Furca bohemica Fritsch, 1908
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publication ID |
https://doi.org/ 10.4202/app.2011.0038 |
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persistent identifier |
https://treatment.plazi.org/id/03B08787-FFDE-FF90-AA66-FAC5FEF0FAE2 |
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treatment provided by |
Felipe |
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scientific name |
Furca bohemica Fritsch, 1908 |
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Figs. 2–7 View Fig View Fig View Fig .
1847 Pharastoma pulchrum ( Barrande, 1846) ; Hawle and Corda 1847:
88, pl. 5: 49a. 1868 Furca bohemica Barrande, 1946 ; Bigsby 1868:192 (nomen nudum). 1908 Furca bohemica Barrande, 1946 ; Fritsch 1908a: 8–9, pl. 11: 1–3. 1908 Furca bohemica Barrande, 1946 ; Fritsch 1908b: 797–798. 1918 Furca bohemica Barrande, 1946 ; Perner 1918: 12. 1919 Furca bohemica Barrande, 1946 ; Perner 1919: 32–33. 1999 Furca bohemica Fritsch, 1908 ; Chlupáč 1999a; 79, 81–82, pl. 3:
1–4, text−fig. 2a. 1999 Furca pilosa sp. nov.; Chlupáč 1999a: 79, 81–82, pls. 2: 1, 3: 5. 1999 Furca sp. ; Chlupáč, 1999a: 79, 82, pl. 3: 6. 1999 Furca bohemica Fritsch, 1908 ; Chlupáč 1999b: 395, fig. affilia−
tion: 1.1.8. 1999 Furca pilosa Chlupáč, 1999 ; Chlupáč 1999b: 396, fig. 1.1.9. 2006 Furca bohemica Fritsch, 1908 ; Van Roy 2006: 68, fig. 4.1b. 2006 Furca pilosa Chlupáč, 1999 ; Van Roy 2006: 68, fig. 4.1c. 2006 Furca sp. ; Van Roy 2006: 68. 2009 Furca bohemica Fritsch, 1908 ; Rak 2009: 15–16, fig. 1B–E, G, H. 2009 Furca pilosa Chlupáč, 1999 ; Rak 2009: 15–16, fig. 1F. 2009 Furca sp. ; Rak, 2009: 16, fig. 1A. 2010 Furca bohemica Fritsch, 1908 ; Ortega−Hernández et al. 2010: 427. Lectotype: NML 27715 from Děd Hill (formerly Drabow), near Beroun,
http://dx.doi.org/10.4202/app.2011.0038
Czech Republic, Late Ordovician. Figured by Fritsch (1908). Lectotype selected by Chlupáč (1999a) from two Fritsch’s (1908) syntypes.
Emended diagnosis.— Furca with vaulted cephalic shield, short anterolateral spines and delicate secondary spines of variable length. Longitudinal sulcus−like depression in the posterior half of the cephalic shield.
Referred material.—A number of collections outside the NML house a limited amount of F. bohemica material. These include the Yale Peabody Museum (YPM IP 014784), the Museum of Comparative Zoology, Harvard University (MCZ 6068a, b, MCZ 6069), and the Natural History Museum in Vienna (NHMW 1901/004/0011).
Description.—As reported by Chlupáč (1999a), the overall morphology of F. bohemica consists of a central body from which three pairs of large spines develop, each with a distinct orientation and length ( Fig. 7 View Fig ). The central portion of the cephalic shield has a slightly elongated subtrapezoidal (wider anteriorly) shape, and is characterized by its conspicuous convexity, evident in both external and internal moulds ( Figs. 2–6 View Fig View Fig View Fig ). The only prominent feature of the central body is a small fusiform depression, or sulcus, that occupies a posterior position in the axial plane of the cephalic shield ( Figs. 2B, C, E, G View Fig , 3B, C View Fig , 5); the location of the median sulcus is suggestive of a role in cephalic muscle attachment, probably functioning as an apodeme, but the quality of preservation does not allow making further inferences about the significance of this structure. The front end of the cephalic shield has a straight margin that develops laterally into a pair of short anterolateral spines, which occasionally display a subtle degree of curvature on their inner facets, giving the impression of backward−facing horns ( Figs. 2A, D–H View Fig , 3B, D, G View Fig , 5A–F, 6B, C View Fig ). Immediately posterior to the aforementioned structures, the cephalic shield extends into a pair of large mediolateral spines. These are considerably arched and face backwards, with the tips reaching beyond the posterior margin of the central body; this represents the widest section of the cephalic shield in most specimens ( Table 1). The bases of the mediolateral spines are widely attached to the main body of the cephalic shield, extending from the base of the anterior spines to the anterior−most tip of the axial sulcus. The posterior of the cephalic shield is characterised by a pair of almost straight spines that are directed backwards, and which are subequal in length to the mediolateral spines. The bases of the posterolateral spines are in mutual contact on their inner faces, and thus the posterior border of the cephalic shield has an inverted “U” shape ( Figs. 3–6 View Fig View Fig ). As in Ma. splendens ( Whittington 1971; García−Bellido and Collins 2006) and Mi. hexagonalis ( Stürmer and Bergström 1976; Kühl and Rust 2010), the bases of the mediolateral and posterolateral spines are not in direct contact with each other, and thus define the slender posterior half of the cephalic shield ( Table 1). The most striking feature of F. bohemica is the presence of a prominent fringe that consists of dozens of delicate secondary spines ( Figs. 4A View Fig , 7 View Fig ). The secondary spines are gently curved backwards and vary in length from 1.5 to 7 mm, depending on their position, and are connected to the cephalic shield margins by distinctly triangular bases. The longest secondary spines are located on the most distal parts of the cephalic shield, such as 1/3 from the tips of the primary spines, giving the latter a rather leaf−like appearance ( Figs. 4A, B View Fig , 7 View Fig ); this contrasts with Chlupáč’s (1999a) interpretation, which portrayed the spines as having very little variation in length and perfectly straight outlines. In most specimens the secondary spines are lost or badly preserved, and the only traces left consist of the triangular stubs ( Figs. 5E, 6A–C View Fig ); however, the outline of the secondary spines can be clearly traced by the dark halo that surrounds the specimens ( Figs. 3C–I View Fig , 4E, F View Fig , 5C–I, 6A View Fig ). Secondary spines have been described in Mi. hexagonalis ( Stürmer and Bergström 1976; Kühl and Rust 2010), although in this case the spines consist of less numerous, but longer, straight spines respectively. Ma. splendens shows the presence of small serrations restricted to the posterolateral primary spines ( Whittington 1971; García−Bellido and Collins 2006); despite their resemblance to the triangular stubs observed in various specimens of F. bohemica in which the secondary spines have not become completely preserved, these structures are probably not homologous (see character 5).
Remarks.—From the studied material, a single specimen (NML40860a, b) displays a peculiar morphological abnormality ( Fig. 5A, B); the left side (right in counterpart) of the posterolateral spine base bears a short but conspicuous structure whose shape closely resembles the tip of a backward facing supernumerary spine. This extra spine is preserved as a flat impression in both part and counterpart, which is consistent with the preservation of the other spines on the specimen, where most of the convexity has been lost. The homogeneous colouration between the supernumerary spine and the rest of the fossil adds support to its authenticity, as an alien superimposed fragment would be expected to have a different preservation, as is the case for a small piece of trilobite shell on the left anterior spine ( Fig. 5A, B). The extra spine differs morphologically in the absence of any traces of the marginal fringe of secondary spines, or at least their triangular bases, which are clearly observable in the posterolateral spines. Apart from this asymmetrical structure, however, the rest of the fossil does not have any further malformations or traces of injury.
The occurrence of abnormalities in Palaeozoic arthropods is a relatively infrequent phenomenon that has been most extensively reported in trilobites (e.g., Owen 1985; Babcock 1993), although a single case is known in marrellomorph arthropods ( Whittington 1971: pl. 18: 3). In most cases, however, it is difficult to assess the origin of the malformation.
http://dx.doi.org/10.4202/app.2011.0038
Some asymmetrical abnormalities can be broadly explained through predation and injury healing ( Babcock 1993), but many malformations have a developmental origin (i.e., teratologies) and do not necessarily follow this rule. The specimen of Ma. splendens illustrated by Whittington (1971) bears a pair of supernumerary processes attached to the anterior margin of the lateral spines; in this case, it seems plausible to argue that this is the result of a teratological condition as opposed to a healed injury. Van Roy (2006) discussed Whittington’s (1971) specimen and reached the conclusion that it may represent an atavistic individual, in which a previously suppressed pair of spines associated with an inconspicuous head segment, as indicated by the number of primary spines, reappeared due to a developmental abnormality (see character 3). The model proposed by Van Roy (2006) is supported by the overall correlation between the number of head segments and primary spines in the cephalic shield of various marrellomorphs, as well as the highly symmetrical nature of the malformation in the aforementioned Ma. splendes specimen. In the case of the aberrant F. bohemica , however, the situation is not as straightforward, as the malformation is clearly confined to the left side of the cephalic shield and there are not any other abnormal features. It seems rather unreasonable to rely on the same argument as Van Roy (2006) to account for this additional structure, as that would imply a more primitive four−segmented condition for the marrellomorph head, which cannot be supported due to the absence of any appendage data for Furca or the recognition of other phylogenetically related organisms. As such, the simplest solution is to interpret this structure as a localized mutation, the result of an abnormal healing process due to injury or even a case of parasitism.
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