Ptychodus

The distinctive crushing / grinding teeth of Ptychodus are amongst the most familiar of Chalk fossils, but the taxonomic placement of the Ptychodontid sharks is an ongoing point of controversy. They possess a problematic combination of characters which suggest a number of non-compatible affinities - their tooth microstructure like Hybodontids, their tooth morphology like rays (Batomorphii), their jaw structure like Heterodontiformes, and their vertebrae like Lamniformes. They are widely thought of as rays by amateur collectors, though the academic community seems to have dismissed this option, largely viewing them as relatively 'primitive' euselachians, close to the Hybodontids. However, recent work based on vertebra from well preserved US material has concluded that they are neoselachians (Everhart & Caggiano, 2004).

Spectacular complete and articulated sets of Ptychodus teeth are encountered occasionally in the US. Associations of teeth and vertebrae are also known from the UK, as well as exceptionally preserved jaw sets retaining the teeth (see below).

Generally teeth comprise of a large robust crown, approximating a rounded square in plan view, with a blocky rounded-cube shaped root tucked underneath. Tooth form is highly variable and a perhaps surprising number of Ptychodus species are recognised from the Chalk, each defined on overall tooth form. The main species concepts can be summarised as follows:

- P. decurrens – This is the common form from the Grey Chalk and lowest White Chalk and is highly variable. Flattened to bulbous in lingual profile, but without a depressed marginal area. Crown crossed by a large number of transverse ridges, often finely bifurcated towards the margins, and passing diffusely into a narrow and poorly defined marginal area with granular ornament. A var. depressus form is recognised with a flattened central area, and a var. multistriatus form where ridges join at the ends to form concentric rings. [Cenomanian to Turonian].

- P. marginalis – Central area is broad, flattened and moderately elevated in lingual profile, with a broad depressed marginal area. The central area is crossed by a large number of longitudinal ridges. [White Chalk: Turonian to Coniacian].

- P. mammillaris – Central area is narrow, rounded and highly elevated in lingual profile, with a depressed marginal area. The central area is crossed by a small number of longitudinal ridges. [White Chalk: Turonian to Coniacian].

- P. rugosus – Like P. mammilaris, but with poorly defined ridges on the central area passing into a rugose ('lumpy') ornament. [White Chalk: Coniacian to Santonian].

- P. latissimus – Low rounded profile in lingual view. The central area is crown crossed by a relatively small number of highly pronounced longitudinal ridges, triangular in section. Relatively wide marginal area, not depressed, and with fine granular ornament. P. 'dixoni' form with finer ridges to central area, P. 'paucisulcatus' form with very few ridges. [White and grey Chalk: Cenomanian to Santonian].

- P. polygyrus - Low in profile, with broad, highly flattened central area, and typically very narrow marginal area. Central area ornamented by a relatively small number of pronounced longitudinal ridges, often joined to one another at the ends giving concentric ringed effect. [White Chalk: Coniacian to Santonian].

- P. concentricus - Like P. polygyrus, but with a highly domed central area. Scarce in the UK. [Grey Chalk: Cenomanian].

- P. oweni - Low and rounded in lingual profile, with no depression of the narrow marginal area. Ridged ornament of the central area passes into a finer ridged ornament of the marginal area, all loosely arranged in a part-radial, part-longitudinal fashion. [Grey Chalk: Cenomanian].

- P. mortoni - A US form, virtually unknown from the UK. Rounded to sub-triangular in side profile. Central area bears a radial ornament of coarse ridges converging towards a central apex, with a broad margin with granular ornament.

A	C
B	D

1). (A,B) Ptychodus cf. marginalis - (A) oral (biting surface) and (B) side views of a large tooth - note broad margin and raised central area (x1.4, Booth Museum, BMB 016994, by kind permission of John Cooper); (C, D) Ptychodus latissimus; (C) oral (biting surface) and (D) side views of a large tooth - note weakly raised central area crossed by a relatively small number of broad ridges (x1.4, Booth Museum, BMB 016998, by kind permission of John Cooper).

2). Ptychodus decurrens; oral (biting surface) views of an exceptional articulated portion of the lower dentition, fused by pyrite / marcasite. The front of the animal presumably lies towards the top of the image. The single central row of larger teeth indicates that this is the lower dentition (Grey Chalk of Southerham, near Lewes, Sussex, Willett Collection, Booth Museum, BMB 007331, by kind permission of John Cooper); (A) general view of specimen (x0.9); (B) detail, with central row of enlarged teeth on the right (x1.5).

3). (A,B) Ptychodus decurrens - (A) oral (biting surface) and (B) side views of a typical tooth - note narrow, indistinct margin and slightly rounded character (x1.6, Booth Museum, BMB 016996, by kind permission of John Cooper); (C, D) Ptychodus cf. polygyrus - (C) oral (biting surface) and (D) side views of a large tooth - note flattened profile, and central ridges looped-back and joined at through the margins giving a concentric pattern (x1.6, Booth Museum, BMB 011247, by kind permission of John Cooper).

4). Ptychodus decurrens - partial articulated palate from the upper jaw. Tooth impressions at the base of the specimen indicate that significantly more teeth were originally present (x1.0, Grey Chalk, Maidstone, Kent, NHMUK (British Museum (Natural History) London) PV P40056). Image © 2012 The Natural History Museum, by kind permission.

6). Ptychodus decurrens var. multistriatus - associated teeth - note that the fine ridges link to the sides in crown view to form a concentric pattern (x1.3, Grey Chalk, Kent, NHMUK (British Museum (Natural History) London) PV P2681). Image © 2012 The Natural History Museum, by kind permission.

7). Ptychodus decurrens var. multistriatus - (A) Crown (biting surface), (B) lateral (side), and (C) labial views - note that the fine ridges link to the sides in crown view to form a concentric pattern (x1.8, Grey Chalk, Kent, NHMUK (British Museum (Natural History) London) PV P2681). Images © 2012 The Natural History Museum, by kind permission.

8). (A,B) Ptychodus latissimus - (A) oral (biting surface) and (B) side views of a typical tooth - note gently raised central area with broad pronounced ridges (x2.1, Booth Museum, BMB 016995, by kind permission of John Cooper); (C, D) Ptychodus latissimus - (A) oral (biting surface) and (B) side views of a 'paucisulcatus'-type tooth - note small number of broad pronounced ridges over central area, with pronounced triangular profiles (x1.8, Booth Museum, BMB 016997, by kind permission of John Cooper).

9). Ptychodus polygyrus - A unique specimen constituting a near complete association of teeth from the lower jaw, now reconstructed in approximate life arrangement (Seaford Chalk, Margate, Kent, Slade & Stilwell Collection, currently on display at Monkton Nature Reserve); (A) overview of specimen (x0.7); (B) Detail of teeth from the central row - note the classic concentric patternation to the ridges of the central area (x1.3); (C) Detail of tooth in central row showing damage to the tooth (pre-mortem?) (x4.0). Images used by kind permission of Ron Stilwell.

10). Ptychodus polygyrus - Low angle view of the specimen figure above - note flattened profile to the teeth (distinguishes Ptychodus polygyrus from Ptychodus concentricus (see below). Note also that teeth in the central row have well developed marginal areas, whilst marginal areas are highly reduced to absent in the more lateral rows (x0.7). Image used by kind permission of Ron Stilwell.

11). Ptychodus polygyrus - (A) Crown (biting surface), and (B) lateral views of a very large tooth - note that the relatively coardridges link to the sides to form a concentric patterns(x1.3, Northfleet, Kent, NHMUK (British Museum (Natural History) London) PV P33231). Images © 2012 The Natural History Museum, by kind permission.

12). Ptychodus polygyrus - (A) Crown (biting surface), and (B) lateral (side) views of a very large tooth, associated with the teeth embedded in flint seen below (x1.3, White Chalk, Grays, Essex, NHMUK (British Museum (Natural History) London) PV P12863). Images © 2012 The Natural History Museum, by kind permission.

13). Ptychodus polygyrus - associated teeth embedded in flint (x0.8, White Chalk, Grays, Essex, NHMUK (British Museum (Natural History) London) PV P12863). Image © 2012 The Natural History Museum, by kind permission.

14). Ptychodus marginalis - a large association of teeth (x0.8, Beachy Head, Sussex, NHMUK (British Museum (Natural History) London) PV P6141). Image © 2012 The Natural History Museum, by kind permission.

15). Ptychodus mammillaris; an association of teeth from the Grey Chalk of Sussex (Willett Collection, Booth Museum, BMB 007345, by kind permission of John Cooper); (A) general view of specimen (x0.9); (B) detail (x1.5).

16). Ptychodus decurrens; part and counterpart of a unique juvenile specimen, preserving both the upper and the lower cartilaginous jaws (palatoquadrate and Meckel's cartilage) with accompanying articulated dentitions. This specimen was the basis for Woodward's much reproduced Ptychodus reconstruction (1904) (Grey Chalk, Glynde, near Lewes, Sussex, Willett Collection, Booth Museum, BMB 008524, by kind permission of John Cooper); (A) counterpart: oral (biting surface) view of the frontal portion of the lower dentition and Meckel's cartilage (x1.1); (B) part: ventral / external (underside) view of the Meckel's cartilage with oral view of the medial portion of upper dentition (x1.1).

17). Ptychodus decurrens; more views of the above specimen. (A) dorsal (upperside) view of the combined part and counterpart. The central and right-hand portions of cartilage are the palatoquadrate, whilst the left-hand portion is the Meckel's cartilage. Teeth exposed in oral (biting surface) view (on the left) are the lower dentition, whilst those exposed in root (underside) view (on the right) are the upper dentition (x1.1); (B) detail of the lower dentition, with central row of enlarged teeth on the right. Two teeth of the upper dentition (root view) are seen far right (x5.0).

18). Ptychodus decurrens; more views of the above specimen. (A) oral (biting surface) view of the medial portion of the upper dentition (x3.5); (B) oral view of the lower dentition (left hand side of image) and root view of the upper dentition (right hand side of image) (x2.5).

19). Ptychodus decurrens; a unique specimen with the articulated partial lower and upper dentitions of a moderate sized individual, the front of the animal presumably lying towards the top of the image. The teeth exposed in oral (biting surface) view are of the upper dentition, whilst the overlying teeth exposed in root view are of the lower dentition. The teeth are fused by pyrite / marcasite (White Chalk of Brighton, Sussex, Willett Collection, Booth Museum, BMB 008605, by kind permission of John Cooper); (A) general view of the specimen (x0.8); (B) detail, mainly showing the left hand side of the upper dentition (x1.5).

20). Ptychodus decurrens; more views of the above specimen; (A) Side view, with the left hand portion of the upper dentition in the fore of the image. Note that in this image the specimen is in the reverse of life position, with the biting surface of the upper dentition facing upwards. In this view, the front of the animal is to the right of the image (x1.0); (B) Detached medial portion of the lower dentition (x2.0).

21). Ptychodus decurrens - A remarkable and seemingly overlooked specimen preserving the cartilaginous jaws, with the teeth retained in semi-articulation (x1.2, 'Willett Collection', Booth Museum, BMB 008605 X & XIII, by kind permission of John Cooper): (A) Overview of the main part of the specimen. The teeth exposed seem to comprise (mainly? / entirely?) the left and central portion of the lower dentition. The cartilage bottom-centre of the image is likely the left-hand Meckel's cartilage, whilst the cartilage exposed in section top-right of the image is questionably the right-hand palatoquarate (it is unclear whether the upper dentition is actually present); (B) The second part of the specimen. The large tooth seems to have been inserted by the preparator from another specimen. The cartilage is questionably part of the palatoquadrate.

22). Ptychodus decurrens - Details of the above specimen: (A) Detail of the cartilage (inner surface of left-hand Meckel's cartilage?) showing ornament of ring-tubercles (x2.5); (B) Detail of the lower(?) dentition (x2.0).

23). Ptychodus mammillaris - A very large association of teeth (x0.8, Booth Museum, BMB 011272, 016993, by kind permission of John Cooper); (A) The main part of the association; (B) A further part of the association.

24). Ptychodus mammillaris - Series of teeth in (A) oral and (B) side views - note the broad margins and prominently raised central areas crossed by a large number of relatively fine ridges ((A) x1.4, (B) x1.6, Booth Museum, BMB 016999, by kind permission of John Cooper).

26). Ptychodus concentricus - (A) Crown (biting surface) view and (B) lateral (side) views of a large tooth - note the concentric rings formed by the ridges of the crown, the crown's sub-conical profile, and highly reduced marginal area / band (x1.8, Zig-zag Formation, Grey Chalk, Holborough, Kent, NHMUK (British Museum (Natural History) London) PV P10261). Images © 2012 The Natural History Museum, by kind permission.

27). Ptychodus concentricus - (A) Lingual and (B) labial views of a large tooth - note the the crown's sub-conical profile, and highly reduced marginal area / band (x1.8, Zig-zag Formation, Grey Chalk, Holborough, Kent, NHMUK (British Museum (Natural History) London) PV P10261). Images © 2012 The Natural History Museum, by kind permission.

28). Ptychoduc oweni - (A) Crown (biting surface) and (B) lateral (side) views - note that the ridges of the crown radiate towards the margins, and the diffuse transition to a poorly defined, narrow marginal area (x2.0, Zig-zag Formation, Grey Chalk, Halling, Kent, NHMUK (British Museum (Natural History) London) PV P39125). Images © 2012 The Natural History Museum, by kind permission.

29). Ptychodus mammillaris; oral (biting surface) views of two large teeth; (A) part of an association from the Grey Chalk of Sussex - see figure 3 below (x2.5, Willett Collection, Booth Museum, BMB 007345, by kind permission of John Cooper); (B) isolated tooth sourced from an old collection (x1.7, Randell Collection, RR1488).

30). (A) Ptychodus mammillaris; part of an association of teeth (x2.0, White Chalk?, Burham, Kent, in the collection of Maidstone Museum, MM 21-622-625, by kind permission of Ed Jarzembowski); (B) Ptychodus rugosus; detail of an association of teeth, figured below (x3.0, Belle Tout Marls, Seaford Chalk, Seven Sisters, West Sussex, Randell Collection RR1245).

31). Ptychodus rugosus; an association of teeth (Belle Tout Marls, Seaford Chalk, Seven Sisters, West Sussex, Randell Collection RR1245); (A) general view of specimen (x0.8); (B) oral surface of a 'nascent' (incompletely developed) tooth (x3.2).

32). Ptychodus rugosus; details of the above association (Belle Tout Marls, Seaford Chalk, Seven Sisters, West Sussex, Randell Collection RR1245); (A) x4.0; (B) x3.0.

33). Ptychodus cf. mortoni - (A) Crown (biting surface) view and (B) side view profile of an incomplete tooth. This is perhaps the only record of this form from the UK, but it is well known from the US - Note pointed crown with ornament of radial ridges passing into a marginal area with granular ornament (x2.9, Chalk, Winchester area, in the Collections of Oxford Museum). Images (A, B) taken from Woodward (1902-1912, Plate LIV, figures 1 and 1a respectively).