Cheiracanthus, FROM

Burrow, Carole & Newman, Michael, 2020, A redescription of the three longest-known species of the acanthodian Cheiracanthus from the Middle Devonian of Scotland, Palaeontologia Electronica (a 15) 23 (1), pp. 1-43 : 2-5

CHEIRACANTHUS FROM SCOTLAND

Cheiracanthus murchisoni Agassiz, 1835

Cheiracanthus murchisoni was one of the first Devonian fish to be described. It was erected by Agassiz based on IGUN. 66 in the Neuchâtel Museum collection (now MHNN FOS39) from Gamrie ( Agassiz 1833 -1843, plate 1c, figure 3; Figure 1.1, 1.3 ). Cheiracanthus minor Agassiz, 1835 was erected in the same work, figuring one specimen ( Agassiz 1833 -1843, plate 1c, figure 5). This specimen was part of the Traill collection and is now considered lost ( Andrews 1982). Miller (1841, plate 7, figure 1) figured a specimen of Cheiracanthus but did not give it a species name. Agassiz later erected Cheiracanthus microlepidotus Agassiz, 1844 based on three specimens from Lethen Bar, NHMUK P.544 ( Agassiz 1844 -1845, plate 15, figure 2; Figure 1.4, 1.5 ), and MHNN FOS 40, 41 ( Figure 1.6-9 ). Agassiz also produced a restoration of the genus ( Agassiz 1844 -1845, plate D, figure 2). Miller (1847) reproduced his Cheiracanthus figure from his earlier publication, but this time labelled it Cheiracanthus microlepidotus ; NMS G. 1953.4.2 is the specimen on which the figure was based. M’Coy (1848) erected two new species based on specimens from Orkney, Cheiracanthus lateralis M’Coy, 1848 , and Cheiracanthus pulverulentus M’Coy, 1848 , neither of which were figured. He later illustrated one of the C. pulverulentus syntypes SM H4424 (M’Coy 1855, plate 2.B, figure 2; Figure 2.1, 2.2 ; the other syntype is illustrated in Figure 2.3 ) and included a description of the other known species in that work.

Egerton (1860) considered Cheiracanthus lateralis a junior synonym of Cheiracanthus minor as there were no specific characters to separate them. Subsequently, Traquair (1888) considered that the character differences exhibited in Cheiracanthus microlepidotus , Cheiracanthus minor and Cheiracanthus pulverulentus resulted from the different preservation at the various localities, and united them [as well as C. lateralis which had already been dealt with by Egerton (1860)] under the species that had page priority, namely Cheiracanthus murchisoni .

Traquair (1895, plate 2, figure 2) provided an accurate restoration of Cheiracanthus murchisoni . The last published reconstruction of Cheiracanthus murchisoni was by Watson (1935, figure 22). Later, Watson (1937) described the morphology of Cheiracanthus murchisoni in some detail, particularly the head. He noted some morphological differences between C. murchisoni and Cheiracanthus latus . The scales, however, were hardly mentioned at all. Gross (1947) was the first to give detailed descriptions and illustrations of the crown morphology and the histology of the scales, and Ørvig (1951) was the first to investigate the histological structure of the endoskeleton of Cheiracanthus murchisoni , using specimens from Gamrie. He found no traces of perichondral bone, only globular calcified cartilage. He also noted that the rings of Liesegang in the cartilage globules tended to be somewhat polygonal, rather than the circular shape found in the arthrodires.

Miles (1966) briefly compared the Middle Devonian German form Protogonacanthus with Cheiracanthus murchisoni , and later (Miles, 1973) described and illustrated ‘ceratotrichia’ in the fins.

Denison (1979, figures 9E, 10J, 31D) reproduced some of Gross’ illustrations of scale histology, as well as his own illustration of a scale crown, and detailed the diagnostic features of the species.

Young (1995, figure 9) drew and described the scales of Cheiracanthus murchisoni from two regions of the body, above and below the lateral line.

One interesting recent paper described the preservation of internal organs of Cheiracanthus (Davidson and Trewin, 2005) . Two Cheiracanthus specimens from Tynet Burn that were figured by Davidson and Trewin (2005, figure 2A, 2B) (NMS G.2000.65.1, 2) are now known to be Cheiracanthus murchisoni .

Cheiracanthus grandispinus McCoy, 1848

Cheiracanthus grandispinus was erected by McCoy but not figured at that time. Later, he figured the holotype specimen SM H4423 and gave a fuller description, including detail of the ornamentation of the scale crowns (McCoy, 1855, plate 2.B, figure 1, 1a; Figure 3 ). (All the figured specimens in his plate are mirror images of the actual specimens.) Both Egerton (1860) and Traquair (1888) concluded that C. grandispinus was a valid species. No work of note has since been done on the species .

Cheiracanthus latus Egerton, 1861

Cheiracanthus latus was first erected and figured by Egerton (1861, plate 10, figures 1, 2; Figure 4 ), with his description based on some 50 specimens from Tynet Burn. He figured only one specimen, but that half of the original nodule is now lost. However, the other half ( NHMUK PV P3253 ) has been located and is now defined as the holotype. Egerton (1861) distinguished the species from Cheiracanthus latus Egerton, 1861 by the body being relatively short and thick, having a broad tail and large fin webs with the ventral fins closely approximated; he did not recognise the diagnostic character of the ornamentation of the scale crowns. He also listed small conical teeth as a character, although he stated that these teeth were not preserved in any of the 50 or so specimens he examined .

Traquair (1888) concluded that Cheiracanthus latus Egerton, 1861 was a valid species, as did Woodward (1891), who did not identify any type specimens in the NHMUK collection .

Watson (1937) described the morphology of in some detail, particularly the head. He noted some morphological differences between and Cheiracanthus latus Egerton, 1861 . The scales, however, were hardly mentioned at all. Gross (1947) was the first author to describe the crown morphology and the histology of the scales in detail.

Miles (1966) briefly compared the Middle Devonian German form Cheiracanthus latus Egerton, 1861 with Cheiracanthus latus , and later (Miles, 1973) described and illustrated the structure of the shoulder girdle.

Denison (1979, figures 30B, 31A-C) reproduced Miles’ (1973) illustration of the shoulder girdle structure and also figured a scale crown, as well as reproducing Watson’s reconstruction of the fish.

Young (1995, figure 8) described the scales of Cheiracanthus latus and figured the scale crown forms above and below the lateral line anterior the pelvic fins, as well as posterior to the dorsal fin.

Newman et al. (2019) described the histology of Cheiracanthus latus spines when comparing against the genus Haplacanthus . They also discuss the nature of the base of insertion in Cheiracanthus .

Figures

FIGURE 1. Type material of Cheiracanthus murchisoni Agassiz, 1835 and its junior synonym C. microlepidotus Agassiz, 1844. 1-3, Holotype MHNN FOS 39 from Gamrie: 1, specimen in present condition; 2, closeup of midbody scales; 3, image in Agassiz (1833-43, plate 1c, figure 3). 4-9: Syntypes of the junior synonym C. microlepidotus from Lethen Bar: 4, 5, NHMUK PV P544; 6, 7, MHNN FOS 40; 8, 9, MHNN FOS 41. Scale bars equal 20 mm.

FIGURE 2. Syntypes of Cheiracanthus pulverulentus M’Coy, 1855 from Orkney, a junior synonym of Cheiracanthus murchisoni. 1, SM H4424, ‘syntype’ specimen in present condition; 2, SM H4424 specimen and scales as illustrated by M’Coy (1855, plate 2.B, figure 2), showing a mirror image of the specimen; 3, SM H4425, second ’syntype’, present condition. Scale bars equal 20 mm.

FIGURE 3. Cheiracanthus grandispinus holotype SM H4423 from Orkney. 1, mirrored illustration of specimen and scales from M’Coy (1855, plate 2.B, figure 1); 2, specimen in present condition. Scale bar equals 20 mm.

FIGURE 4. Cheiracanthus latus Egerton, 1861 holotype from Tynet Burn. 1, NHMUK PV P3253, half of specimen not previously figured; 2, half of the holotype specimen illustrated by Egerton (1861, plate 10, figure 1, 2), now lost. Scale bar equals 20 mm.