diameter, but tubercles stop. Well defined strong braided keel.

Differential diagnostics : A. subnodosus can be confused with A. striatus at small sizes. A. subnodosus has stronger ribs and tubercles also after 3 cm size. At larger sizes A. subnodosus can be confused with A. margaritatus, but is distinguished by stronger ribbing & tubercels on inner whorls.

Distribution : In Yorkshire the ammonite can be found at Robin Hoods Bay / Hawsker, Staithes 

Synonyms : Ammonites nodosus SOWERBY 1815
           Amaltheus nodifer BUCKMAN 1911

References : 
Schlegelmilch : Die Ammoniten des sƒÆ’‚¼ddeutschen Lias, Stuttgart 1992
M.K. Howarth : A monograph of the Ammonites of the Liassic Family Amaltheidae in Britain, Palaeontographical Society 1958




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