Autecology of selected fossi1 organisms: Achievements and problems. A. Cherchi (ed.). Boll. Soc. Paleont. Ital., Spec. Vol. 3, Mucchi, Modena, 1996, 145-158, 3 pls., 6 text-figs.

The Giordunellu Bornemann, 1891 from Lower of Sat-dinia, (Italy): a discussion on its morphology and possible mode of life

Gian Luigi PILLOLA Dipartimento di Scienze della Terra Università di Cagliari

KEY WORDS - Trilobita, Functional morphology, Palaeoecology, Twace ridges, Pits, Lower Cambnan, Sardinia, Italy

ABSTRACT - The trilobite Giordanella Bornemann, 1891 (Dolero1enidae)fiom the Lower Cambrian Ne& Group of SWSardinia, is represented by two species: G. rneneghinii (Bornemann, 1883) and G. vincii Pillola, 1991 bothlshowing the earliest known examples of illaenid- like morphology. 7%e possibtlity that they spent part of their life in infaunal stance is suggested by the comparison with other with similar adaptivejèatures, some of which are claimed to bave been recovered in l+ position.

Giordanella vincii irobablv, revresents ' an active ebifaunalL, trilobtte., livine close to calcimicrolnal-archaeocvathanmounds ofthe middle and upper Sa Tuvara Member (Matoppa Formation). It occasiomlly rnay bave assumed- a bumastoid stance inclined at low angle. The abundance of Giordanella rneneghinii in higher energy environments seem to be related to its pre-adapted morphology; its burrowing attitude may bave been exploited for protection against water turbulence, but also may bave been used for food gathering. The terrace ridge position and geometry show that the ventral ones and those on the dorsal stde on the pygtdium of G. meneghinii could bave functioned as sediment gripers. Those closely associuted with pits on the cephalic border may bave had a predominantly current-monitoring junction.-.

RLASSUNTO - [Il trilobite Giordanella Bornemann, 1891 (Cambriano inferiore, SW Sardegna, Italia): considerazioni sulla sua mor- fologia e modo di vita] - Giordanella Bornemann, 1891 (Trilobita, Dolerolenidue), proveniente dal Gruppo di Ne& del Cambriano inferiore del SW della Sardegna, è il più antico esempio di trilobite illaeniforme finora noto; esso è rappresentato da due specie: G. meneghinii (Borne- mann, 1883) e G. vincii Pillola, 1991. La possibilità che questi due taxa trascorresseroparte della loro vita in posizione infaunale è suggerita dal confronto con altri trilobiti aventi morfologia e strutture adattative simili; fra questi ultimi alcuni sono stati ritrovati in posizione di vita. Giordanella vincii era probabilmente un trilobtte epibentonico, vivente in prossimità dei mounds a calcimicrobt e archeociatidi del Mem- bro di Sa Tuvara (Formazione di Matoppa), che occasiomlmente poteva assumere la posizione 'bumastoide' inclinata a basso angolo. L'abbondanza di G. meneghinii in ambienti a più elevata energia sembra legata alla morfologia preadattata; l'attitudine endobentonica parziale poteva essere sfruttata per una protezione rispetto alla turbolenza dell'acqua ed, eventualmente, per la ricerca di nutrimento. La posizione e la geometria delle terrazze cuticolari suggerisce che quelle situate sulla superjìcie esterna della duplicatura e quelle nei bordi della faccia dorsale del pigidio potrebbero aver avuto una funzione meccanica di tipo frizionale o stabtlizzante. Le twazze cuticolari intima- mente associate alle perforazioni localizzate al margine del ctphalon potrebbero aver espletato una funzione sensoriale predominante.

INTRODUCTION formation and extends in the younger Monte Azzieddas Mb. and part of the Punta su Pranu Mb. Giordanella Bornemann, 1891, is a redlichioid of the Punta Manna Formation (Text-fig. 1). trilobite from the Lower Cambrian Nebida Group The diagnostic characters allowing distinction of the Iglesiente area (SW Sardinia, Italy). Th'1s genus between G. meneghinii and G. vincii are mostly is placed in a distinct subfamily (Giordanellinae located in the cephalic region: G. menegbinii has Pillola, 1991) belonging to the Dolerolenidae shorter palpebral lobes and a much longer distance Kobayashi 1951; this assignment is mostly based on (exsag.) between the posterior tip of the palpebral the strong similarity between the ontogenic develop- lobes and the posterior margin of the fixed cheeks. ment of Giordanella and Dolerolenus Leanza, 1949 This trend is more pronounced in larger cranidia (Pillola, 1991). and proves an allometric growth for the cephalic Two species of Giordanella have been retained parts mentioned above (Pillola, 1991, fig. 35, herein from the previous literature: G. menegbinii (Bor- Text-fig. 2). nemann, 1883), and relative synonyms (see Rasetti, Some morphological features seem to be peculiar 1972; Pillola, 1991), and G. vincii Pillola, 1991. to G. meneghinii: These two species do not coexist in any known sec- - occurrence of terrace ridges on the cranidial tion: G. vincii occurs in the middle and upper parts border, on the free cheek margins and on the latero- of the Sa Tuvara Member of the Matoppa For- dorsal regions of the pygidium (ventral terrace ridges mation, while G. meneghinii appears in the upper- on the doublure occur in both species). most beds of the Sa Pruixina Member of the same - occurrence of pits (= punctae, fine per- 146 G. L. PILLOLA

Text-fig. 2 - Scatter diagram showing plot of the exsagittal length of the palpebra1 lobe (LLp) against the total sagittal cranidial length (LCr) in Giordanella meneghinii (Bornemann, 1883) and in G. vincii Pillola, 1991. Note: in the representation of the cranidium of G. vincii and of the cephalon of G. meneghinii the vertical mid-lines separates the in- ternal mould (left side) from the external surface (right side). After Pillola, 1991, modified.

forations of the test), covering the whole dorsal carapace. The genera1 aspect of Giordanella and the typical features of G. meneghinii will be described below and discussed in comparison with other effaced trilobites, in order to propose a possible mode of life. The discussion of (1) the functional morphology (considering the balance between well adapted and - unfavorable features for a specific function) and also (2) the analogies with living and (3) the independent geologica1 information (see For- tey, 1985), may provide a reasonable and safer basis . for the deduction of the probable function of some features and, as far as ~ossible,the trilobite ecolog~.

SEDIMENTARY AND FAUNAL CONTEXT

The Cambrian sequence from SW Sardinia is composed of three main lithological units which are, in ascending order: - the Nebida Group (Atdabanian to Botomian), including mainly terrigenous clastics with microbial- archaeocyathan buildups (Matoppa Formation) and mixed siliciclastic and carbonate de~osits(Punta Text-fig. 1 - Schematic column of the Lower Cambrian Nebida Manna Formation); Group from eastern Iglesiente (SW Sardinia, ItaIy), - the Gonnesa Group (Botomian to Toyonian), showing the range of Gi~~~nel~avinci2 and G. consisting exclusively of carbonate sediments, with meneghinii. a) Sa Tuvara Mb.; b) Sa Pruixina Mb.; C) Monte Azzieddas Mb.; d) Punta Su Pranu Mb.; e) dolostones at the base (Santa Barbara Formation) , cuce"Aspu Mb.; 0 Biasterria ~b.~f~~~ and limestones (more or less dolomitized) at the top Pillola, 1991, rnodified. (San Giovanni Formation); THE TRILOBITE GIORDANELLA BORNEMA NN 147

- the lower part of the Iglesias Group (upper- Trilobites and other fauna1 remains occur more most Toyonian to Tremadoc), which begins with or less close to the carbonate mound systems. The the argillaceous or nodular limestones of the Campo trilobite communities are dominated by Meta- Pisano Formation (Toyonian-mid Middle Cam- redlichiinae (Iglesiella, Sardoredlicbia, Metaredlicbia) brian); this latter is overlain by the Cabitza For- and/or Dolerolenidae (Dolerolenus, Gior'danella) mation, represented by siltstones, sandstones and while Pararedlichiinae, Metadoxididae and eodiscids shales, and sporadic limestones, yielding, in its mid- are minor components. Other skeletal remains are dle part, a Lower Tremadoc fossi1 assemblage represented by Archaeocyatha, both inarticulate and (Pillola, 1990, 1991). rare articulate brachiopods, molluscs (Stenotbecoides, Giordanella is the commonest trilobite of the Hyolitha) and Coeloscleritophorans (Cbancelloria, Nebida Group and is widely distributed thorough Ez;ffelia?). the Iglesiente and Sulcis (SW Sardinia). G. vincii oc- During deposition of the Sa Pruixina Member an cur in the middle and upper part of the Sa Tuvara important terrigenous episode, related to a sand shoal Member (lower Matoppa Formation), N2 (pars) environment, stopped definitively the development trilobite Zone. G. menegbinii appear at the top of Sa of the Epipbyton-Renalcis dominated mounds. Later Pruixina Member (upper Matoppa Formation) con- on, this shallowing upwards sequence evolved into comitant with the lowermost occurrences of subtidal to pure oolitic shoal and back shoal environ- Dolerolenus longioculatus Pillola, 1991 and the disap- ments of the basal Punta Manna Formation. pearance of D. aff. courtessolei (N2 to N3 transition) The Sa Pruixina Member is generally poorly and extends in the younger Monte Azzieddas and fossiliferous, except near the top, where the very Punta Su Pranu Member of the Punta Manna For- rich Giordanella menegbinii beds occur. mation (Text-fig. 1). The last occurrence of G. The Monte Azzieddas Member consists of a menegbinii took place in the upper part of the N3 cyclic succession of oolitic-oncolitic grainstones Zone, just below the massive occurrence of Enan- alternating with calcimicrobial-archaeocyathan tiaspis enantiopa (Bornemann, 1891). boundstones, peloidal wackestones to grainstones The Sa Tuvara Member is characterized by the and minor terrigenous layers related to an oolitic occurrence of thick sandy sequences and thin silty shoal belt. These sediments indicate a clear change in layers alternating with finer clastics and local environmental conditions in comparison with the calcimicrobial-archaeocyathan mounds built up underlying Matoppa Formation. The oolitic shoal mainly by Epipbyton and Renalcis and minor Gir- protected a back shoal area situated further to the vanella. east. Peloidal wackestones and grainstones dominate The clastic sediments display principally fine with ooid-grainstones (spillovers) in the shallower laminations: horizontal and low angle cross bedded parts of the back shoal area (inter- to subtidal). laminated sandstones, HCS coarse sandstones. Rare Lagoonal parts of the back shoal area contain calci- ball-and-pillow structures occur. Thinly laminated microbial-archaeocyathan biostromes, dominated facies with some lenticular bedding are quite com- by Girvanella (calcimicrobial boundstones, baf- mon in the muddy deposits; intense bioturbation flestones and bindstones, archaeocyathan floatstones locally caused homogenisation leading to mottling and lithoclastic floatstones). and destruction of primary bedding. The basal oolitic, Monte Azzieddas Member Calcimicrobial-archaeocyathan mounds occur at yields rare Giordanella menegbinii and, occasionally, several levels in the Sa Tuvara Member sequence of Dolerolenus longioculatus. centra1 and western Iglesiente, commonly closely This unit is overlain by the Punta Su Pranu associated with fine deposits, which may laterally Member, which consists of a thick sequence of well become coarser. bedded calcareous sandstones, with subordinate The repeated occurrence of several coarse clastic quartzarenites, oolitic and/or oncolitic limestones, dominated sequences alternating with finer deposits mudstones, local archaeocyathan-dominated and carbonate mounds units, reveals a certain framestones, and siltstones. Trilobites are fragmen- «cyclic» arrangement of the Sa Tuvara Member tary and randomly dispersed in accumulation beds, sequences (Pillola, 199 1). with some traces of erosion and breakage before The sedimentary features of each unit, their burial. Giordanella menegbinii predominates during repetitive occurrence in a vertical profile, and the the deposition of the lower portion of the Punta Su lateral facies distribution, strongly suggest prodelta Pranu Member. Subordinate taxa include (front delta slope) to delta bottomset environments, Dolerolenus longioculatus, Sardoredlicbia arenivaga passing into sub-tidal and tidal mud flats towards the (Meneghini, 1888) and rare Sardoredlichia carinata east. Rasetti, 1972. 148 G. L. PILLOLA

With the exception of the more «diversified» Rasetti (1970), Nicosia (1971), Rasetti (1972), Pillola community of the Sardaspis laticeps (Bornemann, (in Debrenne et al., 1989) and Pillola (1991). 1891) beds, younger communities consist of one or Giordanella is a nearly isopygous, oval trilobite . two dominant species as, in ascending order, reaching 8 cm of total sagittal length (Text-fig. 3);ihe Yiliangella serrae-Longduia henyi (both Pillola, cephalon is semi-circular in outline, with a relatively 1991), Sardaspis laticeps, Enantiaspis enantiopa and strong convexity (both transversally and sagittally), . Dolerolenus zoppii (N4) accompanied by a constant showing a weak difference in relief between the con- background of minor components. tiguous parts of the shield. Rostrum and hypostome The Sardaspis beds locally consist of decalcified are free, but contiguous. marls yielding some branching and solitary Ar- The thorax has 8 segments; the rachis is chaeocyatha probably living in situ, and small patch relatively wide, occupying 25-30% of the thorax reefs, mostly constituted of Protopharetra and width (tr.) and is delimited by deep dorsal furrows; Coscinocyathus; some articulated trilobites occur each axial ring has a well developed articulating half- together with rare Hyolitha and inarticulate ring. Pleurae have wide (exsag.) pleural furrows and brachiopods. Sardaspis laticeps (Wutingaspiinae), wide sub triangular facets, stout short pleural spines Dolerolenus longioculatus and Giordanella meneghinii and pronounced downsloping attitude in their dista1 (Dolerolenidae), Sardoredlichia arenivaga occur in part. almost equivalent proportions, Enantiaspis enantiopa The pygidium is sub-trapezoidal, well rounded, (Metadoxidiidae), Sardoredlichia carinata and with a strong relief, consisting of an elevated rachis Metaredlichia cf. cylindrica (Metaredlichiinae) being with 6 axial rings and termina1 piece, and of the much rarer. The increase in biodiversity is probably pleural regions showing 4-5 ribs; three antero-lateral due to environmental conditions. pygidial spines occur (Pl. 1, figs. 3, 7; pl. 2, fig. 4). The upper unit of the Punta Manna Formation, The segmentation is usually better impressed on named Cuccu Aspu Member, corresponds with the the interna1 mould of the carapace (Text-figs. 2, 4); appearance of dolomitic beds bearing calcimicrobial the test is not preserved in most of the available mats, alternating with quarzitic sandstones (Pillola, material; however, some specimens clearly show 1990; 1991; Pillola et al., 1995). that G. meneghinii possesses a thick cuticle (200-300ym, up to 800ym), a character that has been MORPHOLOGICAL FEATURES OF GIORDANELLA considered as an important adaptation to the high energy environment (Pillola, in Debrenne et al., Genera1 morphological features of both Gior- 1989). danella meneghinii and G. vincii are briefly sum- The similarity between Giordanella and Illaenus marized below; for additional information see Bor- aPsilocephalus» (= Borthaspis) or «Platypeltis» ' nemann (1883; 1891), Meneghini (1888), Nicosia and (Platypeltoides), taxa to which Giordanella was

EXPLANATION OF PLATE 1

(The specimens labelled DSTCP. are deposited in the Dipartimento di Scienze della Terra, Cagliary University, Pillola's collections) Figs. 1-2 - Giordanella vincii Pillola, 1991. 1) Nearly complete exoskeleton, DSTCP. 0517, dorsal view, x4. 2) Damaged carapace, U. Gross collection (Heidelberg University); note the locally well preserved articular facets of the pleurae and the terrace ridges on che pygidial doublure; dorsal view, x4. Figs. 3-9 - Giordanella meneghinii (Bornemann, 1883). 3) left pleural region of a small pygidium, DSTCP. 0283, x14. 4) Rostral plate DSTCP. 0287, showing well defined terrace ridges; ventral view, x10. 5) Incomplete cranidium DSTCP. 2251; note the strong convexity and the terrace ridges on the anterior border; frontal view, x10. 6) Incomplete librigena DSTCP. 0238, preserved in coarse sandstone; note the terrace ridges on the doublure; dorsal view, x4,5. 7) Pygidium, latex cast DSTCP. 1788; ventral view, x3,5. 8) Slightly distorted pygidium DSTCP. 0241; dorsal view, x8. 9) genal region of an incomplete librigena DSTCP. 2252; note the terrace ridges pattern; iateral view, x15.

Lower Cambrian, Nebida Group from SW Sardinia, Italy. Figs. 1,2, Sa Tuvara Member (Matoppa Formation); 1) Cuccu Egai, 2) SE of the Seddas Moddizzis ore concentration plant; Gonnesa. Figs. 3-4, Monte Azzieddas Member (Punta Manna Formation); Medau Arriendi Pasca locality, Gonnesa. Figs. 5, 8-9, Lower Monte Azzieddas Member (Punta Manna Formation); Medau Cocco Paolo-Case San Antonio section, Gon- ' nesa. Figs. 6, uppermost Sa Pruixina Member (Matoppa Formation); Medau Cocco Paolo-Case San Antonio section, Gon- nesa. Figs. 7, lower Punta Su Pranu Member (Punta MannaFormation); Punta Rubicina section, Nebida. G. L. PILLOLA, THE TRILOBITE GIORDANELLA BORNEMANN P). 1 150 G. L. PILLOLA

Text-fig. 3 - Reconstruction of the carapace of Giordanella rneneghinii (Bornemann, 1883); dorsal view, x2. Note: terrace ridges and pits are not figured. assigned by Meneghini (1883, 1888) and Bornemann (1883), is only superficial and represents an example of convergence (see discussion in Fortey, 1990). The weak to obsolete glabellar segmentation in Dolerolenidae is indicative of a peramorphic trend (Pillola, 1991, p. 75); the cranidial features suggest a close relationship between Giordanella and Dolerolenus courtessolei Pillola, 1991 (Text-fig. 4D); however in Giordanella, the peramorphic develop- menta1 trend observed on the cranidium, is accom- panied by a strong paedomorphosis in the pygidium and in the thorax. The result of this combination is a restricted number of thoracic segments, a generalized effacement and the occurrence of ankylosed segments on the pygidium (miome- risation, sensu Zhang Wentang- & Liu, in Zhang Wentang et al., 1980): Giorddnella is one of the first effaced polymerid trilobites of the geologica1 record, showing an Illaenid-like morphology (Text-figs. 3; 4A-C; 6; pl. 1, Text-fig. 4 - A-C) Giordanella vincii Pillola, 1991; A) cranidium figs. 1-2; pl. 2, fig. 6). of the type specimen showing the convexity of the shield, lateral view, x10; B) ibid. posterior oblique TERRACE RIDGES AND PITS view, x1O; C) locally exfoliated incomplete specimen in extended posture, x8. D-E) Dolerolenus Terrace ridges - Terrace ridges are a common courtessolei Pillola, 1991; D) cranidium, internai mould, x1,6; E) detail of the anterior portion of the feature in trilobites; according to Miller (1975) and librigena showing the distribution of pits and Schmalfuss (1981), the term terrace ridge (= terrace ridges, x2. THE TRILOBITE GIORDANELLA BORNEMANN 151 cuticular terrace, terrace line) is applied to a ridge Terrace ridges and pits on Giordanella - The strongly asymmetric in cross section, with a steep following descriptions are mostly based on face (more than 60') on one side and a flat slope (less specimens of G. meneghinii that come from the than 30') on the other. lower part of the Monte Azzieddas Member, crop- Two different types of terraces ridges occur in ping out in the Medau Cocco Paolo small quarry, trilobites: the dorsal terraces and the ventral terraces located near Seddas Moddizzis abandoned mine (on the doublure and hypostome) which, com- (Pillola et al., 1995, fig. 18) and the Medau Arriendi monly, occur independently. Pasca locality (close to Gonnesa town), partially As a genera1 rule, the dorsal terraces are more or figured by Pillola (1991, pl. 25, figs. 10-11, 13). In ad- less transverse to the axis, with the steep faces directed dition, recent findings from these localities and the backwards (with many exceptions, e.g. the classica1 close site of Case Sant'Antonio, from the upper example of Stenopilus pronus); they are much less meneghinii range zone (Sarduspis laticeps beds) have asymmetric in profile and higher in relief. The per- been studied. pendicular distance between two contiguous dorsal Pits and terrace ridges have been recorded on cuticular ridges ranges from 0.5 to 2 mm (Schmalfuss, Giordanella meneghinii specimens coming from san- 1981; Miller, 1975). In some cases the edorsal terraces- dy (more or less calcareous) deposits by Bornemann are arranged in a Bertillon pattern (cf. Whittington, (1891, pl. 26 (41), figs. 24, 27, 36), Rasetti (1972, pl. 1992, pl. SA, or 10, 80-81, 84; Miller, 1975). IO, fig., 47) and Pillola (1991, pl. 25, fig. 12). ~heterrace ridges of the doublure run sub- Terrace ridges occur both ventrally and dorsally parallel to the border of the carapace; usually, they in Giordanella meneghinii and are found only on the are very long, regularly spaced, with low ventral parts in G. vincii. Pits cover the entire anastomosing frequency and with steep faces direc- carapace of G. meneghinii and have never been ob- ted outwards. The space between these terraces served with certainty in G. vincii. varies from 0.1 to 2mm. The schematic profiles of The ventral parts show a regularly spaced terrace terraces have been named by Miller (1975): a) simple, system with a low anastomosing frequency, for in- obtuse; b) chamfered, obtuse; C) right-angled or ver- stance on the rostrum (Pl. 1, fig. 4), on the librigena tical; d) acute or overhanging. The relationships bet- (Pl. 1, fig. 6; pl. 3, fig. 2) and o; thé pygidium (~1.1, ween terraces, pore canal openings and cuticular fig.- 7). structure have been analysed by severa1 authors ~irracesridges on the cranidial edge and on the (Miller, 1975; Wilmot, 1991). anterior border of librigenae run sub-parallel to the Despite the constructional variation, both dorsal perimeter of the cephalon. Steep faces are and ventral terraces have the same basic structure progressively outward on the doublure of librigenae and seem to represent steps in the intergument com- and in the rostrum, upward to downward directed, parable to those seen in living Arthropoda; their on the margin and on the dorsal ridges; in profile, function, not necessarily the same in al1 trilobites, is these ridges are usually of low-overhanging to right still a matter of debate. angle type. The number of ridges on the frontal border Pits (opening pores, punctae) - Pits are funnel- decreases during morphogenesis, passing from 6 in shaped depressions, considered as the openings of the smallest specimen (5,4 mm, cranidial sag. length) the perpendicular canals observed in the cuticle of to 1 on larger cranidia (13 mm). The perpendicular many trilobites; these canals (or canaliculi) may have distance between two contiguous ridges is 60pm on different diameter (Dalingwater, 1973). The pits are small cranidia and become stable in larger adults considered to be either the trace of sensory organs (around 200pm with a maximum of 250pm). This probably in association with setae or similar struc- contrasts with the secondary increase of terrace tures (sensillae) or the openings of glands. However, number during ontogeny and the constant distance in some cases, they may have had an accessory between terraces, reported by Schmalfuss (1981, p. respiratory function, in association with the caecal 337) and Miller (1975, p. 165) and with the increase network (Jell, 1978). in number observed during the ontogeny of Paladin eichwaldi shunnerensis (King, 1914) described by Facets - The facet is usually sub-triangular to Clarkson & Zhang (1991). The height of a single petaloid, smooth or with terrace ridges, slants ridge increases from 25pm to about 50pm and forward-downward, it is located at the extremity of become stable around this value in cranidia larger the pleura and in the anterolateral angle of the than 6mm. A similar pattern and size of terrace pygidium. The facet facilitated the gliding of the free ridges occurs in the anterior external border of the extremities of the pleurae over one another. free cheek. 152 G. L. PILLOLA

A particular arrangement of terraces occurs near The density varies from 100 on the cranidial bor- the genal angle and genal spine (Pl. 1, fig. 9; pl. 2, fig. der to 120 on the occipital ring and falls to 60 in free la-b), where the ridge changes in direction, passing cheek (for specimens with inferred same size). from the ventral to the dorsal side, still retaining the About 100 pits per mm2 occur on the anterior outward to backward orientation of the steep face. border and preglabellar field in specimens 5.4 long These ridges may have a lesser dissimetry. A similar (sag.) (Pl. 2, fig. 3), passing to 80 on cranidia of 8.0 pattern of terrace ridges occurs librigenal border of mm (Pl. 1, fig. 5) and to about 50 in larger specimens Eobronteus laticauda (Wahlenberg, 1821) figured by reaching 10.0 mm of total cranidial length. Miller (1975, fig. 19 A-B) and in the genal region of A similar trend is observed on the posterior bor- some paradoxidiids. der of the pygidium: small specimens (4.7 mm of On the pygidium, the adorsal* terrace ridges are sagittal length) have between 80 and 90 pits per located only on the dista1 portion of the pleural square mm (Pl. 1, fig. 3), passing to 80 (Pl. 2, fig. 4; regions and are more abundant on the anterior pl. 3, fig. 3b) 70 and 65 for pygidia respectively of segments, where the bifurcation and the addition of 5.8, 6.0 and 7.0 mm. some short ridges occur (Pl. 1, figs. 3, 8; pl. 2, figs. 4, The terrace ridges and pits on the cephalic bor- 5; pl. 3, fig. 3). On the doublure, the ridges are der of G. meneghinii can be compared to those ob- closely spaced and run sub-parallel to the inner served in Dolerolenus zoppii, D. bifidus and in D. cour- margin (Pl. 1, fig. 7). Except the interruption due to tessolei. the more or less developed spiny ends, it is clear that Severa1 specimens of Dolerolenus zoppii and D. the dorsal terraces are connected with the ventral biftdus show fine sub parallel terrace ridges on the ones, and maintain the posteriorly directed steep doublure and around the cephalic border; in some face. cases, fine pits occur at the base and on the flat slope Pits can be observed on the whole dorsal part of between ridges, included on the doublure, i. e.on the the exoskeleton of Giordanella meneghinii; usually librigenae, on the thoracic pleurae and on the they are larger on elevated or convex parts of the rostrum of D. zoppii (Pillola, 1991, pl. 14, figs. 1-5,9; shield (e. g. cranidial and pygidial border, occipital pl. 16, fig. 1). ring and axial areas, etc.), less frequent and smaller In D. courtessolei the pits are larger and more on the furrows; no pits have been discerned on the abundant than in zoppii and, like in G. meneghinii, steep faces of the terrace ridges and on the ventral they cover the whole dorsal carapace (Pillola, 1991, side of the carapace. pl. 21, figs. 1-6,9; pl. 22, figs. 1-2). More particularly, The pits have a certain tendenc~to range into on the librigenae of D. courtessolei (Text-fig. 4) there two different size-classes: the largest and most abun- is a wide dorsal field covered by coarse closely dant, are 90-IOOpm in diameter, the smallest are spaced pits, associated with terrace ridges on the ex- 20-50pm. Their density and size vary not only on ternal border. Smaller and spaced pits occur on the ' the same specimen (Pl. 1, fig. 8; pl. 2, fig. la, 3) but genal field, which become rarer in a narrow band also during ontogeny. bordering the facial suture. The pits are arranged in The density (expressed in number of pits per rows especially between and in the vicinity of the . square mm), tested on the same region of different more or less dicontinuous raised ridges. A similar specimens (e.g. anterior cranidial border, pre- variation in size pattern of these sculptural features glabellar area, occipital ring, antero-lateral area of the occur in G. meneghinii. However, it is difficult to pygidium) diminishes in larger adults. decide if the development of ridges originates from

EXPLANATION OF PLATE 2

Figs. 1-6 - Giordanella meneghinii (Bornemann, 1883). la-b) Librigena DSTCP. 2253; a) external surface showing pits and terrace ridges; dorsal view, x10; b) detail of the genal spine, slightly tilted comparatively to the inferred life position; x18. 2) Damaged cranidium, DSTCP. 2254, with finely pit- ted test, locally weathered, and terrace ridges preserved along the anterior left margin; dorsal view, x6. 3) Small cranidium, DSTCP. 2255, with relatively well defined glabella; note the different sized pits; dorsal view, x10. 4a-C)Nearly complete pygidium, DSTCP. 2256, with well preserved terrace ridges and pits; a) postero-lateral view, x7; b) dorsal view, x7. C) detail of the antero-lateral pleural area, x15. 5) Pygidium DSTCP. 2257, with larger pits, dorsal view, x6. 6) Flattened and distorted specimen with damaged cranidium, interna1 mould DSTCP. 0317; dorsal view, x2,5. Lower Cambrian, Nebida Group from SW Sardinia, Italy. Figs. 1-5, lower Monte Azzieddas Member (Punta Manna Formation); Medau Cocco Paolo-Case San Antonio section, Gon- nesa. Fig. 6, upper Sa Pruixina Member (Matoppa Formation); Medau Cocco Paolo-Case San Antonio section, Gonnesa. G. L. PILLOLA, THE TRILOBITE GIORDANELLA BORNEMANN ' P]. 2 154 G. L. PILLOLA the fusion of such sensillar rows or if the alignment terrace ridges observed on the ~etaloidarticulating of pits is subsequent to the incipient appearence of facets of Sympbysurus palpebrosus, where the criss- ridges. cross relationshi~of terraces, when the trilobite was . enrolled, allowed it to maintain a limited contact INTERPRETATION OF TERRACE RIDGE AND PIT with the surrounding oxygenated water and to rein- FUNCTIONS ON TRLLOBITES force the binding between pleurae; in addition, some terraces located at the margin of the facets and along - Various theories have been proposed to explain the cranidial rim were connected with detecting the the terrace ridges functions on trilobite (Miller, state of enrollment (Fortey, 1986). 1975; Osm6lska, 1975; Stitt, 1976; Schmalfuss, 1981; According to Fortey (1985), the proximal parts Fortey, 1985, 1986; Seilacher, 1985); by contrast, of the broad doublure of some asaphids, having well there is a genera1 consensus for a prevalent sensory developed terrace ridges, were unlikely to have made function (tactile, mechano- chemio-receptors) for contact with the sedirnent and consequently were setae or similar organs, pits and pore canals; this in- improbable to have had a gripping function. Thus, a terpretation is also corroborated by strong analogies function of terrace ridges directly involved in grip- with extant arthropods (Miller, 1975; Haas, 1981; ping the sediment during suspension feeding on the Wilmot, 1991). sea floor seems possible only for benthic filter- The gripping and/or current-monitoring func- feeding trilobites having doublure of rather uniform tion for the terraces ridges are frequently invoked to width around the perirneter of the . explain this peculiar feature of trilobites. Miller The prevalent current-monitoring function of (1972,1975) suggested a current-monitoring function the dorsal terrace system (including terraces, pits and in close association with pits and pore canals (and connected sensillae) in Eobronteus laticaudu has been related setae) by comparison with sculptural features experimentally tested; strong connections between present in modern arthropods, and concluded that the terrace course pattern and flow patterns have most of them performed a sensory function. Schmal- been pointed out (see Miller, 1975, figs. 13, 19, 20). fuss (1978, 1981) indicated that dorsal terraces were The tendency to even ridge separation in parallel generally supposed to help burrowing process, while terrace zones has been related not only to the flow- the ventral ones were adapted to consolidate the direction detection but also to the flow velocity walls of a filter chamber beneath the anima1 (see also changes perception. Seilacher, 1985). According to Miller (1975), it is possible that the The carapace function (Whittington, 1962; terrace system in trilobites firstly appeared on the Wilmot, 1990), and the possibility of terrace ridges doublure, later migrating dorsally for sensory pur- controlling in some way sediment passage or poses connected with specific habitats. The develop- removal of surplus sediment from the exoskeleton, ment may have proceeded from an arrangement of has been minimized or rejected in further seta1 pits, granules or tubercles in rows to exploit the discussions (Miller, 1975). Other functions, such as flow-rate measuring possibility of this configuration, camouflage, sexual recognition, protection against subsequently resulting in the fusion of such sensillar epifaunal setting, might also be possible (Fortey, rows into ridges. However, these assertions need 1985). proper investigations in order to elucidate develop- A few examples, among the classica1 ones, provides ment polarities and relationships between sculptural some clarification of these disparate opinions and features during ontogeny. their implications for trilobite mode of life. It is clear that severa1 functions may have been The suggestion that the terrace ridges in exerted by terrace ridges, some of these are suppor- Stenopilus pronus (cf. Text-fig. 4) may be sediment ted by convincing arguments; the decision to opt for grippers, used when this trilobite burrowed back- one rather than another is just a question of balance wards into the sediment (Stitt, 1976), is supported by between competing possibilities. the three criteria proposed by Fortey (1985). Such gripping function would not apply to the ventral GIORDANELLA LIFE HABITS terraces of a pelagic trilobite like Pricyclopyge, where the well developed terraces, located between the Considering the three main areas of information hypertrophied eyes, were unlikely in contact with necessary for the deduction of trilobite ecology (fun- the sediment (Fortey, 1985, fig. IO), or to the dorsal ctional morphology, analogy, geological evidence), antero-lateral terraces of the cephalon of a vagrant the following aspects must be emphasized in benthic anima1 like Paladin (see Clarkson, 1991). propose the possible mode of life of Giordunella vincii A similar contrasting deduction concerns the and G. meneghinii: THE TRILOBITE GIORDANELLA BORNEMA NN 155

- Articulating half-rings and facets on the gripping function during burrowing or infaunal at- pleurae are well developed in Giordanella meneghinii titude. and in G. vincii (Pl. 1, fig. 2); the partial overlap of - No complete specimen of G. vincii has been the dista1 portion of each pleura (as tested in segmen- found, and the lack of free cheeks in connection ted paper models) produces a considerable shor- with the cranidium strongly suggests that the ar- tening of the distance between the cephalon and the ticulated individuals represent exuvial remains and/ pygidium. We can suggest a high flexibility of the or partially reworked dead individuals (Pillola 1991, exoskeleton, and a possible complete enrolled at- pl. 23, figs. 1, 6-7; pl. 24, fig. 2; here P1. 1, figs. 1,2). titude of Giordanella cannot be excluded. Except the specimen on P1. 2, fig. 6, no other ar- - The cuticle thickness of Giordanella vincii ticulated G. meneghinii has been recorded; so, (data obtained from partially exfoliated specimens unequivocal remains in life position are not and from the empty space between the interna1 and available. the external mould) is lower than in G. meneghinii; - The genera1 morphology of Giordanella is in this latter a thicker cuticle (800pm) occurs in close to that of early illaenids or some nileids (e.g. cranidia found in the Monte Azzieddas Member. Platypeltoides); the relatively smooth cephalon has a The trilobite cuticular thickness has been related significant convexity in al1 directions (Rasetti, 1972; to palaeoenvironment (Fortey & Wilmot, 1991): Pillola, 1991, pl. 24, figs. la-d; here Text-fig. 4A-B). thin cuticles dominate in the early off- Considering the palpebra1 lobes as parallel to the shore Olenid Biofacies, thick cuticle occurrences in- base of the visual field, in horizontal position, crease progressively in the Nileid, Illaenid-Cheirurid which, according to Clarkson (1966, 1969), is the and the most onshore Bathyurid Biofacies respec- most favourable attitude for a maximum field of tively. In a schematic environmental profile, the view, the thorax and the pygidium of Giordanella Illaenid-Cheirurid Biofacies tends to be associated may have been partially buried into the sediment with alga1 mounds near the platform edge and in (Text-fig. 6). higher energy environments (see Fortey, 1975). The bumastoid stance (sensu Fortey, 1986) is a There is no reason to exclude the same relationship common posture in trilobites with high cephalic between cuticular thickness and water turbulence convexity and illaenid-like morphology (Text-fig. 5). during the Lower Cambrian. Such stance has been recorded in several Illaenidae - A sensory (tactile and/or current-monitoring) (Illaenus sarsi, Panderia megalophthalma, Bumastoides function for the cephalic terrace ridges and pits on tenuirugosus, ...), as reported by Bergstrom (1973) Giordanella meneghinii seems plausible; never- and Westrop (1983), in the nileid Symphysurus theless, the lack of pits on the doublure, and near the palpebrosus (see Fortey, 1986 and cited references), in steep faces of dorsal pygidial terraces, may be related Stenopilus pronus (Plethopeltidae, Ptychopariida) to a possible reduction of the sensory requirements (Stitt, 1976) and in several other illaenimorph or to the inefficiency of setae-like organs in such species. areas. In this case, the ventral and pygidial terrace In agreement with Westrop (1983) and Fortey ridges on G. meneghinii may have had a prevalent (1986) this possible infaunal habit was poly-

Illaenus sarsi Panderia megalophth alma Stenopilus pronus

Text-fig. 5 - Infaunal attitude of some trilobites cited in the text: Illaenus saniJaanusson, 1954;Panderia megalophthalma Linnarsson, 1869 and Stenopibspronus Raymond, 1925 in their inferred life position during infaunal attitude. After Bergstrom, 1973 and Stitt, 1976. 156 G. L. PILLOLA

Text-fig. 6 - Extended posture of Giordunella menegbinii (left) and its possible stance during infaunal attitude (right).

phyletically derived, so that, it may have developed CONCLUSIONS in different trilobite families. - Illaeniform trilobites with similar habits have The Lower Cambrian dolerolenid trilobite Gior- been found quite commonly in the more or less soft danella show both effacement and strong, convex terrigenous and/or calcareous bottoms between reef morphology, like that of some illaenids, and mounds and stromatolitic domes (Bergstrom, 1973; primitive nileids. The gross morphology, the con- Stitt, 1976); many species are frequent in reef or peri- vexity of the cephalon, the orientation of the visual reefal deposits (Lane, 1972). field and the well-developed articulating facets and However, in Giordanella the convexity of the half-rings suggest high flexibility of the exoskeleton. cephalon is lower than in Panderiu megalophthalma The possibility that it spent part of life time in in- (see Bruton, 1968) or in Stenopilus pronus, and, in al1 fauna1 stance is suggested by the comparison with likelihood, the thorax and pygidium did not dip other trilobites with similar adaptive features, some down so strongly. of which are claimed to have been recovered in life - The Illaenid aspect of Giordanella is a mor- position. This burrowing attitude may have had ex- phological innovation which occurred early in the ploited for protection but .also for food gathering. Lower Cambrian; however, Giordanella vincii did The terrace ridge position and geometry show not have a particular (numerically) advantage against that the ventral ones and those on the dorsal side of the rest of the trilobite community living in the the pygidium only of G. meneghinii could have fun- more or less soft terrigenous to marly bottoms bet- ctioned as sediment grippers. Those closely ween mounds of the Sa Tuvara Member. G. associated with pits on the cephalic border may have meneghinii, since its first occurrence at the top of the exerted a predominant current-monitoring function. sandy shoal of the Sa Pruixina Member, represents a In al1 likelihood, Giordanella vincii was derived minimum of 95% of the trilobite remains. from Dolerolenus courtessolei, or a common ancestor, The calcimicrobial-archaeocyathan and oolitic by paedomorphosis leading to miomerisation and a limestones of the basa1 portion of the Punta Manna strong peramorphosis of the cephalon. It represents Fm. (Monte Azzieddas Mb.) yield rare trilobite a vagrant benthic trilobite, living close to remains. G. meneghinii constitutes again the calcimicrobial-Archaeocyathan mound, which majority of the trilobite remains. The predominance burrowed in soft, fine-grained sediments, spending of this taxon persisted during the deposition of the most of its time in active epifaunal habits. It assumed lower part of the Punta Su Pranu Member, with the bumastoid stance for specific purposes. similar environmental and taphonomical charac- The successful adaptation of Giordanella teristics, and gradually faded away. meneghinii to higher energy environments was

EXPLANATION OF PLATE 3

Figs. 1-3 - Giordunelkz meneghinii (Bornemann,l883). 1) Detail of the anterior margin of the cranidium DSTCP. 2251 (see also pl. 2, fig. 5), x90.2) Fragmentary librigena DSTCP. 2252, showing the outer surface of the doublure with only ventral terrace ridges, and the outer impression of the dorsalside with large sized pits, latex cast, x40. 3a-b) Pygidium DSTCP. 2256 (see also pl. 2, fig. 4); 3a) Antero-lateral corner, showing the terrace ridges, note the abrasion of the terraces edges and che lack of pits at the vicinity of the steep faces, x60. 3b) Posterior lateral margin, x30. Lower Monte Azzieddas Member (Punta Manna Formation); Medau Cocco Paolo-Case San Antonio section, Gonnesa. THE SRILOBITE GIORDANELLA BORNEMA NN 155

- Articulating half-rings and facets on the gripping function during burrowing or infaunal at- pleurae are well developed in Giordanella meneghinii titude. and in G. vincii (Pl. 1, fig. 2); the partial overlap of - No complete specimen of G. vincii has been the dista1 portion of each pleura (as tested in segmen- found, and the lack of free cheeks in connection ted paper models) produces a considerable shor- with the cranidium strongly suggests that the ar- tening of the distance between the cephalon and the ticulated individuals represent exuvial remains and/ pygidium. We can suggest a high flexibility of the or partially reworked dead individuals (Pillola 1991, exoskeleton, and a possible complete enrolled at- pl. 23, figs. 1, 6-7; pl. 24, fig. 2; here P1. 1, figs. 1,2). titude of Giordanella cannot be excluded. Except the specimen on P1. 2, fig. 6, no other ar- - The cuticle thickness of Giordanella vincii ticulated G. meneghinii has been recorded; so, (data obtained from partially exfoliated specimens unequivocal remains in life position are not and from the empty space between the interna1 and available. the external mould) is lower than in G. meneghinii; - The genera1 morphology of Giordanella is in this latter a thicker cuticle (800pm) occurs in close to that of early illaenids or some nileids (e.g. cranidia found in the Monte Azzieddas Member. Platypeltoides); the relatively smooth cephalon has a The trilobite cuticular thickness has been related significant convexity in al1 directions (Rasetti, 1972; to palaeoenvironment (Fortey & Wilmot, 1991): Pillola, 1991, pl. 24, figs. la-d; here Text-fig. 4A-B). thin cuticles dominate in the early Ordovician off- Considering the palpebra1 lobes as parallel to the shore Olenid Biofacies, thick cuticle occurrences in- base of the visual field, in horizontal position, crease progressively in the Nileid, Illaenid-Cheirurid which, according to Clarkson (1966, 1969), is the and the most onshore Bathyurid Biofacies respec- most favourable attitude for a maximum field of tively. In a schematic environmental profile, the view, the thorax and the pygidium of Giordanella Illaenid-Cheirurid Biofacies tends to be associated may have been partially buried into the sediment with alga1 mounds near the platform edge and in (Text-fig. 6). higher energy environments (see Fortey, 1975). The bumastoid stance (sensu Fortey, 1986) is a There is no reason to exclude the same relationship common posture in trilobites with high cephalic between cuticular thickness and water turbulence convexity and illaenid-like morphology (Text-fig. 5). during the Lower Cambrian. Such stance has been recorded in several Illaenidae - A sensory (tactile and/or current-monitoring) (Illaenus sarsi, Panderia megalophthalma, Bumastoides function for the cephalic terrace ridges and pits on tenuirugosus, ...), as reported by Bergstrom (1973) Giordanella meneghinii seems plausible; never- and Westrop (1983), in the nileid Symphysurus theless, the lack of pits on the doublure, and near the palpebrosus (see Fortey, 1986 and cited references), in steep faces of dorsal pygidial terraces, may be related Stenopilus pronus (Plethopeltidae, Ptychopariida) to a possible reduction of the sensory requirements (Stitt, 1976) and in several other illaenimorph or to the inefficienc~of setae-like organs in such species. areas. In this case, the ventral and pygidial terrace In agreement with Westrop (1983) and Fortey ridges on G. meneghinii may have had a prevalent (1986) this possible infaunal habit was poly-

Illaenus sarsi Panderia megalophth alma Stenopilus pronus

Text-fig. 5 - Infaunal attitude of some trilobites cited in the text: Illaenus sarsiJaanusson, 1954;Panderia megalophthalma Linnarsson, 1869 and Stenopiluspronus Raymond, 1925 in their inferred life position during infaunal attitude. After Bergstrom, 1973 and Stitt, 1976. G. L. PILLOLA, THE TRILOBITE GIORDANELLA BORNEMANN P1. 3 158 G. L. PILLOLA probably induced by its pre-adapted morphology, Greenland, with a note on trilobite faunas in pure with the addition of some refinements represented iimestones: Paiaeontoiogy, 15: 336-364. by ridges around the perimeter of the cephalon and MENEGHINI,G., 1883, Nota alla fauna Cambriana delllIglesiente: pygidium and numerous pits especially on the Atti Soc. Toscana Sc. Nat., Processi verbali, 4: 7-9. Pisa. -, 1888, Paleontologia dell'Iglesiente in Sardegna. Fauna Cam- perimeter of the anima1 and on elevated parts of the briana. Trilobiti: Memorie Regio Comitato Geologico exoskeleton, which according to Wilmot (1991), are d'Italia, 3 (2): 1-51. Roma. in the best situation for sense organs. The reduced size MILLER,J., 1975, Structure and function of trilobite terrace lines: of the palpebra1 lobes (and probably of the visual sur- Fossil and Strata, 4: 155-178. face) and the thickset cuticle represent further well NICOSIA,M.L., 1971, Giordanella, trilobite caratteristico del adapted features against water turbulence in sandy Cambriano sardo: Geo-Archeologica, 1: 27-29. Roma. -, & RASETTI,F., 1971, Revisione dei Trilobiti del Cambriano and oolitic shoal to back-shoal environments. dell'Iglesiente (Sardegna) descritti da Meneghini: Memorie, Atti Accademia Nazionale dei Lincei, 8 (1/2): 1-20. Roma. ACKNOWLEDGEMENTS PILLOLA,G.L., 1990, Lithologie et trilobites du Cambrien in- férieur du SW de la Sardaigne (Italie): implications I wish to tanks M. Robardet and F. Paris, CNRS URA D1364, Rennes University, for helpful criticism and technical paléobiogéographiques: Comptes Rendu Académie des Sciences, Paris, 310: 321-328. support during the preparation of this paper; R.A. Fortey for suggestions and improvement on the type-script. -, 1991, Trilobites du Cambrien inférieur du SW de la Sar- This paper is a contribution to the I.G.C.P. Project 366 daigne, Italie: Palaeontographia Italica, 78: 1-174. «Ecologica1 aspects of the Cambrian Radiation,,. -, & G~oss,U., 1982, Stratigrafia del Membro di Matoppa della Formazione di Nebida (Cambrico inferiore) nell'area M.te San Giovanni- M.te Uda. In Carmignani, L., et al., REFERENCES (eds), Guida alla geologia del Paleozoico Sardo. Guide geologiche regionali: Soc. Geol. Italiana: 79-82. BERGSTR~M,J., 1973, Organization, life, and systematics of LEONE, F. & LOI, A., 1995, The Lower Cambrian Nebida trilobites: Fossil and Strata, 2: 1-69. -, Group of Sardinia. 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