Wonderful Life: The Burgess Shale and the Nature of History (29 page)

3.47.
Molaria
, a unique arthropod of “merostomoid” form (Whittington, 1981).

3.48. The tuberculate arthropod
Habelia
. Drawn by Marianne Collins.

Habelia
has the same basic shape as
Molaria
, but Whittington also described an impressive set of differences, some of high taxonomic significance. The carapace is covered with tubercles—a superficial though visually striking difference (figure 3.48). The trunk has twelve segments, with no cylindrical telson. The extended tail spike, ornamented with barbs and ridges, is unsegmented, but has a single joint about two-thirds of the way back. The head has a pair of antennae and only two pairs of subsequent ventral appendages. The first six trunk segments bear biramous appendages, but the last six probably bore gill branches only (in
Molaria
all eight body segments bear biramous appendages.)

Whittington also discovered a new arthropod genus—a complex, tiny creature less than a half inch in length (figure 3.49). This unique and peculiar animal, named
Sarotrocercus
, has a head shield followed by nine body segments and a tail spike with a tuft of spines at its tip. A large pair of eyes, borne on stalks, protrudes from the bottom front end of the head shield (
Molaria
and
Habelia
are blind). In addition, the head carries one pair of thick, strong appendages terminating in a two-pronged segment. Whittington also found ten very different pairs of appendages (one pair on the head and one on each of the nine body segments)—long comblike structures, presumably gill branches, but without any evident trace of a leg branch. Whittington reconstructed
Sarotrocercus
as a pelagic animal, swimming on its back with
Amiskwia
and
Odontogriphus
among the rare Burgess organisms that probably lived in the water column above the stagnant basin that received the mudslide.

3.49. The tiny arthropod
Sarotrocercus
, swimming on its back. Note the large eyes, the strong pair of feeding appendages, and the gill branches, presumably used for swimming, on the body segments behind. Drawn by Marianne Collins.

Actaeus
, based on a single specimen two inches long, has a head shield with a marginal eye lobe, followed by eleven body segments and an elongate, triangular terminal plate (figure 3.50). The head bears a pair of remarkable appendages, each with a stout initial portion, bent and extending downward, ending in a group of four spines. Two very long whiplike extensions attach to the inner border of the last segment, and run down and back. Behind this structure, the head probably carried three pairs of ordinary biramous appendages.

Alalcomenaeus
has a basically similar look and arrangement of appendages (see figure 3.50), and may be related to
Actaeus
. A head shield, bearing a marginal eye lobe, is followed by twelve body segments and an ovate terminal plate. The head bears a pair of large appendages, each with a broad initial section followed by a long thin extension—not nearly so complex as in
Actaeus
, but similar in style and position. The head also carries three pairs of biramous appendages. One specimen reveals an impressive set of spines on the inner surfaces of the walking legs—in proper position for passing food forward to the mouth. “These remarkable appendages,” Whittington wrote, “suggest a benthic scavenger, able to hold on to, and tear up, a carcass” (1981a, p. 331).

Aside from a very tentative relationship between
Actaeus
and
Alalcomenaeus
, each of the five genera presented a highly specialized design based on unique features and arrangements of parts. Whittington concluded, echoing the now-familiar Burgess story:

In 1983, Bruton and Whittington combined to deliver the
coup de grâce
by describing the last two major Burgess arthropods—the large
Emeraldella
and
Leanchoilia
, last two members of Størmer’s discredited Merostomoidea.

3.50. Two arthropods that may be closely related (Whittington, 1981). (A)
Actaeus
. (B)
Alalcomenaeus
.

3.51.
Emeraldella
, seen from above (A), and from the side (B), resting on the bottom. The very small gill branches of the biramous appendages indicate that this animal walked on the sea floor.

Emeraldella
possesses the basic “merostomoid” form, but accompanied by yet another set of unique structures and arrangements. The typical head shield bears a pair of very long antennae, curving up and back, followed by five pairs of appendages, the first short and uniramous, the last four biramous (figure 3.51). The first eleven trunk segments are broad, though progressively narrowing toward the rear, and each bears a pair of biramous appendages. The last two segments are cylindrical, and a long unjointed tail spine extends at the rear.

Leanchoilia
also shares the superficiality of general “merostomoid” shape, with a triangular head shield (terminating in a curious, upturned “snout”), followed by eleven body segments, narrowing and curving backward beyond the fifth. A short triangular tail spine with lateral spikes caps the nether end (figure 3.52).
Leanchoilia
bears thirteen pairs of biramous appendages, two at the rear of the head shield, one on each of the eleven body segments.

But
Leanchoilia
also possesses the most curious and interesting appendage of any Burgess arthropod—an exaggerated version of the frontal structure of
Actaeus
, a possible relative. Borrowing a term from
Yohoia
, and in the absence of any appropriate technical name, Bruton and Whittington simply called this structure the “great appendage.” Its basal part contains four stout segments facing down at first, but bending through ninety degrees to run forward. The second and third segments end in very long, whiplike extensions, annulated over the last half of their length. The fourth segment has a tapering shaft ending dorsally in a group of three claws, and extending ventrally as a third whiplike structure with annulations. The different orientations of various specimens indicate that this great appendage was hinged at its base (figure 3.53) and could extend forward, to help
Leanchoilia
repose on the substrate (figure 3.54), or bend back, perhaps to reduce resistance in swimming. Further evidence for swimming as a primary mode of life comes from the biramous appendages. Unlike
Emeraldella
, with its long walking legs and small gill branches,
Leanchoilia
bears such large gill branches that they form a veritable curtain of overlapping, lamellate lobes, completely covering and extending beyond the shorter leg branches underneath.

The completed redescription of all “merostomoid” genera prompted Bruton and Whittington to reflect upon the incredible disparity uncovered beneath a superficial similarity of outward form. Consider only the arrangement of appendages on the head—an indication of original patterns in segmentation, and a guide to the deep anatomical structure of arthropods.
Sidneyia
has a pair of antennae and no other appendages.
Emeraldella
also bears pre-oral antennae, but has
five
additional pairs of appendages behind the mouth, one uniramous and four biramous.
Leanchoilia
does not possess antennae, but bears its remarkable “great appendages,” followed by two biramous pairs behind the mouth.

3.52. Top view of
Leanchoilia
. Note the three whiplike extensions of the great appendage in front and the triangular tail spine behind.

The Burgess had been an amazing time of experimentation, an era of such evolutionary flexibility, such potential for juggling and recruitment of characters from the arthropod grabbag, that almost any potential arrangement might be essayed (and assayed). We now recognize clear groups, separated by great morphological gulfs, only because the majority of these experiments are no longer with us. “It was only later that certain of these solutions were fixed in combinations that allow the present arthropod groups to be recognized” (Bruton and Whittington, 1983, p. 577).