Isotelus-Type Hypostomes and Trilobite Feeding

The function of forks: Isotelus-type hypostomes and trilobite feeding

THOMAS A. HEGNA

Hegna, T.A. 2010: The function of forks: Isotelus-type hypostomes and trilobite feeding. Lethaia, Vol. 43, pp. 411–419.

Despite previous investigations, the function of the forked morphology of asaphid trilobite hypostomes is enigmatic. The focus of this study is the large and robust forked hypostome of the largest known genus of trilobite, Isotelus, and the independently- derived forked hypostome of Hypodicranotus, the longest hypostome relative to body size of any trilobite. Although the trilobite hypostome is analogous to the labrum in other arthropods, forked hypostomes lack an obvious modern functional counterpart. The Isotelus hypostome is distinguished from other trilobite hypostomes by closely-spaced terrace ridges on a greatly thickened inner surface of the forked posterior margin, with the scarp of the terrace facing antero-ventrally. This is compatible with a grinding function, suggesting possible limb differentiation to complement this structure. The inner face of the tine (one of the two, prominent, sub-parallel posterior projections) is also unique in that it has a microstructure which is evident in section, running perpendicular to the surface. Macropredatory and ﬁlter-feeder roles are ruled out, and previous charac- terizations of the hypostome as knife-like or serrated are rejected. Its function is incom- patible with that of other non-asaphid trilobites with forked hypostomes, like the remopleuridid Hypodicranotus, which lack similar terrace ridges and thickened inner- edge cuticle. h Arthropoda, Asaphida, ecology, functional morphology, Trilobita.

Thomas A. Hegna [[email protected]], Geology & Geophysics Department, Yale University, PO Box 208109, New Haven, CT 06520-8109, USA; manuscript received on 14 ⁄ 1 ⁄ 2009; manuscript accepted on 15 ⁄ 8 ⁄ 2009.

Forked hypostomes represent a derived morphology 1994; Vannier & Chen 2002; Bergstro¨m et al. 2007; in several disparate groups of trilobites, like asaphids Lin 2007), trace fossils (Campbell 1975; Jensen 1990; (Figs 1–2) and remopleuridids (Fig. 4). The posterior Brandt et al. 1995; Rydell et al. 2001; English & margin has two prominent, backward-facing projec- Babcock 2007) and unique exoskeletal morphology tions. Forked hypostomes are always rigidly attached (Bergstro¨m 1972; Campbell 1975; Sloan 1992). to the cephalic doublure, limiting their mobility but The general mode of feeding in trilobites appears to increasing their strength. Trilobites with a forked have involved a ventral food-groove between the limb hypostome range from the Cambrian to the Upper gnathobases – passing food forward along the axis to a Palaeozoic. This hypostomal feature is much more posteriorly-directed mouth. Some trilobites may have common in its weaker incarnations (i.e. subtly generated a feeding current similar to that of some forked), where it is represented by a notched posterior modern anostracan crustaceans (Cannon 1928, 1935a) margin (Fortey & Owens 1999). to propel food particles towards the mouth. Trilobites have been attributed to all the major guilds of vagile, benthic marine invertebrates (i.e. predator, scavenger, Background detritivore and filter-feeder). Spiny gnathobases, as in Olenoides serratus and Naraoia compacta, have been The trilobite hypostome is a small, sclerotized plate cited to as evidence of predatory ⁄scavenging habits, under the head, positionally (if not functionally) while sediment-infilled guts have been interpreted as analogous to the crustacean labrum. Whether the indicative of deposit feeding (Bergstro¨m et al. 2007; hypostome and head appendages interacted during but see Vannier & Chen 2002; for an alternative taph- feeding is unknown, as is the exact position of the onomic interpretation of sediment-filled guts). Filter- mouth relative to the hypostome. Because of these feeding has also been suggested as a life mode for uncertainties, research on the feeding habits of trilo- some trilobites on the basis of an inferred filter-cham- bites has focused on hypostome attachment style ber (Fortey & Owens 1999) or a planktonic lifestyle (Fortey 1990; Ivantsov 1990; Fortey & Owens 1999), (Schoenemann et al. 2008). For a review of trilobite limb morphology (Raymond 1920; Babcock 2003), feeding, see Fortey & Owens (1999) and Hughes preserved gut traces (Sˇnajdr 1987; Chatterton et al. (2001).

DOI 10.1111/j.1502-3931.2009.00204.x 2009 The Author, Journal compilation 2009 The Lethaia Foundation 412 T. A. Hegna LETHAIA 43 (2010)

AB C

F G

Fig. 1. All ﬁgures are of an Isotelus sp. hypostome. Left scale bar (2 cm) applies to (A, B, F) upper right scale bar (1 cm) applies to (C–E), lower right scale bar (1 cm) applies to (G). A–B, F, hypostome YPM 7473 in ventral, dorsal, and posterior (ventral up) view. C, dorsal view of left fork tine YPM 7473. D–E, dorsal and ventral view of right fork tine YPM 7473. G, section through hypostomal fork tine YPM 37253 etched for SEM photographs and thin section images in Fig 2.

prominent, sub-parallel posterior projections (tines), Methods triangular in section with widest angle facing dorsally (Fig. 1). Smooth ventrally with slight raised lip near Specimens are housed at the Yale Peabody Museum posterior tip (ventrally). Surface of hypostome with (YPM) and Harvard Museum of Comparative Zool- irregularly spaced, wavy terrace ridges sub-parallel to ogy (MCZ). Photographs were taken with a micro- long axis of fork. Inward face of tine covered with scope-mounted digital camera or with a macro lens. dense, parallel terrace ridges, asymmetrical in section, SEM photography was conducted at Yale (all SEM roughly parallel to dorsal edge of fork-tip, ridges specimens save Fig. 2J, K are gold coated). A single strongest ventrally becoming effaced dorsally; steepest posterior fork from the YPM collections was trans- edge of terrace facing towards crotch of fork; ridges in versely sectioned with one-half becoming the petro- plan view with gentle curve opening antero-ventrally graphicthinsectioninFigure2A,Bandtheotherhalf (Fig. 2A). Terraces on inner surface generally more etched in dilute HCl and illustrated in Figures 1G and asymmetrical in section than terraces elsewhere on 2C–I. Petrographic thin sections were viewed under hypostome. Terrace density decreases through ontog- both polarized and cross-polarized light. Usage of the eny (Fig. 3), but difﬁcult to assess at what rate further term ‘cuticle’ instead of ‘exoskeleton’ follows Daling- terraces are added, if at all, because of the paucity of water & Mutvei (1990). pristine specimens. Terraces on the inner surface of the tine occasionally interﬁnger. Cuticle with inner terrace ridges as much as seven times thicker than Hypostome morphology cuticle elsewhere on hypostome; petrographic thin- sections (Fig. 2A, B) reveal a perpendicular micro- Isotelus structure manifest under cross-polarized light which matches the regularity of the terrace ridges (section Hypostome suturally joined to anteromedial portion perpendicular to long axis). Etched cuticle under SEM of the cephalic doublure; surface covered in terrace shows same perpendicular structures corresponding ridges with varying degrees of regular spacing. Length to terrace ridges (Fig. 2C; section perpendicular to of hypostome roughly half of cranidium, bearing two long axis), with no similar structures visible on other LETHAIA 43 (2010) Asaphid hypostome functional morphology 413

AB C

DE F

JK L

M N O

Fig. 2. Images (A–I) are from YPM 37253. A, B, petrographic thin section of an Isotelus sp. hypostomal fork tine perpendicuar to the long axis, in the medio-ventral corner of the fork tine. Images show plain and polarized light. Section made from specimen in Fig. 1G. Images are approximately 4-mm across. C, etched section of thickened inner surface of hypostomal fork tine, scale is 1 mm (see Fig. 1G). Note terrace ridges on left side. D, close up of etched terraces perpendicular to the cuticle’s edge in C, scale is 200 lm. E, magnified image of etched terrace ridge cuticle, scale is 50 lm. F, magnified image of etched cuticle on the thickened inner surface of hypostomal fork tine cut perpendicular to the tine’s longest axis, scale is 20 lm. G, SEM images of transition from thick to thin cuticle, scale is 500 lm. H, image of etched cuticle from thin wall of hypostome, scale is 100 lm. I, SEM image of transition from thick to thin cuticle, scale is 500 lm. J, SEM image of right fork tine, inner surface terrace ridges, anterior to right, scale is 1 mm. K, magnified view of terraces in J, scale is 200 lm. Images (L–O) are from YPM 510851. L, close up of terrace ridges on the inner surface of the immature Isotelus sp. hypostome. Anterior is up. Image is approximately 28-lm across. M, magnified surface of left fork tine of immature hypostome (N). Image is approximately 490-lm across. N, dorsal view of immature Isotelus sp. hypostome from the Walcott Rust Quarry, NY, scale bar 500 lm. O, close up of terrace ridges on the inner surface of the immature Isotelus sp. hypostome (N). Anterior is to the upper right corner. Image is approximately 54-lm across. sectioned areas of hypostome. Perpendicular features length of tines ventrally with the steep side facing away are not manifest on the innermost layer of cuticle – it from midline, deep terrace ridge at inner edge of tine most likely represents the endocuticle. Microstructure creating a marginal border-like structure on the ven- (Fig. 2E, F) appears to represent the central laminate tral surface; medial body of hypostome slightly raised zone of Dalingwater & Mutvei (1990). and faintly bilobed ventrally. Posterior tips of tines pointed and flattened dorso-ventrally, weak keel runs Hypodicranotus length of tines medially on dorsal surface with no other ornamentation (Fig. 4). When viewed laterally, Differs from Isotelus with a hypostome equipped with tines curve slightly dorsally. Cuticle on fork not thick- two long, posteriorly-directed projections nearly equal ened (Fig. 4G, H), roughly uniform thickness around to thoracic length, sub-parallel terrace ridges run the circumference of each tine. 414 T. A. Hegna LETHAIA 43 (2010)

Hypostome terrace density suggest that they opposed motion consistently from 25 one direction along their entire length. Wavy terrace lines would be better able to counteract forces from different directions, but only most effectively along 20 the portion of the ridge that was perpendicular to the force. The hypostome is neither sharp nor serrated (Figs 1 and 2J–O), contrary to the views of Babcock 15 (2003) and Sloan (1992). The thickened nature of the inner surface, coupled with the facing direction of the terrace ridges that oppose posterior movement in 10 the food groove, also support a grinding function. The orientation of the microstructure perpendicular Increasing density

Number of terraces per 2 mm 5 to the surface may have strengthened it to lateral forces. Several aquatic crustaceans have mandibles with superﬁcially similar surfaces that they use for

0 grinding food (Ocioszyn´ska-Bankierowa 1933, 1936; 2 4 6 8 10 12 14 Mahoon 1960; Tyson & Sullivan 1981; Mura 1995, Transverse width of terraced surface (mm) Increasing size 1996). The size of the terraces on a grinding surface bear a relationship with the size of the objects to be Fig. 3. Graph of terrace ridges density (on the hypostomal inner ground – the objects must be larger than the height of surface) relative to size of the hypostome. As the hypostome grows larger, the density of the terrace ridges decreases. This is not a the terrace, otherwise they will clog the surface (this is strictly linear relationship, as hypostomal terrace ridges are added the same relationship discussed by Savazzi et al. 1982, during growth. Measurements were made from YPM specimens, between sediment size and the terrace ridge ⁄sediment with separate measurements from right and left fork tines where possible. The drawing in the upper right hand corner is a sagittal ratchet function). This also makes filter-feeding unli- section through an asaphid cephalon showing the orientation of kely – food filtered out of the water column or sus- the inner edge of the hypostome relative to the rest of the head pended sediment will generally be soft and not require (modified from Ivantsov 2002). grinding. Furthermore, soft food pushed against a rigid grinding surface will clog the surface by filling in the troughs. The density of the terraces decreases with Functional interpretation body size, suggesting that food size increased with body size. This distinctive Isotelus-type hypostome is present in Grinding surfaces require a force to act against many of the Asaphidae (Fortey & Chatterton 1988; them, presumably supplied by the limbs. This could Ivantsov 1990, 2002; Krueger 2003), although dying have occurred in several ways: an enlarged, specialized out with them at the end of the Ordovician. The size coxa or an anterior limb with a modified orientation. and shape of the hypostome would have made it diffi- The few examples of asaphid trilobites known with cult to position and handle large items of food – par- limbs are too poorly preserved to reveal cephalic limb ticularly struggling prey items, or scavenging on large differentiation (Woodward 1870; Walcott 1884; carcasses. The buttressing of the hypostome to the English & Babcock 2007); the presence of a forked anterior margin in asaphids implies a need for hypostome with a grinding surface, however, suggests strength (Fortey 1990), as do the preferentially thick- that these limbs may have been specialized in order to ened inner surfaces of the tines. The long series of operate against the grinding surface. If such a limb homonymous limbs in trilobites suggests a coxal food was attached near the base of the fork, motion in a processing mechanism shared by disparate groups of posterior arc (pivoting at the coxal joint posteriorly modern arthropods (limulids, branchiopods, etc.) and from a sub-vertical orientation to horizontal) would many early fossil arthropods. The two modern ana- directly oppose the terrace ridges. If this hypothesis is logues notably do not rely on their vision for locating true, it would be the first example of trilobite cephalic their food – suggesting by analogy that food detection limb differentiation to be discovered. Furthermore, it in asaphids must have been mainly chemosensory or would be the only known example of limb motion tactile. directly opposing the exoskeleton (rather than another The closely-spaced terrace ridges on the inner sur- limb) in an arthropod. face of the hypostomal tines are strongly reminiscent Asaphid trilobites may have sought out small, lightly of a grinding surface. This is in contrast to the wavy, cuticularized animals (live or dead) from the sedi- irregularly-spaced terrace ridges on the ventral surface. ment-water interface. The large, broad exoskeleton of The parallel nature of the ridges on the inner surface the trilobite blocked escape upward as the many legs LETHAIA 43 (2010) Asaphid hypostome functional morphology 415

ABC

Fig. 4. Hypodicranotus striatulus, scale bar in lower left (1 cm) applies to all images save (G–H). A, D–E, lectotype MCZ 1616, dorsal (A), left oblique (D), and anterior views (E). B, left oblique view of paratype MCZ 1618 showing in situ hypostome. C, F, dorsal and posterior views of hypostome MCZ 1617. G–H, anterior and posterior ground sections of a Hypodicranotus striatulus hypostome YPM 510850, scale bar is 5mm. searched for food via tactile or chemical sensation. limbs without hypostomal involvement. The tines of Once located, the food was propelled anteriorly by the the fork would have approximately lain beneath the coxae, and the cuticle was broken against the hypo- coxae. If the viscera and limbs took up enough space stome before consumption. Such a scenario is more such that the coxae directly abutted the dorsal surface likely than macropredatory (i.e. feeding on large prey of the tines, then the elongated hypostomal forks may items like other trilobites) or filter-feeding ecologies. If have actually functioned as a boundary between the this hypothesis is true, one would expect to find wear coxae and the distal portions of the limbs. This would patterns on the inner surface of the hypostome. have prevented the distal portions of the limbs from However, the identification of wear-patterns on fossil easily passing food to the food groove and may have hypostomes is complicated – to do so, one needs to restricted the distal portions of the limbs to a locomo- rule out the effects of mechanical preparation and molt tory role (as reaching ventrally around the tines would stage. Unambiguous wear patterns were not observed have been difficult). This would seemingly preclude on the specimens examined during this study. any sort of filter-feeding. If, however, the viscera and Hypodicranotus does not possess any of the special- limbs did not fill all of the space between the hypo- ized hypostome features observed in Isotelus.Itslong tome and the thorax, the distal portions of the limbs hypostome would have made feeding on large food could have been directly involved in feeding (by items difficult, as in Isotelus, but it does not show any reaching above the hypostomal tines). adaptations for grinding. Shorter forked morphologies The hypostome in Hypodicranotus may not have may have played a role in food ⁄prey positioning, played any role in feeding – instead it may have served restraint (Fortey & Owens 1999) or bracing (perhaps to protect the animals’ ventral side while still allowing analogous to the chilaria of living xiphosurans, see access to the ventral food groove. But, this protection Manton 1964), but the elongated tines of Hypodicr- would have come at the cost of the ability to enroll anotus appear too long for such a function. Food pro- (Whittington 1952), something that asaphid trilobites, cessing seems to have been accomplished solely by the despite their forked hypostomes, could do. Sacrificing 416 T. A. Hegna LETHAIA 43 (2010) the ability to enroll suggests that the long fork was Ivantsov (1990) noted the rasp-like surface and thicker very important. Further evaluation of its autecology inner hypostomal wall – hypothesizing that the Isote- must await the discovery of specimens with preserved lus-type hypostome functioned as an immobile grater appendages. for larger food particles. An alternative role for terrace ridges is as a burrowing sculpture. The function of terrace ridges (as sedi- Discussion ment ratchets) has been explored by several authors in several situations (Schmalfuss 1978a,b, 1981; Savazzi The function of the Isotelus-type forked morphology 1981, 1982, 1991; Savazzi et al. 1982). In burrowing, has been considered by several workers (Ivantsov the terraces seem to function as a ‘sediment ratchet,’ 1990; Sloan 1992; Brandt et al. 1995; Fortey & Owens providing purchase in the sediment to aid movement 1999; Babcock 2003). Brandt et al. (1995) docu- in the direction of burrowing. The terrace ridges on mented putative Isotelus trace fossils juxtaposed with theinnersurfaceofthehypostomearedifficulttopic- worm burrows and interpreted the association as ture playing any role in burrowing. Terrace ratcheting evidence of predation. Yet, this example lacks any is advantageous when the motion is perpendicular to evidence of a struggle – either the trilobite changing the terrace. its body position to facilitate the kill or the worm A number of workers have conducted detailed attempting to escape (Neto de Carvalho 2006). Fur- functional studies on a variety of arthropods that feed thermore, the timing of the traces may be very differ- with a ventral ‘food groove’ (Cannon & Manton 1927; ent (see Rydell et al. 2001) with the worm deforming Cannon 1927, 1928, 1932, 1935a,b; Cannon & Leak the trilobite trace long after it had been made. 1932; Fryer 1962, 1966, 1988; Manton 1964; Attramadal Sloan (1992) suggested that the edges of the hypo- 1981; Waloszek et al. 2007). These arthropods (e.g. stomal forks acted as cutting edges. Babcock (2003) Chirocephalus, Limulus, Triops,etc.)likelyprovidethe and English & Babcock (2007) expanded this Isotelus- closest analogueues for how trilobites may have fed. qua-predator idea further, working from observations However, considering the known limb morphology of made on a specimen of Isotelus maximus originally trilobites, they differ in several important ways. First, described by Mickleborough (1883; see also Walcott modern arthropods have a highly differentiated set of 1884). Babcock (2003) described ‘…the bases of rather head appendages (Manton 1977; Schram 1986), some- robust spines…’ (p. 77) which he speculated were thing that trilobites lack. Furthermore, not all arthro- used in capturing and manipulating prey. He also pods with apparent ventral ‘food grooves’ fed by noted that the inner blade of the hypostome is ‘sharp’ moving food anteriorly within the food groove (Can- and the edge is ‘serrated’ (p. 80), which together with non 1927; Attramadal 1981) – the mere presence of a a gut tract devoid of mud or shell debris, he inter- ‘food groove’ does not necessarily indicate the mode preted as evidence of non-durophagous predatory of feeding. The feeding mode of certain filter-feeding habits. However, when considered as a three- crustaceans, like the branchiopod anostracan, Chiro- dimensional object, the hypostome would be a poor cephalus diaphanous (see Cannon 1935a), may be facsimile for a serrated knife (Fig. 1). The presence of applicable to certain trilobites (inferred ‘pelagic’ or robust limb spines in also in doubt. Mickleborough’s nektic trilobites, see Fortey 1985), but are inapplicable (1883) specimen is preserved like the trilobites from to the known range of trilobite limb morphology. the Walcott-Rust Quarry (Brett et al. 1999) as a calcite Two groups of modern arthropods may serve as liv- spar filled cast of the animal. The bases of broken ing models for aspects of trilobite feeding: horseshoe spines should, therefore, appear as circles of spar on crabs (i.e. Limulus) and branchiopods. Horseshoe the limbs set apart from the brown matrix – no such crabs (Limulidae: Chelicerata), like trilobites, have a structures are present, implying that the features posteriorly-directed mouth and spiny gnathobases on described by Babcock and English as spine bases are their walking limbs which are used to shred food. bits of matrix on the limbs. However, horseshoe crabs only have five walking Fortey (1990) had previously characterized the limbs, half the number possessed by most trilobites inner hypostomal surface as ‘rasp-like,’ and suggested (between 10 and 25 for most species). Food is that it may have functioned in concert with the limbs located and manipulated into position by the chelicera to aid in the shredding of food. He supported this and then the animal sits atop its food and either observation with the fact that the hypostome is shreds and chews it with the gnathobases or subjects it strongly braced to the anterior margin of the cranidium to the ‘nut-cracking apparatus’ (chilaria) at the sixth (implying a passive rather than active function for the coxae (Manton 1964). The gnathobases of the five hypostome). Fortey & Owens (1999) hypothesized walking limbs make for an effective ‘conveyor belt’ that longer tines may have helped prevent prey escape. that shreds food on its way to the mouth. LETHAIA 43 (2010) Asaphid hypostome functional morphology 417

Branchiopod crustaceans, in contrast, have a much dalmanitids (Budil et al. 2008) had a tuberculate ⁄den- longer series of homonymous thoracic limbs creating ticulate region medially on the posterior margin of a ventral ‘food groove’. The branchiopod notostracan their hypostome – possibly used in food manipulation Triops envelops its prey or food with its anterior (i.e. shredding or grinding). Budil et al. (2008) limbs, tearing at the prey with its thoracic limb spines broadly compared the denticulate posterior margin in (Fryer 1988) – similar to feeding in Limulus. Suitably dalmanitids to the forked margin in asaphids, and sized particles are then moved dorsally into the food suggested a possible common function (i.e. food groove and conveyed towards the mouth. The opera- manipulation) supported by their proximity to the tion of the thoracic limbs in notostracans resembles food groove. the operation of the walking limbs of xiphosurans; but Doublures present elsewhere on trilobites suggest: notostracans are distinguished by the presence of (1) that they were exposed like the dorsal exoskeleton crushing mandibles and two pairs of maxillae. The as is suggested by the presence of terrace ridges; and mandibles and maxillae are incorporated into an (2) that they sheltered soft-tissue attachment sites. external throat-like chamber called the atrium oris. Therefore, by analogy, the hypostome overhung the The atrium oris is immediately posterior to the rear- mouth with the dorsal doublure exposed. The func- facing mouth, where the mandibles process food. It tion of this sclerotized lower lip would have been dif- essentially envelopes the mandibles between the body ferent than that of the flexible, fleshy labrum in living wall (dorsally), labrum (ventrally) and paragnaths crustaceans, but both would create sheltered spaces in (two triangular fleshy sternal outgrowths positioned which to process food. Additionally, it is possible that opposite the posterior edge of the labrum). The maxil- the mouth was situated much farther anteriorly lae serve to force food into the atrium oris. underneath the hypostome, and that the hypostome The closest analog in modern arthropods to was only covered dorsally by tissue creating a ‘labrum- the hypostome is the labrum. However, following Wal- like’ structure with a hard ventral surface and a soft ossek & Mu¨ller (1990); see also Walossek 1993), it is dorsal surface – a ‘floor’ to the atrium oris. Function- regarded as a positional analogy only. Walossek & ally, this would be like a modern crustacean labrum Mu¨ller (1990) define the crustacean labrum as ‘…a with a sclerotized outer surface. However, known complex organ with sensorial and glandular func- mouths in both Agnostus pisiformis (Walossek 1993) tions…’ (p. 416). It forms the ‘floor’ of the atrium oris. and the stem crustaean Oelandocaris oelandica (Stein In contrast, the trilobite hypostome is a skeletal ele- et al. 2005) emerge at the posterior margin of the ment of the ‘forehead’ (Mu¨ller & Walossek 1987) as the hypostome ⁄labrum. mouth is thought to have been positioned immediately The position of the mouth notwithstanding, the posterior to the hypostome rather than underneath. hypostome of Isotelus bears an uncanny resemblance As an exoskeletal element, it is hard to imagine the to a grinding surface. This level of hypostome differ- hypostome possessing the range of sensory ⁄glandular entiation is otherwise unknown in trilobites, and functions of the crustacean labrum (although these would hint at a hitherto undocumented level of functions could have been performed by tissue imme- cephalic limb specialization. However, this line of rea- diately adjacent to the hypostome). However, the soning cannot be extended to all forked hypostomes; morphology of certain trilobite hypostomes suggests nothing about the long-forked hypostome of Hypod- that they may have possessed an atrium oris. Well- icranotus suggests that it possessed differentiated preserved, silicified trilobite hypostomes of certain cephalic limbs. A detailed examination of other dis- odontopleurids (Chatterton & Ludvigsen 1976; Chat- tinctive hypostomes may yield further insights into terton & Perry 1983; Chatterton et al. 1997), lichids trilobite ecology. (Chatterton & Ludvigsen 1976; Chatterton et al. Acknowledgements. – D. E. G. Briggs, U. Farrell, E. Lazo-Wasem, 1979), calymenids (Chatterton et al. 1990), cheirurids R. Lerosey-Aubril, J.-P. Lin, A. Seilacher, O. E. Tetlie and J. Wolfe (Chatterton & Ludvigsen 1976) and encrinurids provided helpful discussions. G. Kloc provided measurements of (Edgecombe et al. 1998), for example all have a signif- an unusually large Isotelus hypostome. S. Butts helped with specimen access. T. Lioubetskaia aided with Russian translations. icant dorsal doublure on the posterior margin of the P. Budil, R. Owens and associate editor A. Owen provided helpful hypostome. This may indicate that soft tissue attached reviews. to the hypostome significantly in front of the hypostomes’ posterior margin. Some lichids had small recesses in the dorsal doublure of their hypostome, suggesting References an as yet unknown function (Chatterton & Ludvigsen Attramadal, Y.G. 1981: On a non-existant ventral filtration current 1976; fig. 19.19). Likewise, the hypostome of some in Hemimysis lamornae (Couch) and Praunus flexuosus (Mu¨ller) odontopleurids (Chatterton & Perry 1983; fig. 23.12) (Crustacea: Mysidacea). Sarsia 66, 283–286. phacopids (Bruton & Haas 1997, 2003), and 418 T. A. Hegna LETHAIA 43 (2010)

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