Onychodictyon gracilis, Liu & Shu & Han & Zhang & Zhang, 2008

Liu, Jianni, Shu, Degan, Han, Jian, Zhang, Zhifei & Zhang, Xingliang, 2008, The lobopod Onychodictyon from the Lower Cambrian Chengjiang Lagerstätte revisited, Acta Palaeontologica Polonica 53 (2), pp. 285-292 : 286-291

Onychodictyon gracilis sp. nov.

Figs. 2A 1 –A 7, 3A 1 –C 2.

Holotype: ELIJSO−0003 ( Fig. 2A 1 –A 7); from the Jianshan section, Haikou county, Lower Cambrian.

Derivation of the name: Gracilis refers to its delicate appearance.

Diagnosis.—Relatively small to up to 50 mm, the body comprises a rounded “head” without “antennae”, the trunk with 10 segments and 12 pairs of limbs, the posterior of the trunk bear−

Fig.1. Early Cambrian lobopod Onychodictyon ferox Hou, Ramsköld, and Bergström, 1991 , from Haikou, Kunming, Yunnan, China. A. Specimen Ą ELIJSO−013; A 1, part ELIJSO−013A, a nearly complete specimen, dorso−ventrally preserved; A 2, Camera lucida drawings of A 1, showing nearly all the main features of Onychodictyon ferox , note the paired “antennae”, 10 pairs of plates, 11 pairs of limbs, the annuli and tubercles of both the trunk and limbs,

the fine canal and claws of limbs; A 3, counterpart ELIJSO−013B showing the paired “antennae”, 10 pairs of plates, 11 pairs of limbs and the gut; A 4, enlargement of the head area of A 1, note the paired “antennae”; A 5, enlargement of the head area of A 3, note the paired “antennae”. B. Specimen ELIJSO−001;

B 1, enlargement of the posterior area of ELIJSO−001A, showing the trunk end with the last pair of limbs and the limbs with fine canals; B 2, enlargement of the limbs and appendicula of Onychodictyon ferox ELIJSO−001B, note the annuli and tubercles of the limb and appendiculae with fine canal of the trunk.

Abbreviations: an, “antennae”; app, appedicules; ann, annuli; cl, claws; fc, fine canal; lb, limb, lb1, the first pair of limb, lb2, the second pair of limb, and so on; p, plate, p1, plate 1, p6, plate 6, and so on; ri, rim of the plate; sp, spines; tu, tubercles.

ing a short projection. Compared to Onychodictyon ferox , O. 2A 1, A 3). The limb−pairs 2 to 10 are located ventro−laterally gracilis is smaller, more fragile−looking with more pairs of of the plate−pairs 1 to 9, respectively ( Figs. 2A 1, A 3, 3B 1, B 3);

limbs. limb−pairs 11 and 12 are based anteriorly and posteriorly be−

Description.—Three complete specimens (ELIJSO−0003, low plate−pair 10 ( Fig. 2A 1, A 3). As in the limbs of O. ferox ,

ELIJSO−0005, and ELIEJO−0017) are known. All three were there are about 30 rows of thin, transverse annuli from the laterally compressed. The body can be divided into a well− base to the tip ( Fig. 2A 1, A 3, A 6). Tubercles are present in two defined head area, a trunk with 10 segments and a small pos− rows along the lengths of the limbs, similar to those of the terior projection. The maximum length (sagittal) of the three trunk ( Fig. 2A 1, A 3, A 6), the number of series being about 10

specimens is 50 mm (ELIJSO−0003) ( Fig. 2A 1), 35 mm to 20, and small appendicules are observed protruding from

(ELIEJO−0017) ( Fig. 3B 1) and 12mm (ELIJSO−0005) (Fig. these tubercles ( Fig. 2A 1, A 3, A 6). A pair of claws occurs at

3A 1), transverse width amounts to 5 mm (ELIJSO−0003) the tip of Limb 8, and these two claws point in the opposite

( Fig. 2A 1), 3.5 mm (ELIEJO−0017) ( Fig. 3B 1) and 1.5 mm directions ( Fig. 2A 1, A3, A6).

(ELIJSO−0005) ( Fig. 3A 1). A short posterior projection is well preserved in specimens

The head area is clearly differentiated from the trunk, ELIJSO−0003 ( Fig. 2A 1, A 3) and ELIJSO−0005 ( Fig. 3A 1, A 2,

the ridge on the “head” is very smooth on the antero−ventral A 4), and is similar to that in Microdictyon . The projection of side, and a little sclerotized on the dorsal side of specimens Onychodictyon gracilis protrudes postero−ventrally from the

ELIJSO−0003 ( Fig. 2A 1, A 3 –A 5) and ELIEJO−0017 (Fig. margin, slightly ventral to the posterior end of the body, and is

3B 1 –B 3). However, there is no sign of “antennae” or any about 2.5 mm long by 2 mm wide in specimen ELIJSO−0003

other structures on the “head” ( Figs. 2A 1, A 3 –A 5, 3A 1 –A 3, ( Figs. 2A 1, A 3). Probably because of the state of preservation,

B 1 –B 3). According to the black trace interpreted as the gut, the ridge of the projection in ELIJSO−0003 shows some signs the mouth is located on the antero−ventral side of the “head” of small spines ( Fig. 2A 1, A 3), but that of the projection in

( Figs. 2A 1, A 3 –A 5, 3A 1 –A 3, B 1 –B 3). ELIJSO−0005 is smooth ( Fig. 3A 1, A 4).

The trunk region comprises 10 segments, each bearing a The alimentary canal is expressed as a trace with a dark number of strong annuli ( Figs. 2A 1 –A 3, A 6, 3B 1, B 3, C 1, C 2), band about 1mm wide. It is simple and straight and extends which range between 1 and 6 in number ( Fig. 2A 1, A 3). The through much of the body, mostly in a subventral position number of annuli is in direct proportion to the longitudinal ( Figs. 2A 1, A 3, 3A 1, A 2, B 1, B 3). No anus is apparent.

length of the segments. There are only 1–2 in the first segment,

increasing to 4 in the 5 th segment. The 6 th, 7 th, 8 th segments have 4–6 annuli. The remainder, except for the small projec− Comparison tion, all have annuli, the number decreasing from 5 to 2 ( Fig.

2A 1, A 3). As in Onychodictyon ferox , each annulus has a verti− The similarites of both Onychodictyon ferox and O. gracilis cal row of “tubercles”, and long, finely annulated appen− are clear, implying a close relationship for the two species of dicules extend from these ( Figs. 2A 1, A 3, A 6, 3A 1, A 2, B 1, B 3, Onychodictyon : (1) both are subcylindrical in outline; (2)

C 1, C 2). Similar to those of O. ferox , the plates are rounded, there are similar dorsal plates and appendicules; (3) both bear dorso−ventrally elongate, convex outward, with a ridge near lobe−like limbs with tubercles and appendicules; and (4) two the edge, with the structure of plates showing the same pattern claws are present in these two species. However, there are also

(convexity, ridges, rims) ( Figs. 2A 1, A 3, 3A 1 –A 3). A pointed significant differences between them: (1) O. ferox possesses a spine protrudes from each plate ( Fig. 3A 1 –A 3, B 1, B 3, C 2), ac− pair of “antennae” on the “head”, while O. gracilis does not;

cording to preservation, in some cases only the basal plate of (2) 11 pairs of limbs occur on the trunk of O. ferox , with 12

the spines is observable ( Fig. 2A 1 –A 3). pairs of limbs only observed in O. gracilis with the first pair of

Twelve pairs of limbs are observed in Onychodictyon limbs located anterior to the first plate pair; (3) the last limbs of gracilis , the first pair located anterior to the first plate pair, O. ferox are located at the end of the trunk with the claws usu−

and, compared to the other limbs of the trunk, this pair of ally bent and directed anteriorly, while in O. gracilis the direc−

limbs is fat and strong ( Figs. 2A 1, A 3 –A 5, 3B 1 –B 3). As Ram− tion of the claws of last limb bent according to the posture of sköld and Chen noted, this pair of anterior limbs is set at a the trunk; and (4) O. gracilis bear a small projection on the greater distance from Limb1 than Limb1 is from Limb2 (Fig. trunk, while O. ferox does not. Accordingly, we interpret Ą Onychodictyon ferox and O. gracilis as two different species. tennae are not homologous with those of arthropods; they are However, because different sexes within the same species innervated by protocerebral and deutocerebral nerves respecprobably had different morphologies, there is the alternative tively ( Scholtz and Edgecombe 2005, 2006). Evaluation of possibility that the two species represent sexual dimorphs of which part of the cerebrum innervated the antennae of O. ferox Onychodictyon . and Miraluolishania is beyond the scope of this paper, but considering other distinct differences between Onychodictyon and onychophorans, such as no jaw or oral papillae in Onycho− Discussion dictyon, we are inclined to believe that the antennae of lobopods are closely implicated in the origin of antennae in arthropods. Concerning the “jaw” or “head shield” we think that Onychodictyon were first described, based on two specimens,

both phenomena are artifacts of preservation. Although the as a caterpillar−like animal with “papillae” surrounding the

dorsal cephalic ridge of O. gracilis is a little sclerotized, the mouth and a pair of “antennae” on the “head” (Ramsköld and

head area of both O. ferox and O. gracilis shows no sign of Hou 1991; Hou et al. 1991). Later, these “papillae” were regarded as jaws ( Ramsköld 1992; Hou and Bergström 1995)

jaw−like structures or a head shield (Figs. 1A1–A5, 2A1–A5, and the pair of “antennae” was thought to be a misinterpreta− 3A1–A3, B1–B3), and this is a typical character of most lobopods, such as Cardiodictyon and Hallucigenia (Ramsköld and tion ( Bergström and Hou 2001; Hou et al. 2004). Then, Rams−

Chen 1998). A pair of anterior appendages occurs on the trunk köld and Chen (1998) and Chen et al. (2004) observed 10 new

of O. gracilis , and it appears that these appendages, which are specimens and interpreted the “jaw” structure as a head shield

based laterally on the trunk, and also anterior appendages in with a pair of appendages anterior to the pair beneath the first

the same are present in Aysheaia ( Ramsköld and Chen 1998) .

plate pair. Based on this information, we can see that the focus

We conclude that, thus far, the common features of O. of the debate is: (1) whether Onychodictyon possessed “anten−

ferox and O. gracilis strongly indicate that they belong to the nae” or not; (2) is there a jaw, a head shield, or anything else;

same genus. However, the significant differences between (3) is there a pair of anterior appendages? After examination of

them show that they are likely to be different species or possi− 70 specimens, we consider that there indeed exists a pair

bly sexual dimorphs. Together with Miraluolishania , the “anof “antennae” on the “head” of Onychodictyon ferox (Fig.

tennae” of these species are crucial to any exploration of the 1A 1 –A 5), which is quite similar to that of the coexisting lobo−

origin of antennae in arthropods.

pod Miraluolishania haikouensis in both its location and morphology ( Liu et al. 2004; Xiao 2004; Brigitte et al. in press),

both lum−like of them in shape located

. Furthermore on the dorsal

, the of

“antennae the “head

”

”

of and

Miraluo−

flagel− Acknowledgments lishania are regarded as quite similar to those of arthropods,

such as Fuxianhuia ( Hou and Bergström 1997) . Thus , this pair This work was supported by the National Natural Science Foundation of China (grants 40332016 and 04062003), the Program for Changjiang of “antennae” is not only an homologous structure in both O .

Scholar and Innovative Research Team in the Universities, and Minisferox and Miraluolishania , but also suggests that such “anten− try of Sciences and Technology of China (PCSIRT, grant 2000077700), nae” might be a primitive link between lobopods and arthro− “973” Program (grants 2006CB806401), Ph.D. Innovative Program of pods. Arthropod researchers generally agree that the anten− Northwest University (grant 07YYB02). The authors are indebted to nules, or first antennae, of crustaceans are homologous to the Simon Conway Morris (Department of Earth Science, University of antennae of other arthropod groups. Moreover, the antennae Cambridge, Cambridge, UK) for helpful discussion, Susan Turner of all arthropods seem to be associated with the unfused ante− (Queensland Museum, Australia) for improving the English, Zheng−

qian Luo for improving the line drawings, Paul Taylor (Natural History rior−most monosegments ( Edgecombe 1998; Schram and Hof

Museum, London, UK) for linguistically adjusted our text, and 1998). Besides, it is virtually certain that antennae of all ar− Yanchun Yao (both Northwest University, Xi'an, China) for field work thropods were originally uniramous (Hou and Bergström help. Special thanks are given to Huilin Luo and Shixue Hu (both 1997; Dzik 2003). New data indicate that onychophoran an− Yunnan Institute of Geological Science, China) for advice.

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