Trilobitenkunde von PETER A. JELL

PHYLOGENY OF EARLY CAMBRIAN TRILOBITES ES

Special Papers in Palaeontology, 70, 2003, pp. 45-571

by PETER A. JELL

 

ABSTRACT. From consideration of earliest Olenellina and Redlichiina versus later members of those groups it is argued from ontogenetic, outgroup, and stratigraphic data that the detached (natant) hypostomal condition is primitive and the attached (conterminent) condition derived. Several early redlichioid genera are demonstrated to resemble earlier olenelloid genera more closely than they do other redlichioids, begging the question 'did facial sutures evolve more than once among trilobites?' At least five separate lineages, from different very early Olenellina (Fallotaspidoidea), are identified, in each of which dorsal facial sutures appeared independently. These involve transition between the following pairs of genera or very closely related forms: Profallotaspis and Bigotina in Siberia; Archaeaspis and Uktaspis in Siberia; Eofallotaspis and Lemdadella in Morocco; Repinaella and Elganellus in Siberia; Choubertella and Yunnanocephalus in Morocco and China. Thus the Redlichiina, as constituted in the 1997 Treatise, is considered polyphyletic. Some preliminary speculations on evolutionary pathways between major groups in the Early Cambrian are outlined.

 

KEY WORDS: Cambrian, trilobites, evolution.

 

High level classification within the Trilobita, as proposed by Fortey (1990) and employed in the Treatise (Fortey and Owens 1997; Fortey 1997), accorded considerable importance to the position and mode of attachment of the hypostome relative to the rostral plate and the dorsal exoskeleton. In particular, members of the Order Ptychopariida were credited with a distinctive derived character, the hypostome not being exoskeletally connected to the cephalic doublure (i.e. detached). This paper sets out to re-examine that contention because it has been recognised that earliest Redlichiida (both Redlichiina and Olenellina) almost certainly shared the detached hypostome condition and thus it seems more likely to be the primitive condition. This proposition means that docking of the hypostome against the rostral plate in the Judomiidae led to all its descendants in the Olenelloidea having attached hypostomes and there was similarly a single event in each of the other lineages into the Redlichiina and Corynexochida when the hypostome became attached. This transition, occurring so often in later trilobite groups (Fortey 1990) thus always proceeded in the same direction and the need for Fortey (1990) to have claimed the later transitions as secondary reversals is removed. It is suggested that at least one lineage, giving rise to the Ptychopariida, retained the primitive detached hypostome throughout the Cambrian and beyond and that those groups with attached hypostomes evolved iteratively from one or a very few lineages. It also means that homoplasy was a strong factor from the very beginning of trilobite evolution, being exhibited in the acquisition of dorsal sutures and attached hypostomes on several occasions in the Lower Cambrian.

As a consequence of this study I also outline a new phylogenetic concept for earliest Cambrian trilobites, identifying some generic level transitions to derive major groups. This concept involves evolution of several different lineages beginning with different genera of the Fallotaspidoidea (sensu Palmer and Repina 1997) and leading to well-differentiated groups of later Lower and Middle Cambrian trilobites. If proven valid, these lineages would make the Redlichiina, as presented in the Treatise (Zhang et al. 1997), polyphyletic. The Olenellina is considered to have a common ancestor in Profallotaspis Repina (in Khomentovsky and Repina 1965) from the Profallotaspis jakutensis Biozone on the Siberian Platform, currently considered the earliest known trilobite (Palmer and Repina 1993, 1997). By extrapolation down the stratigraphic column, through more completely known descendants, Pro­fallo­taspis is assumed to have possessed those characters which Fortey and Whittington (1989) used to define the Trilobita. It is considered the most primitive trilobite known and a genus from which all other trilobites may have evolved.

Early evolution of the trilobites has been the subject of several cladistic analyses (e.g. Lauterbach 1983; Fortey and Whittington 1989; Lieberman 1998) but all have been seeking relationships of higher level groups within the Trilobita or, more commonly, of Trilobita with other early Arthropoda. Thus the Redlichiina has not been internally analysed and those that have dealt at the generic level within the Olenellina (Lieberman 1998, 1999) have concentrated on the Olenelloidea rather than the Fallota-spidoidea. The Olenelloidea is a further derivative from the Fallotaspidoidea (not shown on Text-fig. 1 as it is separate to the main thrust of this paper) through the Judomiidae, with the character transitions well outlined by Palmer and Repina (1997). Identification of lineages herein has similarly sought to identify important characters and their transitional states rather than a pigeonholing classification as attempted by the cladistic analyses mentioned above. So, for example, Lieberman's (1998, fig. 1) cladogram showing the Judomiidae as an entirely separate group from the Olenelloidea and from the Fallotaspidoidea misrepresents the valid conclusions of Palmer and Repina (1993) that there is a demonstrable transition in glabellar and palpebral features from the Fallotaspididae through the Judomiidae to the Olenelloidea. It is outside the scope of this paper to address the problems of applying cladistic analysis to early trilobite phylogeny. In so far as the success of a cladistic analysis depends heavily on the characters used (and characters not used) in the analysis I do not employ that technique, believing the approach taken by Palmer and Repina (1993, 1997) is more appropriate in our present state of knowledge and noting that Whittington (1989) did not offer a cladistic analysis in his discussion of olenelloid phylogeny.

THE PRIMITIVE HYPOSTOME ATTACHMENT CONDITION IN TRILOBITES
Fortey (1990), in what was a landmark paper on higher level trilobite systematics, considered the primitive hypostome attachment condition to be exemplified by the Redlichiida, that being the sister taxon of all 'higher' trilobites (i.e. excluding only the Agnostida and olenelloids). He relied on Redlichia longtangensis Zhang and Lin (in Zhang et al. 1980), with an attached hypostome, as an example for the entire Redlichiida to assert, along with other evidence, that the attached hypostome condition was primitive. However, the earliest Redlichiina and Olenellina with comparatively long preglabellar fields and short anterior borders and rostral plates (where known), for which a detached hypostome condition is almost certain, argue against it being a valid exemplar. Thus, it is necessary to examine the lines of reasoning employed by Fortey (1990) in drawing his conclusion.

The primitive state: Redlichiida
Under this heading Fortey (1990, p.540) argued that the attached or conterminent state was primitive because it was characteristic of the redlichiids. His 'Hypothesis of relationships placing Agnostida and Libristoma in the Trilobita as a whole' (Fortey 1990, text-fig. 14) showed the redlichiids as sister group to all trilobites except the olenelloids and Agnostida. This hypothesis showed the detached (or natant) hypostomal condition as character No. 5 and only in the 'Libristoma' (essentially the Order Ptycho­pariida, but considered by Fortey (1990, p. 550), even more inclusive, the inference being that the remainder of the groups shown had attached, or conterminant, hypostomes - except Agnostida, in which he emphasised that the detached condition is not homologous with that in his Libristoma).

However, there is good evidence that some Redlichiida have the detached hypostomal condition, including all the earliest Redlichiina and Olenellina. Ushbaspis (Zhang et al. 1980, pl. 30, figs 5-6), Wutingaspis (Zhang et al. 1980, pl. 41, fig. 12), Parabadiella (Zhang et al. 1980, pl. 46, fig. 1) have short rostral plates each with their posterior margin an even curve parallel to and beneath the border furrow, and have the anterior of their glabella well behind this line. In short they must have had detached hypostomes. While I would interpret Eoredlichia (Zhang et al. 1980, pl. 34, fig. 5; pl. 35, fig. 11; pl. 36, fig. 7) in the same way, I am aware that Shu et al. (1995, pp. 209, 218, fig. 4B) inferred a probable interrostrohypostomal plate in Eoredlichia intermedia based principally on the occurrence of 'fractures that mimic a plectrum-like extension of the anterior border' (Shu et al. 1995, fig. 4B). These fractures both continue in a straight line across the border at a low angle to the margin, and certainly at least the right-hand one continues beyond the margin into the matrix. The lateral margins of such a plate as illustrated by Zhang et al. (1980, pl. 20, fig. 9) and Fortey (1990, fig. 6) between hypostome and rostral plate is hardly likely to have been so well sclerotised as to cause such extensions of the fractures- However, the anterolateral margins of the hypostome leading along the posterior of the anterior wings could be expected to be well enough sclerotised to cause such fractures when crushed under the cranidium. Thus I suggest that the hypostome in the Shu et al. (1995, fig. 4B) specimen is twisted slightly to the left and pushed forward up against the rostral plate. The distinct straight ridge joining these two oblique fractures is probably the anterior of the hypostome interpreted as having moved forward because the anterior wings would, in life, have been beneath the anterior corners of the glabella farther back from the anterior border as in other specimens of Shu et al. (1995, pl. 1, figs 6-7). I conclude that none of the specimens illustrated by Shu et al. (1995) demonstrates a sclerotised exoskeletal plate between the rostral plate and anterior of the hypostome. Their interpretation of such a plate was clearly as a result of a preconceived acceptance of Fortey's (1990) interpretation for all Redlichiacea. They confirmed this preconceived acceptance in their treatment of Yunnanocephalus yunnanensis (Mansuy, 1912), which they incorrectly assigned to the Redlichiacea (=Redlichioidea). Assuming this assignment, they inferred a conterminant hypostomal attachment (Shu et al. 1995, p.227, bottom of left column) but in the same paragraph conceded that 'Neither of the conditions can be proved from the studied Chengjiang material.' Yunnanocephalus belongs to the Ellipsocephaloidea (Zhang et al. 1980), which has a predominantly detached hypostome condition and this is confirmed by their illustration (Shu et al. 1995, fig. 17A) of a rostral plate no longer than the anterior border, and the hypostome beneath the glabellar anterior, behind a long preglabellar field.

A range of early Siberian redlichiids [Elganellus Suvorova, 1958 (Repina 1966, pl. 7, figs 4-16), Tungusella Repina, 1960 (Repina in Khomentovsky and Repina 1965, pl. 5, figs 8-12; Repina 1966, pl. 5), Sajanaspis Repina, 1960 (Repina 1960, pl. 11, figs 1-2; Repina in Khomentovsky and Repina 1965, pl. 14, figs 7-12; pl. 15, figs 1-8) and Siberiaspis Repina (in Khalfin 1960; Repina 1966, pl. 15, figs 13-15; pl. 16, figs 1-8) among others], must also be interpreted as having detached hypostomes because of the relative position of glabellar front and anterior border even though their preservation in carbonates reveals no direct information on the ventral structure.

Bigotina bivallata Cobbold, 1935 was shown by Pillola (1993) to have a preglabellar field as long as, or longer than, the anterior border, which is connected to the glabellar anterior by a low plectrum. Pillola (1993) did not describe a rostral plate for Bigotina bivallata. However, his material, which I examined at The Natural History Museum, London, in April 2001, contains a few rostral plates which must be assigned to this species because they are identical to each other and no other trilobite species occur in the collection. Pillola presumably used the same reasoning to assign librigenae to this species. These rostral plates, one of which is numbered B48992b in the collection of the Museum National d'Histoire Naturelle, Paris. are as long as the anterior border and have no medial expansion that would suggest an attached hypostome. Bigotina bivallata is interpreted as having had a detached hypostome.

Redlichia itself, which was used by Fortey to characterise the redlichiids, is not the first Redlichiina to appear in the fossil record. It first appeared in the Chinese Changlangpu well above the first appearance of the group in the Chiungchussu. Its attached hypostome can be considered a shared derived character. Even within Redlichia where most species have attached hypostomes, some may not. Redlichia murakamii Resser and Endo (in Kobayashi 1935) from Liaoning (Zhang and Jell 1987, pl. 9, figs 1, 3-4) shows the ventral doublure with its inner margin evenly curved parallel to the margin and not expanded medially beneath the plectrum. To counter an argument that the medial posterior expansion of the rostral plate may not preserve well and be broken off these specimens, the dissociated rostral plate of Redlichia majori Lu, 1961 (Zhang et al. 1980, pl. 16, fig. 2) is preserved with the medial extension preserved, thus suggesting that if present it should be preserved, particularly on those specimens where the rostral plate is still in position beneath the cranidium.

Profallotaspis Repina (in Khomentovsky and Repina 1965), Pelmanaspis Repina, 1990, Eofallotaspis Sdzuy, 1978. Parafallotaspis Fritz, 1972, Lenallina Repina, 1990, Daguinaspis Hupe and Abadie, 1950, Choubertella Hupe, 1953, Archaeaspis Repina (in Khomentovsky and Repina 1965), Paranevadella Palmer and Repina. 1993, all illustrated by Palmer and Repina (1993), some Holmiidae and Repinaella Geyer, 1996 are among the Olenellina that can be interpreted as not having had the attached hypostomal condition. Geyer (1996, p. 114) made the point that no fallotaspid hypostome has been found and drew the assumption that it may not have been a mineralised sclerite but rather an unsclerotised labrum. Most importantly, Geyer (1996, fig. 21.1) showed Fallotaspis plana with the rostral plate in place and a long way in front of the glabellar anterior. Whether sclerotised or not the hypostome of F. plana and, by inference, those genera listed above, if situated under the anterior of the glabella, must have been separated from the rostral plate by quite a distance. Furthermore, attached hypostomes are generally more sclerotised than detached ones and so the assumption that fallotaspid hypostomes were unsclerotised allies them more closely with detached forms. This suggests that they were functionally detached hypostomes even if they are in fact unsclerotised labrums. Further study of this group and near relatives could be pivotal to understanding the origin of the sclerotised hypostome and its functional inception. Future discoveries could alter this interpretation but there are too many genera with long preglabellar fields for all of them to later prove to have attached hypostomes. It is significant that these taxa are among the earliest in the various sections in which they occur and the fallotaspidids in particular are the very earliest trilobites known.

Therefore, I contend that the character distribution (specifically character 5) illustrated for Fortey's (1990) text-figure 14 is flawed, making it inappropriate to be cited as the basis for concluding that 'the Redlichiida are ... the sister taxon of all 'higher' trilobites' (Fortey 1990, p. 540). I argue below that the ptychoparioids may have evolved from the ellipsocephaloids which in turn evolved from the bigotinids (Geyer 1990), which in turn evolved from the fallotaspidoids (below) or from a Yunnanocephalus-like form that also evolved from the fallotaspidoids, namely, the Daguinaspidinae. The redlichiids evolved from the fallotaspidoids along separate lineage(s) so that they are unrelated to the great bulk of trilobites with detached hypostomes, in particular the ptychoparioids.

NATANT HYPOSTOMAL CONDITION AS A SHARED DERIVED CHARACTER
Under this heading Fortey (1990) examined other accepted criteria for determination of polarity of a character shift, in this case that the detached hypostomal condition is a derived character. He quoted his comparison with Redlichia (referred to above) as exemplifying the criterion of comparison with the outgroup. However, I contend that Redlichia is an inappropriate outgroup since it is not part of the descent of ptychoparioids and is itself a derived form (in respect of this character) from earlier redlichiids. He then considered whether the detached hypostomal condition was unusual by con­si­der­a­tion of a wider range of arthropod outgroups.

How unusual a character is the natant hypostome?
Fortey (1990, p. 544) appealed to the generality of the attached hypostome in crustacean groups to argue that the detached condition was advanced and apomorphic. The space between the anterior of the hypostome and the rear of the rostral plate was occupied by a ventral membrane. Hypostomes are termed detached because the ventral membrane is not fossilised and thus they appear detached but a membrane of some sort must have enclosed the underside of the cephalon forward of the hypostome. The ventral membrane between the anterior margin and the anterior of the labral plate in the notostracan Triops is quite soft and flexible, much more so than the labral plate itself, which is well sclerotised in some places. It could be expected that the labral plate would fossilise well, but the more anterior ventral membrane may not fossilise at all. I am not aware of any fossil notostracans preserving the ventral structure to show what does or does not fossilise but the question of whether Triops has an attached or detached hypostome remains uncertain.

Perhaps more important than what may or may not fossilise is the search for functional homologues to the trilobite hypostomal conditions among other crustaceans. For example, in Triops, with a flexible ventral membrane anteriorly beneath the head (Jell 1975, pl. 1. fig. 2), there is the capacity for the labral plate to rock backwards and forwards on its fixed anterior wings as fulcral points. This suggests functional homology with the detached hypostomal condition in trilobites where the anterior and posterior ends are apparently not anchored and some rotation of the hypostome about the anterior wings has been postulated (Whittington 1988, p. 335). Thus the feeding habit of Triops, stirring up clouds of sediment beneath its dorsal carapace, and passing the edible particles from suspension to the mouth along the ventral food groove (Fryer 1988) is likely among trilobites (Fortey and Owens 1999, p. 430) with detached hypostomal condition some of which (e.g.Maladioidella) are fine particle feeders from sediment (Fortey and Owens 1999, p. 446). In most trilobites with an attached hypostomal condition the hypostome was held rigidly in place by its sutured margin to the rostral plate or doublure. This is an adaptation (conferring mechanical advantage for muscle attachment) to a predatory or scavenging (able to deal with larger food particles) feeding habit (Fortey and Owens 1999). Thus the widespread occurrence of the labral plate well back from the anterior border in the crustaceans quoted (Fortey 1990, p. 544) may be considered homologous in both position and function with the detached hypostomal condition and not the attached condition as argued by Fortey (1990, pp. 544-545). From a functional point of view, this suggests that the detached (or at least its functional equivalent) hypostomal condition was widespread among primitive crustacean groups rather than unusual as he concluded.

Ontogenetic evidence for character polarity
The ontogenetic argument that a progression from attached to detached hypostomal condition during ontogeny reflects phylogeny and thus attached ancestors for all trilobites is cast into doubt by the observations of McNamara (1986) that numerous Early Cambrian species, though not the oldest, acquired a well-developed preglabellar field (almost certainly indicative of a detached hypostome) during early meraspid growth and even the observation of Fortey (1990, p. 547) that Mindyallan Auritama trilunata Opik, 1967 had a similar early meraspid preglabellar field. This meraspid pre­glabellar field was later lost as the glabella once again grew forward. One could argue that this detached stage in ontogeny was the remnant of the detached ancestor and that the attached hypostome of the protaspid was related to necessities of size and mode of life rather than a reflection of ancestry.

Other noteworthy morphological observations associated with the protaspis to meraspis transition are:
1. The hypostome is larger relative to whole animal size in protaspides than later in development and its relative size decreases throughout growth. We can draw the broad analogy with human growth where the head is well-developed early in growth and assumes a progressively smaller percentage of the overall size. In the case of humans, the early head growth is thought to be driven by the early need to establish the housing for the large brain of the nervous system. In trilobites, I suggest that early growth of the hypostome has to do with early establishment of the alimentary system, to provide nutrition to the rapidly growing protaspis about to make a major change in feeding habit as suggested by Fortey and Owens (1999, pp. 453-454). I suggest further that food particles available to the earliest trilobites may have been of uniformly small size so that protaspides dealt with food particles larger relative to their body size than did meraspides. In this case the attached hypostome may have been necessary to give a musculature to deal with relatively larger particles. With growth to the meraspid and holaspid stages, dealing with particles of the same small size may have been more efficient with the detached hypostome arrangement because the food particles were relatively smaller. As animals became larger during the Early Cambrian available food particle size would have increased and some trilobites learned to deal with these progressively larger food particles. These trilobites gradually evolved the attached hypostomal condition for mechanical advantage to deal with this change in food size. Others retained their feeding habit on small particles of food and continued with the unattached hypostomal condition. Thus I suggest that where food particles were large in comparison to the consumer's body size (i.e. in protaspides and in scavenger/predator species), the attached hypostomal condition prevailed, whereas those species and growth stages that fed on tiny (relative to their body size) food particles had the detached hypostomal condition. I, therefore, contend that attached hypostomes in protaspides was a primitive feature to do with feeding at that body size and does not reflect phylogeny within the trilobites.

2. McNamara (1986, pp. 131-133) noted a major change in the glabella at this protaspis to meraspis transition, namely, in his terms a change from a glabellar stasis to retraction. It involves the glabellar furrows of most species changing from transverse to being angled adaxially to the posterior. He attributed this change to 'a posterior increase in the length of the stomach and the hypostome and a posterior displacement of the cephalic appendages'. I would agree except that I see a posterior movement of the hypostome rather than an increase in its length, acknowledging that either provides the same result of moving the rear hypostomal margin further from the anterior cephalic margin. Taking his recognition of the different attitudes of the glabellar furrows with what we know of hypostome attachment con­di­tions it appears that some corynexochids and certain other Early Cambrian trilobites that retain the stasis stage through to holaspides can be inferred to have had attached hypostomes throughout life and almost certainly evolved from ancestors with attached hypostomes. Using this tool it is possible to infer that the Dameselloidea evolved from ancestors with detached hypostomes as their inclined glabellar furrows clearly indicate McNamara's (1986) Retraction Stage in their ancestry and probably also in their ontogeny. This inference is contrary to the position taken by Fortey (1990, p. 565). However, he (Fortey 1990, p. 564) may be correct with regard to the Leiostegioidea, in which group are many forms retaining McNamara's (1986) Stasis Stage, suggesting attached hypostomes through its ancestry.

McNamara acknowledged the roles of both paedomorphosis and peramorphosis in trilobite evolution and emphasised the morphological plasticity of Early Cambrian species. Taken together these data sug­gest that we do not know enough to assemble a cogent ontogenetic argument on the polarity of this character.

Stratigraphic evidence for natant condition as a derived character
Stratigraphic distribution of attached and detached hypostomes in the lowermost Cambrian supports the detached hypostome being primitive and evolving to the attached hypostome state. Earliest trilobites such as all fallotaspids (except Fallotaspis itself) probably had detached hypostomes; certainly Pro­fallo­taspis, Eofallotaspis, Repinaella and Pelmanaspis must be interpreted as having had detached hypostomes although not directly observed. The earlier Bigotinidae and those early redlichiids men­tioned above, had detached hypostomes.

Fortey's (1990, p. 548) observation (quoted from Brasier 1989), that in no relatively complete section does a ptychoparioid occur before a redlichiid, is valid, but irrelevant because as shown above there are ample trilobites of other groups with detached hypostomes at the bases of various key sections around the world to support the detached condition as being primitive and evolving to the attached condition as an evolutionary move to feeding on larger food particles in changed life habits.

In conclusion, the strong stratigraphic evidence along with comparison with other arthropods and a better understanding of the problems associated with ontogenetic evidence combine to conclude that the detached hypostome was primitive for trilobites and that various groups attained the attached hypo­stomal condition independently in the Lower Cambrian as well as in later times as outlined by Fortey (1990). The above arguments and the considerations of Early Cambrian lineages below would neces­si­tate a revision of Fortey & Owens' (1997) figure 196 as shown in Figure 1.

PHYLOGENY OF THE EARLIEST CAMBRIAN TRILOBITESWITH DETACHED HYPOSTOMES
In a Russian language paper Repina (1990) first drew attention to the possibility of repeated appearance of cheek-bearing trilobites of the Redlichiina from representatives of the Olenellina. On the day after the Third International Symposium on the Cambrian System in Novosibirsk, Lada Repina, A. R. (Pete) Palmer, Zhang Wentang and I discussed this concept and identified further lineages between members of these two suborders. We decided to publish our collective conclusions in English so I agreed to draft the paper and those plans were alluded to by Palmer & Repina (1993, p. 18). Repina was describing some superb­ly preserved early redlichiids from western Mongolia in 1990 and I intended to wait for that pub­li­ca­tion before concluding this paper. However, Repina's Treatise contribution and then her untimely death prevented conclusion of her Mongolian work. In the interim, Geyer (1996. p. 159) quoted a per­son­al communication of these notions from Palmer in his discussion of fallotaspidoid phylogeny and Lieberman (1998) noted an affinity between Fallotaspidoidea and Redlichiina based on earlier papers of Palmer & Repina (1993) among others. Close similarities between particular early Olenellina and Red­lich­iina genera identified the several lineages between the two suborders (Text-fig. 2). Linking of the genera within the Olenellina is based on relative times of appearance and morphological transitions. Sug­gested descent from each of these lineages to major trilobite groups of the later Cambrian is clear in some cases but remains speculative in others and will be the subject of more detailed in­ves­ti­ga­tions.

Profallotaspis - Bigotina - Ellipsocephaloidea lineage
Profallotaspis Repina (in Khomentovsky & Repina 1965) is regarded as the oldest known trilobite on the Siberian Platform, and probably the world (Palmer & Repina 1997, fig. 254), occurring in the Pestrotsvet Formation on the right bank of the Lena River (Zhurinski Mis section) 300 km south-west of Yakutsk. It is succeeded closely by Repinaella Geyer, 1996 and Bigotina (Bigotinella) Suvorova, 1960 in the succeeding Fallotaspis (=Repinaella) Zone. Repina (1990) suggested that Bigotina (Bigotinella) evolved from Profallotaspis because they share the palpebral lobes curving away from and terminating quite some distance from the glabella (thus developing wide interocular cheeks), palpebral lobes joining (or fusing) into the glabella at anterolateral corners and similar preglabellar length. Pillola (1993) discussed Suvorova's (1960, pp. 37, 40) diagnoses in assessing differences between Bigotina Cobbold, 1935 and Bigotinella, which are not very great. They involve the width of the ocular ridges and palpebral lobes, whether these are bipartite or not, length of the frontal area and preglabellar convexity. He did not com­pare either genus with Profallotaspis but rather made comparisons with redlichiinids and ellipso­cephaloids, considering the Bigotinidae provisionally in the Redlichioidea whereas he considered Bigoti­nella an ellipsocephaloid. I assign both to the Bigotinidae, which gave rise to and is part of the Ellipso­cephaloidea (Geyer 1990). I agree with Geyer (1990) that the Ptychoparioidea probably arose from the Ellipsocephaloidea via one or more lineages, although a possible alternative route is mentioned below. I agree with Pillola (1993, p. 859) in considering Bigotina (and I add Bigotinella) as truly primitive non-olenellid trilobites but I consider them unrelated to Redlichiidae as they are at the base of the lineage that retained the detached hypostomal condition from Profallotaspis through the Ellipso­cephaloidea and probably into the Ptychoparioidea, which flourished in the Middle Cambrian. Palpebral lobes (especially their posterior tips) a long way distant from the glabella and thus wide interocular cheeks appear to be a good synapomorphy for this lineage (acknowledging some reversals, i.e. relative decrease in proportions but never the posterior tip approaching the axial furrow) in much later derived families.

Bigotina, Bulaiaspis and origin of the Agnostida
While Profallotaspis and Bigotina, at the base of this lineage, both have straight-sided glabellae, younger bigotinids such as Bigotinops Hupe, 1953, and Pruvostina Hupe, 1953 from Morocco and two species of Bigotinops from Siberia (all refigured by Pillola 1993, pl. 1, figs 3-6) have glabellae that gently taper forward. Bulaiaspis Repina (in Chernysheva et al. 1956) appears in sections on the Siberian Platform in the zone immediately above the first appearance of Bigotina. Its cranidium is very similar to the bigotinids mentioned above and its earlier growth stages (Suvorova 1960, pl. 2. figs 1-19) are also close to those of Bigotina (Pillola 1993, pl. 6). One obvious difference is that the transverse distance be­tween the facial sutures cutting the margin is far less in Bulaiaspis (64 per cent of cranidial width as op­posed to 91 per cent in Bigotina), but in this feature Bigotinops privus Suvorova, 1960 is transitional (70%). The pygidium of Bulaiaspis (Repina 1966, pl. 8, fig. 11) is also different from that of Bigotina (Pillola 1993, pl. 3, figs 3, 8) but again a transitional form may be found in B. prima (Repina 1966, pl. 10, fig. 6). I assign Bulaiaspis to the Bigotinidae and suggest that its assignment to the Chenkouaspidae by Zhang et al. (1997, p. 467) was not with firm conviction because it had earlier been placed under the heading 'Family Uncertain' (Zhang 1992, in the preliminary paper to his 1997 classification). The rela­tion­ship between the Bigotinidae and Chenkouaspidae also requires consideration, particularly in view of the relatively similar pygidia of Bigotina (Pillola 1993, pl. 2, fig. 3) and Chenkouaspis (Zhang et al. 1980, pl. 70, fig. 10) among pygidia that existed at the time.

Early meraspid cranidia of Bigotina (Pillola 1993, pl. 6), of Bulaiaspis (Suvorova 1960, pl. 2, figs 1-19) and of Moroccan fallotaspids (Geyer 1996, fig. 43; specimens in collection of T. P. Fletcher) all have a relatively narrow glabella reaching the anterior border furrow, weak glabellar furrows that are transverse (Stasis Stage of McNamara 1986), a semicircular (to posteriorly pointed) occipital ring and long occipital furrow, a slightly expanded anteriorly rounded anterior glabellar lobe and a short intergenal spine or strong geniculation of the border at that point. These are features of Tsunyidiscus Chang, 1966, the earliest eodiscoid, which was suggested by Jell (1997, p.384) as the most primitive eodiscoid. Jell (1997) made this same comparison between Tsunyidiscus and Dipharus clarki Korobov, 1980 from Mongolia, which he inferred may well be juvenile Bulaiaspis rather than an eodiscoid. This interpretation was based mainly on the pygidial pleural terminations. Pygidia of Bigotina (Pillola 1993, pl. 3, figs 3, 8), with posterodistally directed pleurae, may be more comparable to that of D. clarki and the inflated interocular cheeks of D. clarki are comparable with those of Bigotina coniferica Repina, 1960 (pl. 6, figs 11-13) from the Ketemenskii Horizon on the River Kolba in the eastern Sayan Altay mountains. Whatever the relationships among these species it is very probable that the eodiscoids, and subsequently the agnostoids, evolved from Bigotinidae (quite possibly Bulaiaspis, Bigotina or similar forms) via Tsunyidiscus in this very earliest phase of trilobite evolution by a population becoming reproductively mature while still retaining Meraspid Stage 3 morphology (i.e. by progenesis). Detailed mapping of the origin and evolution of the Agnostida is outside the scope of this paper, but to put forward Bulaiaspis and the Bigotinidae as its most probable immediate ancestors.

Archaeaspis - Uktaspis - Proasaphiscidae lineage
Archaeaspis Repina (in Khomentovsky & Repina 1965) appears in the zone above Repinaella sibirica Repina (in Khomentovsky & Repina 1965) and R. explicata Repina (in Khomentovsky & Repina 1965) or two zones (30-m section) above Profallotaspis on the Lena section and, in the succeeding zone, Uktaspis Korobov, 1963 appears. Archaeaspis, which may be a senior synonym of Bradyfallotaspis Fritz. 1972, is most distinctive in having its glabellar anterior well-rounded. This feature prompted Repina (in Khomentovsky & Repina 1965) to classify it with Callavia and prompts me to suggest it was probably involved in the origin, on the Siberian Platform, of both the Judomiidae (to which Palmer & Repina 1993 allied it in their classification) and the Holmiidae leading to the Olenellidae. Since Bradyfallotaspis is so similar to Archaeaspis it may represent a first migration from Siberia to Laurentia and the Olenellidae, derived from some of the judomiids, may represent a second wave of migration over the same or similar route. In Siberia the immediately succeeding zone yields Uktaspis and Prouktaspis Repina (in Khomentovsky & Repina 1965), which share with Archaeaspis interocular cheeks of similar width (narrower posteriorly than Profallotaspis and Bigotinidae but wider than most other Olenellina and Redlichiina), similar preglabellar fields and most importantly parallel-sided and anteriorly rounded glabellae with the palpebral lobe joining the glabella a little behind the glabellar anterior. The parallel-sided, anteriorly rounded glabella may be a good indicator for this lineage, which may have given rise via forms like Pseudozacanthopsis Repina (in Khalfin 1960) and Chondranomocare Poletaeva (in Chemysheva et al. 1956) to the Proasaphiscidae, Anomocarellidae, Anomocaridae and possibly Asaphiscidae, but the development of the pygidium from that of Uktaspis to Proasaphiscidae is not yet understood. An ancestor for Archaeaspis is not obvious at present.

Eofallotaspis - Lemdadella/Eoredlichia - Redlichiidae lineage
Eofallotaspis Sdzuy, 1978 and Lemdadella Sdzuy, 1978, both among the earliest trilobites in Morocco, share the tapering glabella, broad low bacculae, plectrum, and posterior width of interocular cheeks, but the latter has a dorsal facial suture. Despite this latter distinction these two genera can be regarded as an­other pairing of very similar morphology. Lemdadella and Eoredlichia Chang (in Lu & Dong 1952) are close­ly related (Zhang et al. 1997; Zhang 1986, p. 269) and despite arguments about generic as­sign­ments, it is clear that trilobites very closely related to Eoredlichia were present in China, Australia, Morocco and south-west Europe at about the same time. Thus the Redlichiidae probably evolved in this way from Eofallotaspis in the Moroccan region and migrated to eastern Asia on the other side of Gondwana. On the other hand, Parafallotaspis Fritz, 1972, which is very similar to Eofallotaspis, may have evolved from the Moroccan lineage and migrated to North America without developing a facial suture.

Repinaella - Elganellus - at least some Corynexochida lineage
In Siberia, Repinaella explicata Repina (in Khomentovsky & Repina 1965) and R. sibirica Repina (in Khomentovsky & Repina 1965), which evolved from Profallotaspis (Repina 1990, fig. 1), are ex­treme­ly similar to the redlichioids Elganellus and Tungusella Repina, 1960 (cf. Pegel 2000, figs 7.2 and 7.4 with fig. 5.2, and consider that the cephalic border in R. sibirica is wider than in R. explicata and as wide as in E. elegans) and represent another lineage where the appearance of the dorsal facial suture appeared independently. Tungusella may have given rise to Parapoliella Chernysheva, 1956, which had already as­sumed the attached hypostomal condition and other Siberian corynexochoids, including the Jakutidae Suvorova (in Chernysheva 1960) and Dinesidae Lermontova, 1940 (through Proerbia Ler­mon­tova, 1940) but whether it gave rise to the Zacanthoididae and Dolichometopidae of the Middle Cam­brian re­mains to be determined. The early part of this lineage is characterised by the glabella tapering forward to a truncated anterior, long narrow palpebral lobes with posterior tips well away from the axial furrow, lateral glabellar furrows directed posteroaxially from the axial furrow (certainly L0-L2; L3 and L4 some­times directed anteroaxially) and glabellar furrows well-impressed but rarely continuous across the axis.

Choubertella Hupe, 1953 and Daguinaspis Hupe & Abadie, 1950 are closely related fallotaspids that probably evolved by progenesis (McNamara 1988) so their immediate ancestor is difficult to identify. The similarity of the fallotaspid Daguinaspis and the ellipsocephaloid Yunnanocephalus Kobayashi, 1936 has already been noted by Shu et al. (1995, p. 230) who quoted the lack of genal spines, evenly concave preglabellar fields, and extraocular cheeks and moderately extended pleural spines. I suggest the similarity is probably closer to Choubertella, based on comparison of glabellar shape and size and shape of ocular ridges and palpebral lobes. Shu et al. (1995) sought to relate Yunnanocephalus to Eoredlichia, moving its assignment from the Ellipsocephaloidea (Zhang in Zhang et al. 1980) to the Redlichioidea, quoting two papers by W. T. Zhang as their reference for this. However, I could find no such sub­stan­ti­a­tion in either of the papers quoted and as far as I know Zhang's (in Zhang et al. 1980) assignment to Ellipsocephaloidea remains, as evidenced by its omission from the Redlichioidea by Zhang et al. (1997). Shu et al. (1995) further argued that the similarity between Yunnanocephalus and Daguinaspis was due to convergent evolution because the geographic separation of the genera prevented relationship. They dis­missed the similarities as 'a feeble trend towards overall similarity of the dorsal exoskeleton' and 'a superficial similarity' (Shu et al. 1995, p. 230). On the contrary, the similarities are quite striking, particularly when considered in the context of the very small range of morphology that existed in the trilobites at that early stage in their evolution. Ancestry of the Daguinaspidinae is not certain but can, on negative evidence, be limited to the fallotaspids and probably to a species close to Profallotaspis considering the wide interocular cheeks. From the discussion above, we know that Eoredlichia-like trilobites were able to migrate between China and the western Mediterranean region so it is quite possible for a Moroccan genus to have a descendant in China. I contend that future collecting will reveal these taxa along the migration route. Descendants of this lineage are not immediately obvious but the distinctive glabellar shape, almost transverse eye ridges, glabellar furrows and general aspect of the cephalon are very suggestive of the Ptychoparioidea. The question of whether the ptychoparioids may be polyphyletic is now opened up and since Yunnanocephalus has been classified in the Ellipso­cephaloidea that assignment needs reconsideration. This question is outside the scope of the present paper and will be addressed in a separate paper.


SUMMARY
The two main theses put forward are: (1) The primitive hypostomal condition among trilobites was the detached condition. This condition can be identified in virtually all of the earliest trilobites and while it persisted in at least one lineage throughout the Cambrian, numerous lineages evolving to the attached condition can be identified in the Early Cambrian just as occurred in later geological time. (2) At least five evolutionary lineages in the lower Lower Cambrian trilobite record provide strong arguments for the dorsal facial sutures having evolved independently in each from a fallotaspidoid ancestor with the typical olenelloid marginal suture. Speculation on the possible derivatives of these lineages has major im­pli­ca­tions for higher level trilobite evolution.

Acknowledgements. I am indebted to each of those colleagues mentioned above for sharing their thoughts on this subject with me. Lada Repina, in particular, had developed the best understanding of evolution among these earliest trilobite groups. However, I am entirely responsible for this paper. None of its shortcomings may be attributed to those colleagues who shared their knowledge. I thank Terry Fletcher for his hospitality and the opportunity to examine his collections of Moroccan trilobites. I am also grateful to the organisers of the Third International Conference on Trilobites and their Relatives, at Oxford University, for the opportunity to present these ideas.

 

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P. A. JELL

Queensland Museum
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