Fallotaspis is an early Cambrian trilobite.
|The earliest trilobites appear in the lower Cambrian record. These oldest trilobites include members of Order Redlichiida, Suborder Olenellina, Superfamily Fallotaspidoidea and Order Ptychopariida, Suborder Ptychopariina, Superfamily Ellipsocephaloidea. Even these early representatives bear all of the defining characters of trilobites. Probably the key distinguishing character, one that also allowed trilobites to be preserved so well (and which accounts for their sudden prominence in the Cambrian), is calcification of the exoskeleton. It is interesting that some of the most primitive of trilobites lack a few characters that more advanced trilobites bear. Fallotaspidoids lack facial sutures, for example, and their protaspides are apparently uncalcified. If the ancestors of trilobites in the Precambrian were uncalcified, then their preservation would be restricted to konservat-lagerstätten, which are very rare indeed. Cambrian konservat-lagerstätten such as the Burgess Shale (Canada) and Chengjiang (China) demonstrate the huge diversity of non-calcified arthropods that would have escaped detection if those sites did not exist. Many of these are arachnomorphs closely related to trilobites and relevant in a search for trilobite ancestors. But where did these arachnomorphs come from?|
|Precambrian trace fossils
.Elsewhere on this website, the relationships between trilobites and other Cambrian arthropods were discussed, and it is clear that trilobites were one of several families of arthropods in the early Cambrian oceans. It is reasonable that in the late Precambrian, early arachnomorphs were present, and we can infer their presence through such evidence as ichnofossils (preserved tracks and traces of animals, such as the walking traces to the right) as well as rare fossils of metameric, arthropod-like organisms. The search for the Precambrian ancestors of trilobites rather easily blends into the search for the first unambiguous arthropods in the Precambrian fossil record. There are only a handful of sites on Earth that bear Precambrian metazoan fossils at all, and there are no Precambrian konservat-lagerstätten that quite match the remarkable level of preservation found at Burgess and Chengjiang. The few that exist provide some candidates for arthropod ancestors. Fossils from Australia and Russia are particularly compelling.
Although many of the fossils of the Precambrian seem incomparable to living forms today, quite a few have been interpreted as metameric bilaterians (segmented creatures with a "head" and "tail" end, and mirrored side structures). Much has been made, for example, of the Ediacaran fossil Spriggina floundersi from Australia (top left), which most workers agree shows a cephalic shield with genal spines, and numerous similar segments running backward in a rather worm-like manner (Glaessner 1979). Recent arguments that Spriggina is a trilobitoid ecdysozoan have been made (McMenamin 2003), but the segments of the body alternate along the midline in a decidedly non-arthropod manner, arguing strongly against arthropodan nature (e.g. Ivantsov 2001).
Another very rare Precambrian species cited as an early arthropod, Bomakellia kelleri (top right), bears a semicircular cephalon and a tri-lobed body bearing what might be filamentous gills typical of arachnomorph arthropods. It should be noted, however, that none of the Precambrian species clearly shows antennae or jointed limbs (but then again, such features are rarely preserved even among bona fide trilobites). Assessments of Precambrian taxa such as these and their relationships to arthropods and other groups have often generated as many questions as they sought to answer (e.g., Waggoner 1996).
An un-named "soft-bodied trilobite" from the Flinders
site in Australia (middle left) also might seem a reasonable Precambrian
candidate antecedent to true trilobites (Gehling 1991). At first glance,
it resembles the many-segmented Redlichiida, but it also resembles small specimens of
Archaeaspinus (bottom left), Dickinsonia, or Vendia, which most workers do not consider to be of arthropod affinity.
Nonetheless, this taxon, as well as Parvancorina minchami (Glaessner
1980) both suggest that arthropods did not miraculously appear in the Lower
Cambrian, but took form during the Precambrian. The specimen of Parvancorina
to the right even bears fine lines that some workers have interpreted as
evidence of paired limbs. But how could such a simple body plan be thought
of as arthropodan?
Parvancorina michami (left), Primicaris larvaformis (center), trilobite protaspis (right)
|Parvancorina: a Precambrian trilobite
.The similarity of the Precambrian Parvancorina to the Cambrian Chengjiang arthropod Primicaris larvaformis, and a protaspid of a Cambrian trilobite is seen to the left. All three have an ovoid form, and an anchor-like structure made up of an axial lobe and lateral lobes running along the anterior and lateral edges of the body. Primicaris was first thought to be a larval naraoid (e.g., Hou & Bergstrom 1997), but it was recognized more recently as taxon in its own right (Zhang et al 2003). Recapitulating phylogeny, the trilobite protaspid resembles Primicaris. If the similarity of Parvancorina to Primicaris is more than superficial, it is perhaps the best candidate for an early arthropod in the Precambrian.
Skania fragilis: a Primicaris-like Burgess
|So where did trilobites
The likely scenario is that trilobites arose from Precambrian bilaterians, arguably arthropods, that gave rise to Cambrian arachnomorphs, among them trilobites. The evidence is neither clear nor unambiguous. The fossil record is spotty, but suggestive, and only some remarkable sites such as Chengjiang, Kaili, and the Burgess Shale reveal the rich diversity of non-calcified arachnomorph arthropods. The fossils of the Precambrian reveal some bilaterian diversity, among them a few species that might be candidates for trilobite ancestors. Perhaps it is the simple, dorsally unsegmented Precambrian fossil, Parvancorina, that offers the most reasonable link to arachnomorphs. Lin et al, 2006 strongly linked Parvancorina to an unambiguously arthropodan Cambrian creature, Skania sundbergi, closely related to Primicaris larvaformis. Similar taxa have been documented in Australia, Chengjiang, Kaili, and the Burgess Shale (see image of Skania fragilis, left). If neither Skania nor the protaspid stage of trilobites were preserved, it would have been difficult, if not impossible, to make the link between Parvancorina and trilobites. As it is, both Parvancorina and Skania/Primicaris can be placed in a relationship that might look something like the sequence below.
From Parvancorina to Trilobite
in Four Easy Steps:
When examined comparatively, as above, it is fairly easy to see how separation of cephalon from body, and then addition of additional tagmata (segments) during growth are the key developmental character additions that take us from Parvancorina to a trilobite form. Taken in sequence, and with legs added to accentuate their underlying shared arthropod heritage, the links between Parvancorina, Primicaris, Naraoia, Kuamaia (a helmetid), and trilobites seem easier to visualize in the sequence below:
pygidium & thorax
cephalon, pygidium, thorax
additional trilobite features
Chen, Edgecombe, & Ramskold. 1997. Morphological and ecological disparity in naraoiids (Arthropoda) from the Early Cambrian Chengjiang fauna, China. Records of the Australian Museum 49:1-24.
Gehling, J.G. 1991. The case for Ediacaran fossil roots to the metazoan tree. Memoirs Geological Society of India. 20: 181-223.
Glaessner, M.F. 1979. Precambrian. In: R.A. Robison & C. Teichert (eds.) Treatise on Invertebrate Paleontology. Geological Society of America and University of Kansas, Boulder, Colorado, and Lawrence, Kansas. Part A. 79-118.
Glaessner, M.F. 1980. Parvancorina - an arthropod from the Late Precambrian fauna of the Ediacara Fossil Reserve. Records of the South Australia Museum 13:83-90.
Hou X. & J. Bergstrom 1997. Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils and Strata 45:1-116.
Ivantsov A.Yu. 2001. Vendia and other Precambrian "arthropods." Paleontological Journal 35:335-343.Lin, J. P. 2003. An Ediacaran-like arthropod from the Kaili biota (Middle Cambrian, Guizhou Province, China) - A missing link for assessing the early evolution of arthropods. [unpublished proceedings of the 2003 Geological Society of America conference]. http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_59267.htm
Lin, J.P., S.M. Gon III., J.G. Gehling, L.E. Babcock, Y.L. Zhao, X.L. Zhang, S-X. Hu, J.L. Yuan, M.Y. Yu, & J. Peng. 2006. A Parvancorina-like arthropod from the Cambrian of South China. Historical Biology 18(1): 33–45.
MacNaughton, R. B. A review of the evidence for trilobite predecessors in the fossil record. http://www.swcp.com/~diamond/trilobites.html
Waggoner, B, M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45(2):190-222.
Zhang, X., J. Han, Z. Zhang, H. Liu, and D. Shu. 2003. Reconsideration
of the supposed naraoiid larva from the Early Cambrian Chngjiang Lagerstätte,
South China. Palaeontology 46(3):447-66.