Profallotaspis jakutensis SIBERIA

The International Commission on Stratigraphy has proposed that the start of Series 2 of the early Cambrian is marked by the first appearance of trilobites in the fossil record. Depending on the dating system, this would be about 521 million years ago, on what are currently the continents of Africa (Morocco), Europe (Russia, Spain), and North America (Laurentia). A recent review (Hollingsworth 2008) lists the contenders for the earliest trilobites, including Profallotaspis jakutensis from Siberia; Fritzaspis sp. from the Esmeralda Basin of western Nevada and eastern California; and Hupetina antiqua from Morocco. Recent work in Spain (Linan et al 2008) adds Serrania gordaensis (another bigotinid) to this potential set of first trilobites. By comparison, the oldest trilobites from China and Australia (Abadiella) are significantly younger. Similarly, the Olenellus Biozone of Laurentia lies many meters above the Fritzaspis Zone.

Early Cambrian paleocontinental positions and the first trilobite localities.

Regional stratigraphy of the first trilobites
In Siberia, the first trilobites mark the base of the Atdabanian Stage. Profallotaspis is succeeded by two species of Repinaella, as well as Bigotina. As noted above, this set is most similar to the western Laurentian assemblage. The next zone is characterized by Delgadella (previously Pagetiellus) with Archaeaspis hupei at the base. These early trilobites appear long after archaeocyaths and brachiopods in the same area (Repina, 1981).  In Laurentia, the first trilobites appear below what was once considered the base of the Montezuman Stage, marked by the "Fallotaspis Zone." This Fritzaspis Zone is named for the first appearance of the trilobite Fritzaspis, a form lacking genal spines, which is rapidly followed by Profallotaspis?, Amplifallotaspis and Repinaella? which together comprise the Fritzaspis assemblage. In Morocco, the first trilobites appear simultaneously with archaeocyaths, with Hupetina appearing first,  Eofallotaspis appearing just afterward (Geyer, 1996; Sdzuy 1978). In Spain, bigotinids appear at a similarly early position (Pillola, 1993; Liñan et al., 2005). The first trilobites in south China and in Australia, both species of Abadiella (redlichiids), appear to be younger than the early trilobites of Siberia, Morocco, and western Laurentia (Paterson and Brock, 2007). Additional radiometric dates -- there being none from the Cambrian of western Laurentia -- and geochemical studies should resolve these correlations. For the remaining global regions, the first trilobites include Schmidtiellus in Baltica, and Callavia in western Avalonia. But both are distinctly younger than the old trilobites of Siberia, Morocco, Spain, and Laurentia.

The first trilobite genera  - DRAFT -
Very shortly after the appearance of the first trilobites, there are several other genera that appear in each of the regions. Here is a listing of some of these early genera, in rough order of appearance, oldest at bottom. The three tiers of this table roughly correspond to the  Lower, Middle, and Upper Atdabanian of Siberia. The boldfaced genera are the nominate genera for descibed biostratigraphic zones (e.g., the Fritzaspis Zone of Laurentia, the Eofallotaspis Zone of Morocco, etc.).

The first trilobite orders
Note that within the small set of first trilobites above, four orders are already represented: Redlichiida (represented by fallotaspidoids, redlichioids, and olenelloids), Ptychopariida (represented by ellipsocephaloids), Agnostida (represented by eodiscoids), and somewhat later, Corynexochida (represented by corynexochoids). What this really suggests is that early in trilobite evolution, the distinguishing features between the orders were not clearcut, and trilobites very close to the basal ancestors of those early lineages were developing characteristics that might be applied to several orders. At this early stage of evolution, the orders must be paraphyletic, if not polyphyletic. The similarities can be seen by this comparison of the cephala of a few representative early trilobites:

Fritzaspis LAURENTIA	Profallotaspis SIBERIA	Serrania SPAIN	Hupetina MOROCCO	Eofallotaspis MOROCCO
Archaeaspididae Fallotaspidoidea	Archaeaspididae Fallotaspidoidea	Bigotinidae Ellipsocephaloidea	Bigotinidae Ellipsocephaloidea	Fallotaspididae Fallotaspidoidea

All have parallel-sided or tapering glabellae with simple, even lobation, and well-developed, wide-set eye ridges that converge on and typically merge into the anterior lobe of the glabella. There is often a wide preglabellar area (indicating a natant hypostome). There may or may not be genal spines present, but when present, they can be well-developed. Another notable tendency is for there to be a rearward pointing angle in the posterolateral margins of the cephalon, sometimes bearing a metagenal spine. Some have suggested that the ridges and furrows running from the ends of the ocular ridges to the metagenal angle or spine in fallotaspidoid trilobites are homologous with the posterior facial sutures of suture-bearing trilobites. 

Facial sutures in early trilobites
From the very start of the trilobite record, facial sutures are present in bigotinids. In Spain and Morocco (Western Gondwanaland) Lemdadella is another early trilobite bearing sutures. Jell (2003) noted that sutures seem to appear independently in another major early lineage: the redlichiids of Australia and China, but it is not at all clear from what ancestor they arose. It is quite interesting that the first trilobite found in Morocco, below even the fallotaspidoids, is a suture-bearing bigotinid, Hupetina antiqua. This potentially complicates the idea that the first trilobites were sutureless, and their descendents evolved sutures. Instead it could also be that the fallotaspidoids and olenelloids represent an early lineage that did not develop facial sutures (other than marginal ones), while at least one very earliest clade bore sutures. Geyer (1996) even suggested that holmiid olenelloids may have arisen out of a sutureless variant of the otherwise sutured redlichioid Gigantopygus, which would be a reversal of the prevailing thought that sutureless trilobites preceed sutured ones! Geyer also noted that aside from sutures, it can be very difficult to distinguish between the cranidia of Lemdadella and Fallotaspis. The Bigotinidae were until recently placed in the Redlichioidea, but are now placed in the Ellipsocephaloidea in order Ptychopariida. The ellipsocephaloids are viewed as the bridge group between the orders Redlichiida and Ptychopariida, and in a review of the bigotinids, Pillola (1993) suggested that "bigotinids may represent a morphology close to the common ancestor of the two superfamilies of the opisthoparian trilobites at the beginning of Cambrian times." Indeed, in any chart of  trilobite evolution, if the bigtoinids are considered Ptychopariida, then that order must share the honor of the earliest clades of trilobites. It is more likely true that the ordinal separation of the earliest trilobites is problematic, with otherwise very similar and obviously closely related trilobites separated on the basis of facial sutures and little else. 

Early trilobite evolution
Jell (2003) offered several considerations on the early evolution of trilobites, presenting evidence that the primitive condition for hypostome attachment was natant, not conterminant as proposed by Fortey (1990). In that case, that primitive natant condition is maintained in the Redlichiida and the Ptychopariida, while the conterminant condition becomes the primary state for derived orders such as Lichida, Corynexochida, Asaphida, and Phacopida. Jell also offered an origin of the Agnostida via the Bigotinidae, demonstrating similarities between both the glabellar and pygidial characters of meraspid bigotinids and holaspids of early eodiscines such as Tsunyidiscus, and proposing an origin of the eodiscines via progenesis, retaining bigotinid meraspid morphology while attaining reproductive maturity. He also offered early evolutionary pathways for the origins of the primitive Asaphida and Corynexochida. 

Very shortly after trilobites appeared in the lower Cambrian, they rapidly diversified into the major Cambrian orders: Redlichiida, Ptychopariida, Agnostida and Corynexochida. These orders in turn proliferated, diversified, and gave rise to the rich variety of forms that trilobite workers have described in ten orders, well over 175 families, over 5,000 genera, and over 17,000 species, persisting for a quarter of a billion years before their extinction at the end of the Paleozoic Era.

Fortey, R. A. 1990. Ontogeny, hypostome attachment, and trilobite classification. J. of Paleontology. 33:529-76.

Geyer, G. 1996. The Moroccan fallotaspidid trilobites revisited. Beringeria, 18, 89-199.

Hollingsworth, J.S., 2008. The first trilobites in Laurentia and elsewhere. In: I.Rábano, R. Gozalo and D. García-Bellido (Eds.), Advances in trilobite research. Cuadernos del Museo Geominero, nº 9. Instituto Geológico y Minero de España, Madrid,

Jell, P. 2003. Phylogeny of Early Cambrian trilobites. In: Lane, P.D., Siveter, D.J. and Fortey, R.A. (eds.), Trilobites and their relatives, contributions from the Third International Conference, Oxford, 2001. Special Papers in Palaeontology, 70. The Palaeontological Association, London, 45-57.

Liñán, E., R. Gozalo, M. E. Dies Álvarez, J.A. Gámez Vintaned & S. Zamora. 2008. Nuevos trilobites del Ovetiense inferior (Cámbrico Inferior bajo) de Sierra Morena (España) [New Lower Ovetian Trilobites (Low Lower Cambrian) From Sierra Morena (Spain).] Ameghiniana 45(1)

Pillola, G.L. 1993. The Lower Cambrian trilobite Bigotina and allied genera. Palaeontology, 36, 855-881.

Sdzuy, K. 1978. The Precambrian-Cambrian boundary beds in Morocco (preliminary report). Geological Magazine; March 1978; v. 115; no. 2; p. 83-94. 

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Afterword:
When all is said and done, it is clear that trilobites began their evolution long before their first known appearance in the fossil record, presumably in the form of uncalcified arachnomorph arthropods, such as revealed in the rich assemblages known from Chengjiang and the Burgess Shale. Regional workers like to claim that their first trilobites are the oldest on the planet, but at any time in the future, a locality might be found that pushes trilobites even further back. For example,  the oldest trilobite in Laurentia was long "known" to be Fallotaspis tazemmourtensis, based on a single specimen in California described in 1964. However, this incomplete specimen was shown to be a misidentified product of transport from younger strata. Then, below the Fallotaspis Zone, an older assemblage (the Fritzaspis Zone described above) was discovered and described. So the search for the oldest trilobite will continue to be a work in progress.

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Walking Trilobite animation ©2000 by S. M. Gon III