Coleoidea Bather, 1888
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publication ID |
https://doi.org/10.4202/app.01172.2024 |
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persistent identifier |
https://treatment.plazi.org/id/1F5EE900-8257-8B67-DBE4-FF780908F9C5 |
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treatment provided by |
Felipe |
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scientific name |
Coleoidea Bather, 1888 |
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Subclass Coleoidea Bather, 1888 View in CoL
Order, family, genus and species uncertain Coleoidea indet.
Figs. 4, 5.
Material.— Single specimen ( LYMPH 2023 /77) from the base of Bed 24 of the Planorbis Subchronozone, Planorbis Chronozone, Hettangian Stage , “ Psiloceras Beds ” of Blue Lias Formation ; Foreshore of Doniford Bay [ST 0842 4337], Watchet, Somerset, UK .
Description.— General features: The length of the specimen, from the most distal arm hooks to the posterior of the remaining portion of the phragmocone is 115 mm. With the exception of the ink sac, the specimen is entirely flattened, and the phragmocone wall and septa heavily fractured. There is nothing preserved between the anterior of the ink sac and the dark mass to the posterior of the arm crown, a distance of 60 mm.
Arm hooks and arms: Many of the posterior arm hooks and the dark mass are partially buried in and obscured by a thin layer of finely crystalline material, interpreted as comminuted shell debris. The hooks range 0.6–1.4 mm in length and comprise two general types that may intergrade:
(i) type A: nearly straight, with a slender, faintly curved, short uncinus; a prominent spur is visible on some of the hooks (Figs. 4A 8, 6A 1) and where present, appears to be consistently located at a position 0.75% of the distance from the proximal end of the hook ( Fig. 7 View Fig ). There are also several isolated spurs in the matrix which may have broken away from the hooks. The longest hook of this type is 1.17 mm and the shortest 0.56 mm. The proximal and distal angles (see Fuchs and Hoffmann 2017) measured on one hook are 10° and 36°, respectively;
(ii) type B: larger hooks (up to 1.7 mm), possessing a weakly curved shaft and a long, gently curved uncinus with a short, strongly curved distal end bearing a marked resemblance to a crochet hook (Figs. 4A 11, 6A 2, A 3); although some of the hooks of this type appear to carry spurs, these are spurious and result from the compression of the shaft of one hook onto the uncinus of an underlying hook, creating a projection from the shaft of the upper hook that can be mistaken for a spur (Fig. 4A 11); all the hooks of type B lack spurs; proximal and distal angles measured on one hook are 19° and 26°, respectively.
The wear caused by exposure to wave activity has resulted in the loss of the orbicular scar in all the hooks that are visible. Consequently, it is not possible to obtain proportionate measurements of the relative lengths of the uncinus, shaft and base.
The majority of hooks form three clusters anterior to the dark mass that might represent the buccal mass with the jaws. Each cluster consists of closely packed hooks that are often in contact and/or overlap with each other. Anterior to the clusters, discrete rows of isolated hooks extend forward ( Fig. 5 View Fig : d–h). Both the rows and clusters show consistent alignments and packing order, indicating that they are unlikely to be the product of minor transportation of the hooks, but largely reflect an original configuration; this is not surprising because cephalopod arms have the ability to extremely extend and contract. With few exceptions, the distal ends of the hooks face anteriorly, and the alignment of many of the hooks within the clusters suggests that the hooks form discrete rows. Potentially, nine rows of hooks ( Fig. 5 View Fig : a–i) may be discerned.
The distal ends of the hooks face the anterior in all rows. In rows d–g the bases of the hooks face towards the axis of the brachial crown. In row h, the bases of the hooks, although also facing toward the axis of the crown, are reversed by comparison with rows d–g. This may be because the arm was torted, or it lay on the opposite side of the axis of the brachial crown, i.e., opposite to the other arms.
Hooks of type A appear to be in the minority and are associated with rows f, g and h. They also appear to be present on the left side of cluster 3, which is otherwise dominated by type B hooks. Cluster 1 consists almost entirely of large type B hooks, all of which lack spurs. The hooks are so closely packed that it is possible that spurs, if present, have been obscured from view. Cluster 2 contains a mixture of types A and B, including some that may be intermediate in form. With the exception of row h, cluster 3 also consists of type B hooks and intermediate forms. Hooks of both types appear to become progressively shorter and increasingly slender toward the proximal ends of the arms.
Each of the rows a–i probably represents an arm, but rows d–e and f–g are likely to represent pairs of hooks on the same arm. At a minimum, cluster 1 may represent parts of two arms, cluster 2, parts of three arms, and cluster 3, parts of two arms. Thus, the remains of up to six arms may be preserved. It is unclear whether the other arms were weathered away, remain still covered by sediment, or were detached and fell off during the decay of soft tissue and are represented by the more disordered portions of cluster 1 in particular.
Dark mass: The boundaries of this structure (Fig. 4A 8) are largely obscured by the same layer of shell debris that cloaks some of the posterior arm hooks (Fig. 4A 10). The structure is almost entirely flattened. The main body of the structure is comprised of material the same colour as that of the arm hooks, with a similarly reflective surface, although markedly pitted by comparison. This area is thought to be relatively thick. The margins of the structure, although appearing to comprise a similar material are generally less reflective and have the appearance of being thinner. Parts of both the core and margins are missing in several places and have either been broken off prior to burial or lost to recent erosion.
The core of the structure is 1.7 mm long and 1.5 mm wide. The main feature is a slender projection aligned with the long axis and tapering away from the main body. At the opposite end, a low ridge curves convex outwards symmetrically around the long axis in the plane in which the structure is flattened. The main ridge is associated with two striae or very low ridges parallel with and further outboard of the main ridge. Laterally, the main body extends out to form a pair of broad areas, the margins of which are not visible, but may be up to 0.6 mm long.
The projection is divided anteriorly by a fissure 0.1 mm wide and roughly aligned to the long axis of the structure. The fissure tapers and closes over a distance of 0.55 mm, but a shallow groove, in places filled with shell debris persists and deviates obliquely from the long axis for the length of the body.
Ink sac: The ink sac is 13.2 mm long by 7.7 mm wide and partially flattened. The remaining 4 mm of an ink duct extends from the anterior end of the ink sac. The surface of the ink sac is badly worn though wave erosion, but a number of depressed lobes and shallow groove may indicate that the surface of the sack had been folded prior to or during burial. The posterior end of the ink sac overlaps with the anterior part of the phragmocone by at least 6 mm. The surface of the ink sac is coated in places with calcite or aragonite that has a sucrose texture. These patches may represent the remains of crushed septa.
Conotheca: Nothing remains of the proostracum anterior of the ink sac, and where shell survives, as a consequence of the flattening and crushing of the conch, it is difficult to distinguish between potential proostracum, phragmocone wall or septa. The conotheca is heavily fractured, and matrix is present within the fractures so that there is continuity between the external and internal matrix. This suggests that fracturing probably took place during early burial while the sediment remained relatively mobile. The resultant jigsaw of overlapping fragments creates an impediment to interpretation of the structure of the conotheca.
Underlying and partially obscured by the ink sac, part of the inner surface of the phragmocone wall is visible around the perimeter of the ink sac. Here, the presence of sutures is marked by narrow, shallow grooves that are on average, 1.1 mm apart (Fig. 4A 6). Where seen, the sutures are straight. Between the sutures, the inner surface of the phragmocone wall exhibits faint striae about 0.08 mm apart and parallel with the sutures. These are interpreted as growth increments.
On the inner side of the phragmocone wall, and in places extending onto the margins of the ink sac, two lamellae are visible, each of which is divisible into two fabrics. These are (i) an off-white, finely sucrose-textured fabric, and (ii) a denser, multi-layered fabric that has a nacreous appearance, the edges of which are indicated by arrows in Fig. 4A 3. Fabric 1 coats both sides of fabric 2 but there appear to be traces of matrix between adjacent layers of fabric 1, suggesting the original presence of cavities between the laminae. The most anterior of the lamina composed of fabric 2 appears to be attached to the phragmocone wall at its anterior margin, as may also the second more posterior lamina. The distance
→ Fig. 4. Coleoidea indet. LYMPH 2023 /77 from the Early Jurassic , Hettangian Stage of Doniford Bay, Watchet, UK. A 1, image of whole specimen; A 2, interpretative sketch mapping out the distribution of the surviving components of the organism; A 3, broken edge of phragmocone wall overlain by two sets of lamellae (edges marked by white arrowheads) originating from the phragmocone wall and representing the remnants of probable septa with a thin layer of cameral deposit, edge of ink sac to lower left (black arrowhead); A 4, margin of phragmocone with outer, iridescent layer and fine striations (arrowhead) possibly representing fragment of proostracum.A 5, iridescent layer showing longitudinal striae (black arrowhead) outside phragmocone wall (proostracum) and partially overlain by smooth shell (white arrowhead) that may represent a fragment of the outer shell layer of the conotheca, primordial rostrum, or an anterior part of the orthorostrum; A 6, phragmocone wall underlying ink sac with probable septa (arrowheads) (see A 3) and showing sutures and striae representing probable growth increments; A 7, cluster of closely packed arm hooks. A 8, dark mass possibly representing jaw (see Fig. 8 View Fig for interpretation). A 9, type A hooks with spurs (arrowheads), other hooks mainly type B; A 10, enlargement of brachial crown showing rows and pairs of hooks; A 11, type B hooks with strongly curved terminations of the uncinus .
between the contacts of the two laminae where they are in contact with the phragmocone wall is similar to the distance between the sutures, although it cannot be demonstrated that that they are inserted at the sutures. These structures are interpreted as the remains of two septa; fabric 2 may represent the septum and fabric 1, as questionable cameral deposits (see Bandel and Spaeth 1988; Doguzhaeva et al. 2003: fig. 9.10).
On the outer surface of the phragmocone wall there is a very thin layer of laminated, strongly iridescent material. Parts of the edge of the crushed conotheca can be traced along one side of the specimen although the opposing margin was lost during the extraction of the specimen. The most anterior segment of the conotheca margin appears to have been fractured longitudinally, and the two edges of the fracture are imbricated against each other. The upper facing surface of the conotheca is composed of a material of similar thickness and general fabric to that forming the outer surface of the phragmocone as seen around the ink sac and probably represents the same structure. The iridescent outer surface of the conotheca is covered in traces of striae 0.07 mm apart (Fig. 4A 4) and oriented to face obliquely apically away from the mid-line of the conotheca.
The most posterior portion of the conotheca that is preserved comprises an isolated area 10 mm to the posterior of the rest of the remaining conotheca. This portion is composed of several layers of crushed shell consisting of a chaotic mix of septal and phragmocone wall fragments. The presence of matrix between the conch fragments and in continuity with the matrix external to the conotheca indicates that the walls were ruptured during burial while the sediment remained unconsolidated.
Although this part of the conotheca is so fragmented that it is difficult to identify any features, some fragments show the traces of sutures 0.85 mm apart, while traces of striae, probably representing growth increments are visible in places. Patches of an outer, iridescent layer are also present. One such patch (Fig. 4A 5) shows striations about 0.1 mm distant, parallel to the longitudinal axis of the body.
Toward the left side (anterior down) of the posterior portion of conotheca there is a shallow, elongate trough. The trough is aligned approximately parallel to the longitudinal axis of the specimen. The surface of the trough is largely filled and obscured by matrix except at the margins. At the anterior end of the trough three low ridges each about 0.25 mm apart, form overlapping semi-circular arcs 0.95 mm in diameter which close to the anterior. The nature of this structure is unclear, but the trough may have been caused by flexure during compression of the conch. The anterior ridges may represent truncated layers of the phragmocone wall at the anterior end of the trough.
Since one edge of the flattened conotheca is missing, the width of the anterior end of the phragmocone, estimated as twice the distance between the approximate middle of the ink sac and the surviving margin of the flattened phragmocone is 13.7 mm. Assuming that an originally circular phragmocone cross-section was subjected to brittle deformation the restored diameter is estimated at 17.4 mm. Using the trace of the phragmocone margin and assuming both that the direction of the sutures is normal to the conch axis and that deformation was brittle, the restored alveolar/apical angle is estimated at about 31°. On this basis, the estimated length of the phragmocone is at least 31.5 mm, of which at a minimum, 5mm of the posterior of the phragmocone is missing.
Rostrum: There is little evidence that a rostrum was present, but at the posterior end of the preserved portion of the phragmocone there is a small patch of smooth shell that rests on the outer iridescent layer (Fig. 4A 5), partially obscuring it. Although only a fraction of a millimetre thick, this layer might represent a remnant of the outer shell layer of the conotheca, the primordial rostrum or an anterior fragment of the orthorostrum.
Remarks. —Many of the arm hooks may be assigned to the parataxon Paraglycerites Eisenack, 1939 , and bear a resemblance to hooks assigned to several species of Paraglycerites by Riegraf (1996:figs. 5, 6.1–7)from the bituminous mudstone at the base of the Psilonotenbank (Psilonotum Subchronozone of the Planorbis Chronozone ) in the Tübingen area of southwest Germany. Direct comparison with the arm hooks from the Psilonotenbank is hampered by the relatively fragmentary state of preservation of the latter although they both possess a long base with a low angle between the shaft and base, a long shaft, and an uncinus at a high angle to base. They differ in that spurs appear to be less frequent in LYMPH 2023/77, and where visible, located at a relatively distal position ( Fig. 7 View Fig ). The fragmentary preservation of the Psilonotenbank specimens makes it difficult to judge whether they might be ubiquitous in that material while the compression, partial embedment and crowding of hooks may be obscuring the presence of spurs present on other hooks in LYMPH 2023/77.
Fuchs and Hoffmann (2017: 6) observed that the presence of spurs on arm hooks had at that time, only been reported in rostrum-bearing belemnitids. Engeser (1987) stated that the arm hooks of phragmoteuthids and belemnotheutids lack spurs, although Doguzhaeva et al. (2007: fig. 2b, c) reported the presence of spurs on the hooks in specimens attributed to Phragmoteuthis bisinuata (Bronn, 1859) (see also Hart et al. 2019); now reinterpreted as specimens of Mojsovicsteuthis sp., of uncertain order and family ( Lukeneder et al. 2024). Lukeneder et al. (2024: fig. 3) made no mention of the presence of a spur in their hook type 4, observed by Doguzhaeva et al. (2007: 280) to “bear a pronounced spur on the internal side”. Instead they noted the bifurcated nature of the base of the hooks that here are interpreted as the very prominent inner process of a type of hooks that possesses well-developed inner and outer processes. Under this interpretation, it remains the case that the presence of spurs on arm hooks have only been reported in rostrum-bearing belemnitids. No inference can be drawn from this, however, that LYMPH 2023/77 is a rostrum-bearing belemnitid, as the affinities of many spur-bearing hooks remain unknown—including Paraglycerites (Alexander Pohle, personal communication 2014), and are only known from Acrocoelites and Passaloteuthis ( Reitner and Urlichs 1983) while the hooks in Hibolithes lack spurs (Klug et al. 2010: figs. 2, 6a).
The location of the dark mass immediately posterior to the arm crown suggests that it may be associated with the buccal mass, and in particular the jaws. However, the extremely poor preservation makes this interpretation highly speculative ( Fig. 8 View Fig ). The curved ridge(s) present at one end of the long axis might form a beak-like structure if retro-deformed. Comparison with the jaws of Hibolithes (Klug et al. 2010: fig. 5) from the Kimmeridgian of SW Germany suggests that such an interpretation is feasible if the structure represents the rostrum and part of the hood of the lower jaw while the lateral areas represent parts of the external lamellae. The size of the putative lower jaw is of the same magnitude as that of the Hibolithes although smaller, reflecting the smaller size of LYMPH 2023/77.
The striae present in isolated patches of the outer, iridescent layer of the conotheca are likely to represent growth increments. Since the orientation of the striae on the margin of the conotheca are different from those visible on the posterior portion of the conotheca, these patches are here interpreted as remnants of the proostracum. The striae parallel to the body axis may represent a fragment of the left hyperbolar zone, and those observed on the lateral edge of the crushed conotheca could be a fragment of the right side of the median field, suggesting that the dorsolateral surface of the specimen faces upwards.
The lack of any conclusive evidence for the presence of a rostrum (or a lack thereof), combined with the fragmentary preservation of the conotheca means that ordinal determination of LYMPH 2023/77 is uncertain, while familial and generic determination is not possible. The possession of arm hooks assignable to Paraglycerites suggests a relationship to the specimens described by Riegraf (1996). The latter is known from the Psilonotum Subzone of Southwest Germany. Psiloceras sampsoni , present in the upper part of Bed 24 in West Somerset ( Page and Bloos 1995: fig. 2) is considered a probable senior synonym of P. psilonotum ( Bloos and Page 2000) . LYMPH 2023/77, from the lower portion of Bed 24, is high in the Planorbis Subzone. Thus , there may be little difference in age between LYMPH 2023/77 and the German material. The belemnitid described by Klug and Fuchs (2010) from Blue Anchor (PIMUZ 27078) is associated with specimens of P. planorbis and is likely to be nearly coeval with LYMPH 2023/77.
Klug and Fuchs (2010) assigned the Blue Anchor specimen (PIMUZ 27078) to the Belemnitida on the basis of the morphology of the well-preserved proostracum. No arm hooks are known from PIMUZ 27078. The proostracum of LYMPH 2023/77 is so fragmentary that its overall morphology must remain speculative. Thus, LYMPH 2023/77 can only be compared with PIMUZ 27078 in the general proportions of the restored phragmocones ( Tables 2, 3).
While noting differences which they attributed to the uncertainties of reconstruction and different modes of deformation, Klug and Fuchs (2010) recognised the similarity of PIMUZ 27078 to an Early Jurassic belemnitid specimen described by Hölder (1973) from the Angulata Chronozone of Bünde, Westphalia, Germany. Our attempts at reconstruction of PIMUZ 27078 and Hölder’s (1973) specimen also indicate the similarity of several characters ( Table 3), suggesting that the two specimens might represent different growth stages of the same taxon.
When compared with PIMUZ 27078 and Hölder’s (1973) specimen, LYMPH 2023/77 differs in the larger apical angle of the phragmocone and the depth of the camerae. Assuming that the approximate distance from the anterior end of the phragmocone to the dark mass (jaws?) may represent the length of the proostracum, then it is proportionately slightly shorter than those of PIMUZ 27078 and Hölder’s 1973) specimen although the uncertainty in measurement is likely to be larger than differences in proportionate length. Westermann (1975) observed that for theoretical models of ectocochleate cephalopods, there is an inverse relationship between cameral depth and apical angle. Although this does not hold consistently for fossil ectocochleates, it is possible although untested) that the relationship holds for coleoids and may indicate that the wider apical angle of LYMPH 2023/77 is real rather than a consequence of the deformation of the phragmocone. Thus LYMPH 2023/77 differs from PIMUZ 27078, at least in the in the larger apical angle of the phragmocone and the depth of the camerae.
To date, it seems that no rostra have ever been reported from the Blue Lias Formation. Given the robust nature of the rostrum, it would be expected that after at least two centuries of collecting they would have been recorded if present. Neither can their absence be explained through dissolution since the originally aragonitic phragmocone is preserved in LYMPH 2023/77 and PIMUZ 27078 and would have been more soluble than the calcitic rostrum. Thus, it seems possible that there was no rostrum present or that it formed an extremely thin sheath in the form of a primordial rostrum.
As the differences in phragmocone morphology between LYMPH 2023/77 and PIMUZ 27078 appear to be real, the two specimens seem likely to be taxonomically distinct. Where known in the Phragmoteuthida, the apical angle is large (25–40°, see Fuchs and Donovan 2018) and also in most (but not all) Diplobelida (15–50°, see Fuchs 2019) while the camerae of both orders are generally narrow. The arm hooks of LYMPH 2023/77, however, are quite different to those of the two Early Jurassic, putative diplobelids, Clarkeiteuthis and Chondroteuthis . Instead, they bear some resemblance to the type 3 hooks of Lukeneder et al. (2024: fig. 3c) associated with Phragmoteuthis bisinuata Bronn, 1859 , from the Upper Triassic of Austria (Carnian) Polzber Konservat-Lagerstätte of Lower Austria). Since Phragmoteuthis ranges into the Sinemurian ( Donovan 2006; Fuchs and Donovan 2018: fig. 1), it is possible that LYMPH 2023/77 is a phragmoteuthid. Additional, better-preserved material, if discovered, may confirm such an assignment, but here since very little remains of the diagnostic proostracum, LYMPH 2023/77 is placed in open nomenclature.
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