Chondroteuthidae Jeletzky, 1965

Evans, David H., Klug, Christian, King, Andrew H. & Page, Kevin N., 2024, Hettangian, Early Jurassic coleoids from West Somerset, SW England-filling a gap in the coleoid record of NW Europe, Acta Palaeontologica Polonica 69 (3), pp. 425-445 : 436-442

Family Chondroteuthidae Jeletzky, 1965 Genus Clarkeiteuthis Fuchs et al., 2013

Type species: Onychoteuthis conocauda Quenstedt, 1849 , by original designation ( Fuchs et al. 2013: 246). From a horizon in the Paltum Subzone , Tenuicostatum Zone , lower Toarcian , Lower Jurassic at Pliensbach, Holzmaden area, Baden-Württemberg, Germany

Clarkeiteuthis montefiori (Buckman, 1830)

Fig. 9–12 .

Material.— Two specimens ( LYMPH 2023 /76 and LYMPH 2023 /78) from the base of Bed 24 of the Planorbis Subchronozone, Planorbis Chronozone, Hettangian Stage , “ Psiloceras Beds ” of the Blue Lias Formation ; foreshore exposures in Doniford Bay , Watchet, UK ( LYMPH 2023 /76, ST 0804 4349; LYMPH 2023 /78, ST 0842 4339). Since there are differences in the preservation of the two specimens and they exhibit different features, they are described separately below.

Description

LYMPH 2023/76 ( Fig. 9 , 10 ): LYMPH 2023/76 is 152 mm long and consists of an incomplete and disturbed array of arm hooks extending over a distance of 24 mm; a proostracum, of which 60 mm is visible, an ink sac 42 mm long and 24 mm wide, and a 25 mm long portion of a crushed phragmocone that has an estimated anterior width of 27 mm and retains three septa. The posterior end of the ink sack overlaps the anterior end of the of the phragmocone by 12 mm. There is a 33 mm gap between the most anteriorly visible portion of the proostracum and the posterior end of the arm hook array.

Except for the ink sac, the specimen is entirely flattened and probably presents a ventral aspect since the proostracum appears to lie beneath the ink sac. Other than the ink sac, there appears to be no evidence of the presence of soft tissue. Exposure to wave activity has eroded large areas of the proostracum although its anterior and posterior ends, which remain under the matrix may conserve its full width.

Arm hooks: Approximately 60 arm hooks are visible, as well as the impressions of as many as 20 hooks that have been eroded away at the anterior end of the cluster. Many of the hooks are incomplete and most have been fractured, making it difficult to determine whether variation in curvature between hooks reflects a morphological diversity or is a consequence of crushing. Of those hooks sufficiently well-preserved to assess their morphology, all possess bilobed bases (or expand towards the base, suggesting the presence of bilobed bases) combined with a nearly straight to weakly curved shaft and an uncinus which becomes increasingly curved towards the tip in some individuals ( Figs. 9A 7 , A 9 , A 11, 11 ). Measured from tip to base, hooks range from 1.5–4.0 mm in length, and the base from 1.1–1.3 mm in width. Where preserved, the base appears to being relatively thicker and more robust in larger hooks. In the only hook suitable for measuring the angle between the base and the inflection on the outer side of the hook is 94°, and that between the inflection and the tip of the uncinus is 60°. The distal ends of several hook appear to be more strongly curved and the angle between the inflection and tip of uncinus will be less than 60°. The smaller hooks are more frequent at the posterior end of the cluster. Although many of the hooks appear to have been disturbed, pairs of hooks belonging to the same arm remain evident, and many as four rows of paired hooks may be present ( Fig. 10 ).

Proostracum: The width of the remaining part of the proostracum is 29.3 mm at the posterior end and 18.9 mm at the anterior. It is at least 0.2 mm thick at the lateral margins. The width at the anterior end is likely to reflect the true width since anterior of this point, the proostracum is covered by matrix, so that the width observed immediately adjacent to the matrix cover is unlikely to have been modified by recent erosion. The dorsal surface of the proostracum faces into the matrix and is therefore not visible. The ventral surface is visibly heavily cracked; it consists of apparently semi-translucent material varying in colour from off-white to pale blue, through to a deep brown. It is iridescent in places. The surface, except where eroded, appears smooth. The brown tinted zone forms a longitudinal band about 2 mm wide. The major cracks are 2.0–1.0 mm apart, while more frequent cracks 0.5–1.0 mm distant form a lattice between the major cracks. The minor cracks are most densely distributed towards the lateral margins away from the brown-tinted zone. The distribution of the cracks suggests that either the cross-section of the proostracum was more strongly curved and/or thinner towards the lateral margins.

Under magnification, parts of the ventral surface show a very thin layer that appears to be extremely finely crystalline. This layer may have originally been extensive, covering the whole ventral surface, but has been lost to erosion. Eroded surfaces beneath this outer layer indicate the presence of a stack of thin laminae, each estimated to be no more than 0.02 mm (20 μm) in thickness. The laminae themselves are slightly inclined to the surface of the proostracum such that they are imbricate. Where the laminae intersect with the surface, they are aligned at approximately 60° to the long axis of the phragmocone and dip to the left anterior as viewed from the dorsal surface. Fine cracks, visible as white lines up to 10 mm long follow this alignment.

A pair of depressions are visible on the surface of the proostracum 8 mm in front of the anterior end of the ink sac. The depression on the right (as viewed from the dorsum) consists of series of curved ridges about 0.25 mm distant with furrows in between ( Fig. 9A 5 , A 6 ). The convex sides of the curves face the anterior and the more anterior ridges partially overlap those that are more posterior, particularly where directed inward from the margin of the proostracum. This structure is about 2 mm wide. The inward directed portions of the ridges look as though they should extend further across the proostracum, although they diminish in that direction. About 5 mm laterally from this structure, a second smaller, less prominent depression and set of curved ridges are visible with the convex sides facing the anterior. These are believed likely to form part of the same structure, which may be interpreted as comprising broadly curving ridges and furrows extending laterally across part of the proostracum.

Thin layers of dark, carbonaceous material are visible in between the larger cracks in the proostracum ( Fig. 9A 5 ). They also form small patches on its ventral surface. Whether this material represents originally organic tissue or leaked ink is unclear, but the carbonaceous material associated with the dorsal surface of the proostracum appears to line its surface. It does not extend into the matrix, and would, therefore, seem likely to represent an originally organic structure. Approximately halfway across the width of the preserved part of the proostracum, a narrow carbonaceous band extending the length of the proostracum is visible. This structure appears to be attached to the dorsal surface of the proostracum and is visible between and below a line of longitudinal cracks ( Fig. 9A 5 , A 10 ). The structure shows several ridges and grooves parallel to its axis suggesting that it may be crumpled and folded. This structure may be the same as that visible in Chondroteuthis wunnenbergi Bode, 1933 ( Fuchs 2019: fig. 4c) and Clarkeiteuthis conocauda (Quenstedt, 1849) ( Fuchs 2019: fig. 5b)

Ink sac: The ink sac is partially crushed, and its surface, as currently preserved suggests that the wall may have been folded or even ruptured during burial. Patches of matrix discoloured by carbonaceous material surrounding the ink sac may be evidence of ink leakage. Overall, the shape of the sac is prolate but tapers into a funnel at the anterior end.

Phragmocone: The phragmocone is poorly preserved. The apical portion is missing while the remaining septae and phragmocone wall are completely crushed and flattened ( Fig.9A 3 ). Based on what little remains of the phragmocone, the apical angle is estimated at 20°. The depth of the two remaining camera varies markedly, with the anterior camera having a depth 0.27% of the phragmocone diameter, and the posterior, a depth of 0.16%, giving an average of 0.23%.

LYMPH 2023/78 ( Figs. 11 , 12 ): The specimen consists of an ink sac 25 mm long, ink duct, mantle, and traces of the body extending over a distance approximately 65 mm anterior to the ink sac. The phragmocone is not visible and any shell or tissue to the posterior of the ink sac was either lost prior to burial or to more recent wave action. With the exception of the ink sac and ink duct, the remaining part of the body is largely flattened. The outline of the mantle is 21 mm wide anterior to the ink sac and is constricted to 9 mm, 21 mm anteriorly. The remainder of the outline of the mantle maintains that width for the rest of its length. A jaw-like structure is visible 30 mm anterior to the ink sack toward the anterior extremity of the body. Part of the arm crown is exposed in a break oblique to the lamination of the matrix on the underside of the block. The length of the specimen from the posterior of the ink sac to the exposed part of the arm crown is about 110 mm.

Between the lateral margins of the presumed mantle, with the exception of the ink sac and ink duct, the body has been compressed to a thickness of 0.3–0.4 mm. However, the surface of the compressed body is uneven and consists of a pair of lateral ridges that outline the margins of the body and the mantle, and a pair of longitudinal ridges on either side of the median axis. The latter diverge posteriorly to join the ridges at the margins, creating a cavitiy either side of the anterior end of the ink sac. A similar feature is seen in a specimen of Clarkeiteuthis conocauda figured by Reitner (2009: 290, fig. 1) from the Toarcian, Lower Jurassic, of Swabia. Reitner (2009) described the remains of gills preserved within the cavities of the Swabian specimen.

The ridges and inter-areas that appear to comprise the remains of the body are composed of a black, hard waxy material that is indistinguishable from the that of the ink sac under reflected white light. Leakage of ink into the mantle cavity was reported by Reitner (2009) and Jenny et al. (2019) for another specimen. It is likely that the material lining the mantle cavity is leaked ink.

There are several flattened conchs of Psiloceras associated with the specimen. At least one individual is impressed into the probable leaked ink at the anterior end of the mantle leaving a mould of the of the conch surface.

Arm hooks: Since the arm hooks are only visible in an oblique fracture on the underside of the specimen, the area where they are visible is too small to determine whether they represent individual arms or a disturbed cluster, although at least one example of paired hooks is visible. Many of the arm hooks are represented by fragments, having been broken by the fracturing of the matrix. Of the less damaged hooks, most are partially obscured by matrix. Two better exposed hooks ( Fig. 11A 6 ), measure respectively 0.42 and 0.38 mm at the base of the shaft and the shafts 1.09 and 1.17 mm long. The basal processes have been broken off in both hooks. In a third hook ( Fig. 11A 6 ), the base is complete and has a width of 0.8 mm. The inner process is longer than the outer process. In the first hook, the uncinus is 0.85 mm long but is incomplete in the second hook. The curvature is moderate. The angle of the distal end of the uncinus is at 51° to the shaft in the first hook. Other incomplete hooks have a more strongly curved uncinus such as that in one hook ( Fig. 11A 6 ) the tip is at 70° to the shaft. Several of the hooks show grooves running along their lateral surfaces. These may have resulted from the compression of the hook and the collapse of the internal cavity associated with the basal opening but could also reflect a primary striation.

Jaws?: About 30 mm in front of the anterior end of the ink sac, a structure tentatively interpreted as a jaw floats in the matrix ( Fig. 11A 5 ). The surface of this structure is worn and has probably been deformed while one lateral half remains largely obscured by matrix. If bilaterally symmetrical, the width of the structure would be 3 mm and its length 1.8 mm. The anterior edge is arcuate, but projects forward on either side of the plane of bilateral symmetry to form a sharp apex (compare Klug et al. 2020). Behind the anterior edge, the surface appears relatively smooth although sediment infills parts of the middle zone and is also present in a zone subparallel to the anterior edge of the structure. This suggests the presence of a cavity under the anterior area. In the posterior part of the structure, approximately along the plane of bilateral symmetry, a narrow ridge extends out to the posterior edge. Beyond the anterior apex and floating isolated in the matrix, there is a needle-like structure 0.35 mm long and 0.03 mm wide at its base. This structure points anteriorly and is roughly aligned with the apex of the anterior edge and the plane of bilateral symmetry.

Mantle and mantle muscle: Details of the outline, extent and general features of the mantle were described above. The mantle musculature is described below.

Mantle muscle is preserved anterior to the ink sac forming a band 18 mm long. Traces of muscle are also visible embedded onto the surface of the ink sac. A whitish layer approximately 0.05 mm thick, underlying the leaked ink, may also represent mantle muscle. If this is the case, then the preserved mantle muscle extends around the circumference of the body in this area. The surfaces of the mantle muscles are worn and pockmarked from the holdfasts of marine algae. Despite the wear, faint striations are visible ( Fig. 11A 7 , A 10 , A 11 ) which ( Fig. 11A 7 ) resolve into semi-translucent bands about 0.01 mm across at higher magnifications; these are bordered by bands of more opaque material tentatively interpreted as the remains of muscle fibres as illustrated by Kear et al. (1995; text-fig 4a, b). On the exposed side of the specimen, the muscle varies in thickness from a thin film of about 0.1 mm to zones where it may reach 0.25 mm.

The orientation of the striations indicates that most of the preserved muscle is probably represented by circular muscle ( Fuchs et al. 2016). However, in several places the striations deviate so strongly from circumscribing the body axis that they are oriented longitudinally ( Fig. 12 ). However, there appears to be no consistent pattern to the orientation of the muscle fibres. Much of the deviation from a circumscriptive distribution appears to be associated with the zones where the layer of muscle is thicker. The boundaries between zones where the muscles are thicker or thinner often exhibit cross-cutting relationships in the directions of the muscle fibres ( Fig. 11A 10 ). The thickening of the muscle combined with the relationships of the striations suggest that the mantle has been deformed and twisted in these zones, possibly caused by postmortem shrinking and tearing of the mantle ( Clements et al. 2017). In narrow bands either side of the ink duct ( Fig. 11A 11 ), muscle tissue with a grain parallel to the body axis extends a short distance anteriorly.

Proostracum:Several small fragments of the proostracum remain. The largest of these ( Fig. 11A 8 ) is 20 mm long by 9 mm wide and is comprised of at least three layers. The lowest layer forms a sheet that is probably no thicker than 10–20 μm. The middle layer is a semitranslucent sheet of a thickness estimated to vary from 0.2–0.3 mm and appears to be thickest posteriorly and toward the middle of its width. Over much of the area, the upper surface of the middle layer is exposed as the uppermost layer is missing. The surface, however, consists of a series of linear striations that are oriented roughly parallel to the axis of the body. The striations have a distinct relief and a density of 40 per mm. At an approximately 0.2 mm spacing, the striae are more prominent, and form narrow longitudinal ribs. The outermost layer is extremely thin (possibly <10 μm) and preserved only in small patches where the prominence of the striae appears to be greater. The proostracum contains a network of large matrix-filled cracks as well as much narrower, incipient, and regularly spaced cracks oriented at approximately 30° to the striae. One edge of this fragment of proostracum overlaps with and appears to lie above the compressed body cavity. The other remaining patches of the proostracum consist of no more than it lowest layer. rest on the body cavity, apparently at the same level as the mantle muscle and although there appears to be a marginal overlap with the mantle muscle in places, it is not possible to determine the order in which they lie. It is possible that muscle tissue is preserved above and below the proostracum.

Ink sac and ink duct: The ink sac is 15 mm long and 9.3 mm wide. It is constricted anteriorly to form an ink duct 1.5 mm in diameter. The surfaces of both the ink sac and the ink duct are covered in a tessellation of cracks, indicating that the contents of both had largely solidified prior to the compression of the rest of the remains of the body.

Remarks.—As the remaining large fragment of proostracum LYMPH 2023/78) may appear similar to the pale laminae interpreted as muscle tissue toward the posterior end of the specimen, the difference in fabric is emphasised here. The former consists of a dense and translucent, probably crystalline material, whereas the latter is only marginally translucent and exhibits a very fine striation that completely penetrates the material. Since this specimen lacks the phragmocone, and the proostracum is too fragmentary for its general overall shape to be deduced, identification relies upon the similarity of the arm hooks to those of Clarkeiteuthis conocauda and C. montefiorei (Buckman, 1880) as well as the characteristic shape of the body with the anterior constriction in the diameter of the mantle.

The muscular mantle is inserted into the lateral margins of the proostracum ( Fuchs et al. 2016: 436). Jenny et al. 2016) referred to the mantle associated with the phragmocone of the specimen of a specimen they described C. conocauda as thin or “skinny” (Jenny et al. 2016: 3). Since the mantle muscle was not performing a locomotory function over such areas, powerful muscle was not required. Although in LYMPH 2023/78 the remains of muscle sheets appear to be at their thinnest over the ink sac and duct, where a few fragments proostracum are also present further adorally, it is not clear from the current state of preservation whether these fragments were located above or below the muscle sheet, or whether these proostracum fragments are at their original location. Furthermore, where muscle sheet is preserved over the ink sac, there is no sign of a proostracum located between the muscle sheet and the ink sac. This is difficult to understand and there may be several explanations that are not necessarily excusive to each other:

(i) the fabrics interpreted as muscle sheets are in fact proostracum; this seems unlikely given the strongly contrasting nature of the fabrics attributed to these structures;

(ii) the proostracum originally underlay the muscle sheet but became separated and was extruded during the partial decay of the mantle; there is some evidence for this in that the largest and apparently thickest remaining proostracum fragment appears to lie outside of the body cavity; and

(iii) the proostracum was originally located above the preserved muscle sheets and the muscle preserved below it was attached to its ventral side; note that the grain of some of the musculature in this region is oriented parallel to the body axis as it would if it represented head or funnel retractors.

Additional material may help resolve this conundrum.

Geographic and stratigraphic range.—Early Hettangian to late Sinemurian of Southwest England.

Figures

Fig.9. Coleoid cephalopod Clarkeiteuthis montefiori (Buckman, 1830) fromthe Early Jurassic, Hettangian Stage of Doniford Bay,Watchet, UK.A. LYMPH 2023/76.A1, image of whole specimen;A2, interpretative sketch mapping out the distribution of the surviving components of the organism;A3, enlargement of posterior end of specimen showing undifferentiated remains of septa and phragmocone wall; A4, detail of weathered surface of proostracum showing imbricate tabular layers; A5, posterior end of proostracum showing cracked surface, patches of carbonaceous material underlying the proostracum (white arrowhead), carbonaceous material in a major crack (black arrowhead), and one of the depressions present on the ventral surface of the proostracum (see also A10); A6, detail of depression on the ventral surface of the proostracum showing the arcuate ridges and furrows (arrowhead) probably formed by the migration of muscle attachments;A7, pair of worn and broken arm hooks (white arrowhead inA9) with strongly curved uncinus;A8,A9, explanatory drawing and image of arm hooks forming remains of brachial crown;A10, carbonaceous material underlying crack (black arrowhead in A5) showing linear features that may be corrugation of an organic membrane; A11, arm hook (black arrowhead inA9) showing broad base and basal processes.

Fig. 10. Coleoid cephalopod Clarkeiteuthis montefiori (Buckman, 1830). LYMPH 2023/76. Sketch of distribution of arm hooks and probable hook pairing with likely traces of arms (dashed lines).

Fig. 11. Coleoid cephalopod Clarkeiteuthis montefiori (Buckman, 1830). LYMPH 2023/78 from the Early Jurassic, Hettangian Stage of Doniford Bay, Watchet, UK.A1, image of whole specimen;A2, interpretative sketch mapping out the distribution of the surviving preserved components of the organism; A3, oblique section through the brachial crown, partially exposed on the underside of the specimen; A4, isolated hooks with bases (white arrowhead inA3) and part of the shaft missing but with more strongly curved uncinus;A5, possible jaw with the rostrum below;A6, arm hooks (black arrowhead in A3) with moderately curved uncinus, long tapering shafts and bases that are broken but indicate the presence of broad basal processes;A7, detail of mantle muscle showing division into bands of finely striated muscle, note also the overlap and discontinuity (arrowhead) suggesting tearing or folding of the mantle; A8, fragment of proostracum overlapping and resting on main body of the mantle cavity and exhibiting longitudinal linear banding; A9, detail of proostacum with coarser linear ridges (arrowheads), between which numerous finely striate, parallel and linear longitudinal ridges are visible;A10, thin layer of mantle muscle on lateral flanks showing a nearly homogenous fabric but with some fain banding visible, banding appears to cross the image nearly horizontally while, a separate band (white arrowhead) exhibits vertical bands. One zone (black arrowhead) appears to be corrugated and twisted; A11, mantle muscle developed over and to either side of the ink duct showing a broadly circumferential grain over the ink duct but with a grain parallel to the duct further anterior, possibly representing longitudinal muscle.

Fig. 12. Coleoid cephalopod Clarkeiteuthis montefiori (Buckman, 1830). LYMPH 2023/78. Orientation of mantle muscle fibres. Dark grey, ink sac; grey, leaked ink underneath mantle muscle; beige, mantle muscle; fine dashed lines, alignment of muscle fibres; coarse dashed lines, overfolds or tears in the mantle muscle.

Fig. 7. Coleoidea indet. LYMPH 2023/77. Plot of relative location of spurs on Type A hooks.

Fig. 5. Coleoidea indet. LYMPH 2023/77 from the Early Jurassic, Hettangian Stage of Doniford Bay, Watchet, UK. Sketch of distribution of arm hooks and interpretation of hook pairing and likely traces of arms. 1–3, clusters of arm hooks referred to in the text; a–i, paths of arm pairs as interpreted.

Fig. 6. Coleoidea indet. LYMPH 2023/77. A1, type A hook in lateral view drawn from an image showing the location of spur close to the distal end of the shaft, the most distal part of the uncinus is missing; A2, lateral view of apparently complete type B hook drawn from an image; A3, oblique view of type B hook showing a flatter “platform” zone on the inner surface of the shaft, base appears to be worn. Locations of base, shaft and uncinus marked, but exact lengths are estimates as the orbicular scar has been worn away in all examples.

Fig. 3. Diagrams illustrating methods used to obtain values of the phragmocone diameter, length and apical angle for PIMUZ 27078 and the specimen described by Hölder 1973 (A) and LYMPH 2023/77 (B). The methods assume brittle deformation and originally circular phragmocone cross-section. See text for discussion.

Fig. 8. Coleoidea indet. LYMPH 2023/77. Interpretation of the dark mass posterior to the brachial crown as part of a lower jaw. Dark brown represents heavily sclerotised zone surrounded by more lightly sclerotised (light brown) marginal zone.Except over the purported rostrum, the dashed lines marking the boundary between the two zones are largely speculative (see Fig. 5A8 for comparison).Hatched fill represents layer of comminuted shell debris that obscures parts of the structure and marks out a series of grooves and fissures aligned subparallel to the long axis.