Contributions to Zoology, 71 (4) 159-169 (2002)

SPB Academic Publishing bv, The Hague

Possible lattice in Cretaceous organs Thylacocephala

Sven Lange & Frederick+R. Schram

Institutefor Biodiversity and Ecosystem Dynamics, University ofAmsterdam, Mauritskade 57,

1092 AD Amsterdam, Netherlands

Key words: Thylacocephala, fossils, lattice organs, , Cirripedia, Ascothoracida, Crustacea,

Cretaceous.

Abstract form has come to be recognized (Pinna et al. 1982;

Secretan 1983; Briggs & Rolfe 1983). Typically a

reminiscent of the lattice in thecostracan the entire This Structures, organs large carapace envelops body. cara-

described from the cuticle ofCretaceous , are carapace pace covers the major part of the head and trunk, The lattice like thylacocephalans. new organ structures occur thus obscuring potentially important information similar in pairs along the dorsal midline. While these have a such as Out from under the outline lattice lack do segmentation. carapace to true organs, they seem to pores and two sets of limbs Secretan 1985; Rolfe not occur in the highly apomorphicpattern found in thecostracans. protrude (see

three These discrepancies do not easily support earlier ideas of a 1985): a set of pairs of sub-chelate raptorial

of the within the thecostracans. position thylacocephalans limbs, and another set forming a posterior battery the lattice for The significance of possible organs inferring a of usually eight (but sometimesmore) small swim- relationship between thylacocephalans and thecostracans is ming limbs. Recently, Lange et al. (2001) described discussed. another set of limbs corresponding to antennules

and antennae. In addition to these limbs, a pair of

often out from the compound eyes bulge an- Contents large terior of the A of feather- margin carapace. row

like near the middle Abstract 159 gills regularly appears region

Introduction 159 of the body under the carapace (Rolfe 1985; Secretan

review 160 Lattice organs: a & Riou 1983; Schram et al. 1999). Thylacocephala

Materials and methods 160 are known from the Silurian (Mikulic et al. 1985; Results 161 Van den Brugghen et al. 1997) through to the Cre- Presence and of lattice 161 position possible organs taceous et al. of in (Schram 1999). Morphology possible lattice organs light

microscopy 163 The taxonomic affinities of the thylacocephalans

in SEM 163 Morphologyof possible lattice organs remained ‘uncertain’ (Rolfe 1992), although most Reconstruction ofthe morphology ofpossible lattice authors at least implied some kind of 165 organs affinity. With the recent description of two pairs Discussion 166 of antennae in the Cretaceous thylacocephalan Thy- Acknowledgements 168 al. the References 168 lacocephalus cymolopos Lange et 2001, pos-

sible affinity with Crustacea has beenstrengthened.

determinationof sister However, a crustacean group Introduction for the Thylacocephala remains vexing. A number

of possible and contradictory crustacean relatives

Arthropod fossils now placed within the higher have, nevertheless, been suggested based on vari- taxon Thylacocephala have been known since the ous morphological similarities. Secretan (1983) 19th It is only recently that their unique century. considered the possibility of phyllocarids, mala-

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and conchostracans sister Schram lattice while costracans as taxa. organs a varying number of ‘extra’

drew with malacostracans based fields have been in the of (1990) comparisons pore registered carapace

the sub-chelate on presence of large raptorial limbs other species. Such ‘extra’ fields resemble lattice

and but (stomatopods), large eyes well-developed gills organs display a more irregular layout and lack

and noted the terminal found in ‘real’ lattice (many eumalacostracans), even some pores most or-

potential points of similarity between thylaco- gans (Jensen et al. 1994a).

and Arduini et al. first The outline ofthe lattice varies from cephalans remipedes. (1980) organs very

drew attention to similarities with thecostracan cir- elongate to roundish; and from a deep depression

ripedes, and this was reinforced by Pinna et al. that contains a central raised smooth keel, to simple

Rolfe and in the (1985). However, (1985) Schram (1990) flat pore fields level with surrounding cara-

this cuticle. Intermediates of these rejected possibility, perhaps too precipitously pace principal types

since Schram al. illustrated have less keels with reticulated et (1999: plate 3, fig. 7) may pronounced a

the of cuticular that surface the fields. One so-called presence elongate structures comparable to pore

evocative of lattice distinct theco- terminal were organs, a pore is often associated with each lattice

found the of A rim of cuticle sometimes delimits the lat- stracan apomorphy on carapace some organ.

free of thecostracan larvae tice et al. al. swimming stages (Schram organs (Jensen 1994a; Jensen et 1994b).

et al. Often are minute structures of 5-10 1999). they pm length

and 1-2 width but dimensions with pm vary spe-

cies rather and some ‘gigantic’ lattice organs ap-

50 10 width the Lattice organs: a review proach pm length or pm (including

in of the surrounding rim) some larger lepadid spe-

Thecostracan lattice first described cies et al. organs were by (Jensen 1994a).

New evidence Elfimov (1986), Their status as synapomorphies for demonstrates that the lattice or-

derived setae et in the Cirripcdia and the Ascothoracida (in the latter gans are (Rybakov al. press)

with function group they were initially dubbed cardiac organs, presumably a chemosensory (Hoeg

but al. Jensen et al. (1994a) later identified them as et 1998). Dendrites from sensory cells project

lattice of into the cuticle and terminate in to the organs on grounds homology) was firmly proximity

The terminal the established by Ito & Grygier (1990) and Jensen et pores. larger pore seems most likely

for molecules to reach the ele- al. (1994a). Lattice organs have been found in the pathway sensory

cyprid larvae of thoracican, acrothoracican, and ments in and Ascothoracida. In Thora-

cica and the rhizocephalan Cirripedia as well as in larval stages Rhizocephala, many small pores of the field facilitate ofAscothoracida (Jensen et al. 1994a) andrecently pore may diffusion towards the

elements. These reside in the also in Y-larval Facetotecta (Hoeg & Kolbasov sensory pores exo-

2002). The extant crustacean parasite Tantulocarida cuticle with the thin epicuticle coating the walls

well and the bottom of In as as the Cambrian Bredocaris have been the pores. Acrothoracica, minute

forward as sister of the the but not the brought potential groups pores penetrate epicuticle underly-

exocuticle. The of Thecostraca (Walossek et al. 1996; Walossek & ing possession numerous pores

Muller but both lack lat- be for the lattice 1998), groups definitely may autapomorphic cirripedian

tice within the Thecostraca the et al. & Kolbasov organs. However, organs (Hoeg 1998; Hoeg 2002).

number, position and anatomy of lattice organs

constitute systematically valuable characters (Hoeg

& Kolbasov 2002). Materials and methods

the lattice In thecostracans, organ complex nor-

mally forms characteristic constellations of two Four Cretaceous thylacocephalan species were examined: Proto-

zoeahilgendorfi Dames, 1886; P. damesi 1946; Pseude- clusters containing all together five pairs of lattice Roger, richthus cretaceus Dames, 1886, and Thylacocephalus cymolopos organs. Typically, the anterior cluster contains two Lange et al. (2001). while the includes three pairs posterior usually pairs. Series of possible lattice organs were found in eight speci-

some have a reduced number of However, species mens of; Protozoea hilgendorfi: [BM: J 469-1, J 622-1; OR

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Table 1. Information on fossils well series of lattice like thylacocephalan possessing preserved organs including length (* carapace

without rostrum and in of - length posterior spines, mm); body type (**according to Schram et ah, 1999); position organs (ant anterior, mid - mid dorsal, number of and distance between post posterior); organs neighboring organs (in mm).

P. hilgendorft Carapace length Body type** Position of series Number of llo’s Average distance

mm (estimated*) along midline in each series between illo’s

BM J469-1 (40) globose ant 3 0.5

mid 10 0.55

most 7 0.85

BM J622-I 78 elongate ant 12 0,5

mid 7 0.85

post 11 1.5

BM OR 59245 (2)-l 44 elongate mid ~20 0.5

BM OR 59245 (3)-l (50) intermediate postpost 1212 0.82

BM OR 59554 mid - 15-20 (3) (32) globose mid post 15-20 0.40 '-fc* -1 0.5OLn

MB A 693.3 (24) globose mid ~13 0.6

MSNM i!2172il2172 42 intermediateintermediate mid 5 0.8

MSNM i20556120556 (30) globose post 4-5 0,7

P. damesi

BM OR 59672 15 (4) 8 0.20.2-0.3- 0.3 post ,

59245 (2)-l, (3)-1, OR59554 (3). MB: A 693.3. MSNM: il2172, Results

120556] and one specimen of P. damesi [BM: OR 59672 (4)]

(see Table 1). Presence and position ofpossible lattice Similar structures occurred in organs sporadically some four speci- mens ofP. hilgendorfi: [BM: PI IC 82; In 42507a; OR 59672

and an additional Our lattice (16)] specimen MSNM: i7586-3, which is of putative organs were best observed in uncertain thylacocephalan affinity. P. of this hilgendorfi. Eight specimens species pre- Further possible lattice were noted, but their organs poor served series of lattice while potential organs, only preservation prevents unequivocal identification:P. hilgendorfi-. one P. damesi preserved such a series. Scattered [BM; PI 1C 76, 83; In 48832 (2), (3); OR 59554 (2), OR 59672 of this organs were observed on an additional (14), (15) and (24). MB: A 693.4, 693.5. MSNM: i7586-l]; P. type damesi: [BM; IC 91-1; OR 59245 (4)-l, OR 59672 (25).] four specimens, whereas another 14 specimens

A section (indicated in Fig. 1A) of the posterior showed evidence of what could well also be carapace very along the dorsal midline of one especially well-preserved spe- traces of such structures. In several fossils (BM J cimen (BM: J 622-1) was carefully removed. The section con- 622-1; BM OR 59245 (2)-l and (3)-l), the bilat- lattice taining four pairs of possible organs was subjected to eral of the lat- standard SEM (JEOL symmetrical arrangement possible high voltage JSM-35C), and images were

tice was observed. in in saved in the Semaphore digital image storage system. organs They occur pairs

In addition, ofthe Jurassic Dol- the immediate ofthe specimens thylacocephalan vicinity dorsal hinge-, or mid- locaris ingens van Straelen, 1923 were examined. Structures line with one member ofeach pair on the right valve reminiscent of lattice noted possible organs were on two speci- and the other member of the pair on the left (Fig. mens (MNHN: R.50930 and R.50937). 1A-F). Specimen repositories are abbreviated as follows: BM - The

- Possible lattice were encountered Natural History Museum, London; MB Museum fur Natur- organs any-

MNHN - Museum Natu- where the dorsal midline of the kunde, Berlin; Nationale d’Histoire along carapace but relle, Paris; MSNM - Museo Civico di Storia Naturale, Milan. were never detected on either the rostrum or the

posterior spines. For instance in BM .1 622, which

best the preserves possible lattice organs, the dor-

sal midline is only properly exposed near the ros-

trum, in a few areas along the central part of the

and for carapace posteriorly approximately one

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Fig. I. Prolozoea hilgendorfi. Anterior to the right. A-D, BM J 622-1. A, lateral view ofthe right side (as well asminor part of left side of of lattice both sides of the that marks the dorsal midline. Box indicates carapace) specimen with possible organs on hinge-line

indicate of of the midline anterior position of section that was subjected to SEM, (Arrow locality Fig. ID). B, Part posterior (directly

box in and of like Note also the circular Posterior of lattice like to Fig. 1A) two pairs lattice organs. pit organs (arrowhead). C, pair

from IB. shows the central and the rim. lattice organs fig. elongated depressions surrounding dark D, Smaller anterior possible organs and a pair of slender, oblong structures found only in this position and only in one pair. Arrows indicate position of some possible lattice structures. E and BM OR 59245 end with series oflattice like in close Note organs: o, oblong F, (3)-l, posterior organs. F, up. also the circular BM OR 59554 dorsal midline with numerous pit organs (arrowheads). G, (3), central part of possible lattice organs

the BM J with lattice the mid- and ofthe dorsal midline. Scale bars ofA = along margin. H, 469-1, possible organs at posterior part

=' 10 B = 400 C = 100 D = 500 E and F = 2 G and H 1 mm; pm; pm; pm; mm; mm.

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of the entire of the lattice quarter length specimen (Fig. Morphology ofpossible organs in light

1A). All these exposed stretches reveal possible microscopy

lattice organs (Fig. 1B-C). Series of possible lat-

tice are the midline in other lattice organs preserved along The possible organs appear as oval spots of well in various with specimens as positions respect dark brown color clearly distinguishable from the

1E-H and to the axis Table It conse- cuticle of the body (Fig. I). surrounding light-brown carapace (Fig

quently seems reasonable to assume that the C and The lattice pos- 1B, F). largest possible organs, sible lattice formed organs a complete series from found posteriorly in BM J 622-1, measure 300 x rostrum to none of the ex- 150 consist of posterior spine, though pm. They an outer dark rim with a amined P. such a width of 50 which hilgendorfi actually displayed pm or more, surrounds an ap-

series. 150 x 50 complete proximately pm lighter central area. This

An obvious increment in the distance between elongated, occasionally slightly pear shaped, area

lattice towards the is often delimited wall. neighboring possible organs by a slightly raised ridge or

was observed 1). In BM J 622-1 the lattice in posterior (Table Anteriorly possible organs BM J 622-

1A-D), the first 12 of are I trace dark rim (Fig pairs organs found lack all but a of the and the light

within 5 mm distance between ~ (average adjacent central area typically measures 100 pm in length lattice possible organs less than 0.5 mm). Near the and less in width (Fig. 1D). Probably, the dark rim

center of BM J 622-1, another 7 lattice possible corresponds to an area with a thicker, more heavily

cover 4.5 distance ~0.75 organs mm (average mm), tanned or otherwise differentiated cuticle (Fig. 2).

while the 11 pairs on the 15 mm near the posterior

found spine are with an average distance from each other of approximately 1.5 mm, rising to about 2

mm for the very last pairs. The same trend can be observed for specimen BM .1 469-1, and also in-

ferred from the average distances between pairs in other P. hilgendorfi (Tab. 1).

Apparently, the possession of possible lattice

is restricted neither individuals of organs to a par-

ticular (large) size, nor to individuals of a certain

P. body type. hilgendorji lengths range from 21.5

- 78 mm (Schram et al. 1999), and specimens with

lattice this Fig. 2. Protozoea hilgendorfi. Anterior to the BM J 622- possible organs span range neatly (Tab. right.

SEM photograph.Oblique view of dorsal midline with a 1). The full length of specimen MB A 693.3 is 1. pair of lattice Each consist of central possible organs. a depression estimated at about 24 mm (this specimen lacks the (arrows) more or less surrounded by low ridges. Note also posteriormost part of its carapace), but there is no tendency for lowered areas anterior and posterior to depression. the known doubtthat it to smallest P. Scale bar = 100 belongs hilgen- pm.

dorfi . Specimen BM J 622-1 on the other hand is

a ‘giant’ reaching 78 mm in length. Both speci- lattice Morphology ofpossible organs in SEM mens have lattice Another possible organs. impor-

of tant pattern variation concerns the body type of Pair I P. hilgendorfi, which varies from elongate with a Covers an oval area of 140 x 80 Left: pm (Fig. ratio of length to height of 8:1, to quite globose A 3A). club-shaped trench or of 90 of depression pm specimens with a ratio 2.5:1 (Schram et al. 1999). length is laterally surrounded by two Both longitudinally elongate, globose and intermediary forms are running ridges. Small lines demarcate the periph- represented among the specimens with possible ery outside of the The distance between the lattice ridges. organs (Tab. 1). of the is tops two ridges 40-60 pm, being widest

near the anterior end. From each ridge a steep but

vertical not slope leads toward the more or less Hat-

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Fig. 3. Protozoea hilgendorfi. Anterior to the right. BM J 622-1. SEM photographs (A-C) and (D) line drawing showing reconstruction of with two folds indicated in the anterior ofthe Scale bar = 20 possible lattice organ part depression, r, ridge; d, depression. pm.

the floor of the bottomed trench. Anteriorly trench but a grainy substance obscures most of this terri- is 15 pm wide, narrowing down to 5 pm posteri- tory especially towards the 5 pm wide posterior orly. end.

x Right: Covers approximately 140 70 pm. Its outline resembles the previous. Less pronounced Pair 3 ridges surround the ~ 100pm long depression, which Left: Covers an oval area of 130 x 80 pm. The has a wider anterior part and a more narrow and ridges surround an at least 90 pm long club-shaped

floor trench that of folds shallow posterior part. The of the anterior part preserves traces anteriorly. Near is cast into a few distinct, mainly longitudinal folds. the anterior margin there is a diminutive indenta- \ tion that could represent a pore. Pair 2 Right: A 130 * 60 mm long structure in which

Dimensions x the median is absent. Left: 150 80 pm (Fig. 3B). Ap- ridge practically Neverthe- parently the depression falls in several different parts. less, the remaining slopes delimit a trench of the

Anteriorly there seems to be a short shallow ter- usual shape. A short depression occurs near the 20 race with a bump-like short fold. This is followed pm wide anterior margin followed by a raised area

of the trench that that the short folds mentioned by a deep, only 40 pm long part may correspond to

into the shallow above. The of the trench is extends 60 pm more posterior re- posterior part some-

The width 15 10 gion. narrows from pm to pm. what obscured by sediment and presents little evi-

Weak of floor in folding the occurs both the deep dence for the presence of a pore. part of the trench and the more shallow part. A

is observed the anterior Pair 4 groove near margin.

A 125 trench surrounded Covers an area of 120 x 50 Not as Right: pm long area by Left: pm. diminutive ridges (Fig 3C). The floor of the up to clearly defined as previous due to sediment, but it

wide trench. 20 pm trench shows some folding anteriorly, includes ridges and a club-shaped

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The outline is the but sedi- the cuticle. the Right: general usual, carapace On internal side the slopes obscures details. ment pass into the trench. The floor of the trench is situ-

ated below the external level of the ordinary cara-

cuticle. also lead pace Slopes up from the floor of

Reconstruction the the trench to the both the anterior of morphology of a possible carapace at and

lattice ends. The seem to 20 organ posterior ridges to project up

above the pm carapace surface in its immediate

An attempt to reconstruct the original morphology surroundings. of the lattice of these In the trench has distinct club possible organs Thylacocephala most cases, a shape

be derived from the above with the wider anterior 15-20 while may descriptions (Fig. part reaching pm

3D). The tanned areas observed with light-micros- the width of the more narrow posterior 2/3 of the

surround the trench is less than 10 The copy to possible lattice organs are not predominantly pm. total

SEM and the recognizable in are therefore not be in- length of floor is difficult to asses precisely

in the of it cluded reconstruction. (Cuticle a similar because grades into the carapacecuticle, but typi-

it color occurs in connection with other structures such cally surpasses 100 pm.

the characteristic When covered the broader as pit organs (Fig. IB and F) or not by sediment, an-

the and terior of the trench often folded. along carapace margins, may simply indi- part appears Typi-

in other cate a change thickness or some property cally there are one to a few small and short folds

of the cuticle). (Fig. .4A and B). Slighter folds were observed, but

An individual in the of the trench. possible lattice organ from the rarely, narrow posterior part

of the therefore The search for associated did posterior region carapace can be pores not yield a

It There recognized based on its general relief. covers an consistent result. were no pores appearing

in elongated sub-oval area of approximately 120-150 the same location in all investigated possible

the lattice in pm parallel to longitudinal axis of the organs. However, a few cases pore-like

and width from 60 - 80 both the and a ranging pm. Superficially, grooves were observed, near posterior each lattice consists of central the anterior end of the trench 4B and In possible organ a (Fig C).

other of the elongated trench flanked by low ridges. Though inspected possible lattice organs, how-

these locations the ridges appear clearest along the Tong sides’ of ever, lacked recognizable pores (Fig

the trench, they could actually represent the remains 4A).

of in only one ridge completely encircling each trench. Additional oblong structures were noted P.

To the external side, the ridges gently grade into hilgendorfi (specimen BM J 622-1 and BM PI IC

4. Details of anterior of from Fig. part possible lattice organs Protozoea hilgendorfi. Anterior to the right. BM J 622-1 A, Folds

(arrow). B, Fold and slit C, Short fold and slit Scale bars A and (arrow) pore-like (arrowhead). (arrow) pore-like (arrowhead), r, ridge.

= B = 10 pm; C 5 pm.

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Discussion

The of the lattice position possible organs close to

the midline thylacocephalan dorsal resembles some-

what that of the lattice in the organs thecostracan

the Crustacea. However, characteristic 2 + 3 ar-

rangement found in the latter was not observed in

the examined Thylacocephala.

In a morphological comparison, it is immediately

obvious that the reconstructed possible lattice or-

described gan above does not correspond completely

to of the any principal types of lattice organs oc-

curring in the extant thecostracan crustaceans as

described by Jensen et al. (1994a) and Hoeg & Fig. 5. Dollocaris ingens. MNHN R.50937. Anterior to the right. Kolbasov The lattice (2002). possible organs are indicate start Arrows and end ofanelongate structure occurring different from the field’ with the on the bar = very ‘pore carapace. Scale 2 mm. type characteristic perforated surface (Fig. 6A). Such

pore fields are autapomorphic for the thoracican 82) and probably in P. damesi (specimen BM IC and rhizocephalan thecostracans (Hoeg & Kolbasov around 91-1). Reaching sizes 500-600 they are pm 2002). However, thecostracan lattice organs of the

considerably larger than adjacent possible lattice second principal type do have a smooth cuticle dissimilar organs, though not in of two consisting seemingly ‘folded’ into one elongated keel that sits

juxtapositioned longitudinal ridges. These in This in oblong a depression (Fig. 6B). ‘keel a trough’ structures were found in only one pair indi- per type is considered more plesiomorphic within the

vidual, situated anteriorly immediately ventral to in Thecostraca, occurring the Facetotecta (Hoeg & the series of possible lattice organs in a in bilaterally Kolbasov 2002) as well as the Ascothoracida symmetrical (Fig. 1D). Furthermore, arrangement and the Acrothoracica (Jensen et al. 1994a). Lat- also of the carapace ofDollocaris a pair tice possesses organs from both Acrothoracica and Facetotecta

albeit in a more ventral elongate structures, posi- are more shallow with less pronounced keels than tion; they are slightly curved and seemingly made depicted in Fig. 6B (Jensen et al. 1994a; Hoeg &

up by two parallel ridges (Fig. 5). Kolbasov The 2002). possible lattice organ of the

Thylacocephala resembles the plesiomorphic lat-

tice in organ type having the same basic outline, i.

e., elongated depression of a smooth cuticle with a

tendency to display folds, as observed in its broader

Fig 6. Examples of ‘Keel in thecostracan lattice organs. A, a trough’ type from Ascothoracida. B, ‘Pore field’ type from Thoracica

from & (both Hoeg Kolbasov, 2002) tp, terminal pore.

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anterior region. These folds could the tice represent keel, organs occur in mature specimens of thylaco- but do they not present the same tight fitting keel cephalans.

in sitting a narrow The that sur- Because of depression. ridges these morphological similarities, we round the lattice possible organs be assume that may compa- might the large and numerous possible rable the elevated to rims surrounding the central lattice in organs Thylacocephala are really homolo- area of lattice from reported some theco- the lattice organs gous to organs in the Thecostraca. This

stracans. The lack of definitive evidence of termi- for can a between the argue relationship two groups. nal in the pores thylacocephalans is The troubling. However, it can never serve to include the thylaco- smooth ‘keel in a trough’ type lattice within organ always cephalans the Thecostraca sensu Grygier terminal possess a pore, whereas the terminal (1987) because the pore primarily highly conserved apo- is absent from the apomorphic fields in some of + pore morphic pattern 2 3 lattice organs in Thecostraca species (Jensen et al. 1994a). However, the low- is absent in the Thylacocephala. ered areas anterior and to the If posterior possible a sister is group relationship accepted, an oppor-

lattice (Fig. 2) again bear resemblance to organs to tunity compare the body plan of the Thecostraca lattice organs from some thoracican thecostracans. and the information emerging concerning the body Here the lattice includes an anterior concav- of organ plan the Thylacocephala presents itself. Theco-

ity housing the terminal as well as a pore slight stracans have a comparatively short trunk consist-

posterior concavity et al. (Jensen 1994a). ing of no more than 11 to segments including up In the lattice general possible of the seven organs thyla- in the thorax. Such short bodies also occur

cocephalans are several times larger than the lat- in other crustaceans, which are often placed within tice of organs thecostracans. this size the Perhaps larger (Schram 1986, but see also Schram be could viewed as an allometric of & consequence a Hof 1998). Because most of the adult theco- much size. larger body While thecostracan larval stracans are modified for sessile highly or para- with stages lattice organs measure around 1 mm sitic few lend lifestyles, only themselves to such a (Jensen et al. & Kolbasov the 1994a; Hoeg 2002), direct comparison. The thecostracan ascothoracid examined thylacocephalan fossils from a few ranged genus Synagoga Norman, 1888 has seven thoracic

to to eight cm say nothing of other thylacocephalans; somites and four abdominal in addition to a typical some like Dollocaris and Ostenocaris Arduini et crustacean of cephalic region five segments (Grygier al. 1982 20 cm. In proportion to total 1983; surpass body 1984). The posterior end of a thylacocephalan however, thecostracan lattice length, includes a of organs actually body battery eight (sometimes even

appear slightly larger than those of the thylaco- more) limb-bearing segments. This does not accord

cephalans. Since lattice are de- with of the organs evidently any thecostracans. Furthermore, com-

rived setae et al. in (Rybakov press) it is interesting with the wider of parison array maxillipodans pre- notice that to chemosensory setae reach a may length sents similar problems. To force thylacocephalans

of at least 2.0 mm in of specimens the into these inclusive would decapod more groups require a crustacean Homarus americanus (Milne-Edwards) postulated combination of elements that is diffi- with body sizes comparable to the cult to envision protozoean thyla- at this time, viz., the fusing of two

cocephalans 1982). short (Derby to form in some the 8+ tagmata way segments On the other hand, the number of observed of the pos- posterior thylacocephalan body region. At sible lattice clearly exceeds that of lattice organs present, the Thylacocephala cannot convincingly in the Thecostraca, even if the ‘extra’ organs pore the sister dispute potential group relationship be- fields in observed some thecostracans (Jensen at tween Thecostraca and the Tantulocarida as well al. 1994a) reduce this difference. differ- may The as Bredocaris (Walossek & Miiller 1998).

ent number of observed could on organs depend Attempting to see thylacocephalans in an ances-

body size too, but also be attributed the fact role may to tral with respects to thecostracans also demands that certain small only larval stages in thecostracans' in large hypothetical rearrangements the body plan.

are with lattice equipped organs, while it is likely, Presently, it that the appears thylacocephalans may though not proven, that the observed lat- possible have had at least three pairs of limbs between the

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mandible and the posterior limb battery (Polz 1993, Acknowledgement

Schram 1990, Lange et al. 2001). This suggests that

thylacocephalans had a maxilliped, but speculation We thank Jan van Arkel for graphic assistance. Prof. Derek

Briggs and Dr. Jens Hoeg offered valuable suggestions for on homology to the maxillipeds in Thecostraca to improving the manuscript. We are further indebted Dr. Jens in seems premature. Maxillipeds do occur some Hoeg for permitting us to use the illustrations ofthecostracan but with a maxillopodans, together correspondingly lattice organs (Fig. 6). reduced number ofremaining thoracic limbs (Gry-

gier 1983, Schram 1986). Again, the number of

limbs would seem too low when com- remaining References

pared to the 8+ battery in thylacocephalans.

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