^^^ 4 2006

MCZ

PROCEEDINGS library

°f ^'^^ OCT 1 5 1990

San Diego Society of Natural History

^ ^ -^ HARVARD

Founded 1874 UNIVERSHY

Number 3

15 September 1990

On Mazon Creek Thylacocephaia

Frederick R. Schram

Department of Paleontology. San Diego Natural History Museum. P. O. Box 1390. San Diego. California 92 1 12. USA

ABSTRACT — Three species of the enigmatic arthropod group Thylacocephaia are described from the Middle Pennsylvanian Mazon Creek

biotas: two new species of the genus Coiicavicari.'i. C. gcoriieonim and C. remipes. and one new genus and species. Come^icaris nuizDnensis. There

are problems m the study and intcrprelalion of thylacocephalans because their tbrm is so unusual and they appear to lack characters that would ally

them with known major arthropod groups. A revision of the higher taxonomy of thylacocephalans is proposed and comments are made on possible

affinities of the group.

INTRODUCTION

The class Thylacocephaia as a separate, recognized group of

arthropods is a relatively recent collation. Yet species now included

within this group (Arduini and Pinna, 1989) have been known since

Meek (1872) described what is currently called Concavicaris

hradleyi. However, prior to the work of Briggs and Rolfe (1983),

thylacocephalans were, almost without exception, classified as

phyllocarids even though neither abdomen nor appendages were

known.

In 1982 and 1983, three groups of researchers, working inde-

pendently of each other, published studies of different fossils that

we now realize are related in some way (Rolfe, 1983, presented an

excellent historical review). Each team created a new higher taxon

to accommodate their species. First, Pinna et al. (1982) designated

a class Thylacocephaia to accommodate a new arthropod from the

Jurassic of northern Italy, Oslenocaris cypriformis Arduini et al.,

1980, which they believed had possible cirripede affinities. Second,

Secretan ( 1983) erected a class Conchyliocarida to recognize the

aberrant position of Dollocaris ingens Van Stralen. 1923, from the

Jurassic of France, a recognition facilitated by the study of then

newly made collections (Secretan and Riou, 1983). Finally, Briggs

and Rolfe (1983) described a collection of arthropods from the

Devonian of Australia, but could not attribute their material to any

known class and erected an order Concavicarida of uncertain af-

finities. The disagreements over the interpretation and higher taxo-

nomic placement of these fossils (see Pinna et al., 1985; Secretan,

1985: Rolfe, 1985) in large part reflected differences in the preser-

vation of a combination of peculiar anatomical features unique

among arthropods. Rolfe (1985) weighed the available evidence

and concluded that all three groups of researchers had been working

on closely related fossils, which by priority took the name of

Thylacocephaia.

Quite independent of the above, amateur collectors of the fa-

mous Middle Pennsylvanian Mazon Creek faunas of the Francis

Creek Shale long have puzzled over peculiar fossils they referred to

by the common name "flea shrimp." Results of the study of these

flea shriinp fossils reveals that they are a varied group composed of

three distinct species assignable to two genera of Thylacocephaia.

These fossils suggest a reorganization of the higher taxonomy

within the Thylacocephaia.

Specimens used in this study are designated by the following

prefixes: SDSNH, San Diego Society of Natural History paleon-

tology collections; PE, Field Museum of Natural History inverte-

brate paleontology collections: MCP, Northeastern Illinois State

University, Mazon Creek Study Center.

SYSTEMATIC PALEONTOLOGY

CLASS THYLACOCEPHALA Pinna, Arduini, Pesarini, and

Terruzzi, 1982

Diagnosis. — These are arthropods with a small to large bilobed

carapace enclosing the entire body. Compound eyes are very well

developed as either large, sessile bodies situated in optic notches or

as organs that virtually cover the surface of a protrudant, sac-like

cephalon. Body appendages are of two types: anteriorly 3 pairs of

subchelate, raptorial limbs, and posteriorly a set of from 8 to 16

pairs of apparently paddle-like limbs. Eight sets of well-developed

gills are associated with the subchelate limbs. The posterior trunk is

marked with pronounced pleurites, probably developed internally

as an endophragmal skeleton, associated with the posterior limbs.

Remarks. — This definition is emended from that implicit in

Rolfe ( 1985), who attempted to reconcile the divergent opinions of

Pinna et. al (1985) and Secretan (1985). The result of this study

suggests that the thylacocephalans can be separated into two orders:

the Concavicarida, with a large, well-developed optic notch bearing

a discrete compound eye, and the Conchyliocarida, lacking the

optic notch as a distinct structure and having the eyes located on the

surface of a protrudant, sac-like cephalon.

Despite the fact that some 24 genera have been assigned to the

Thylacocephaia (see Arduini and Pinna. 1989). the affinities of

Frederick R. Schram

several of these still remain uncertain. These include Dioxycaris,

Saccocaris. and Tuzoia from the Cambrian, Galenocaris from the

Devonian, Nothozoe from the Cambrian through Devonian,

Coreocaris from the Permian, and Riii;iiccirls from the Jurassic,

ORDER CONCAVICARIDA Briggs and Rolfe. 1983

Dia:^nosis (after Briggs and Rolfe. 1983), — Thylacocephalans

with a carapace bearing a prominent concave optic notch, typically

developed with a fused rostrum that may curve ventrally to occlude

the notch anteriorly. From 8 to 16 homonomous well-demarcated

trunk segments diminish in height anteriorly and posteriorly.

Remarks. — This order includes such forms as Hanycaris from

the Devonian, Concavicaris from the Devonian through the Carbo-

niferous, Dollocaris from the Jurassic, and an undescribed form

from the Silurian (Mikulic et al.. 1985). In addition,

Ankitokazmaris from the Triassic, Microcaris from the Jurassic,

and Protozoe from the Cretaceous might also belong in this order.

GENUS CONCAVICARIS Rolfe. 1961

Diagnosis. — The carapace has a fused hinge line, a rostrum

extended anteriorly, pronounced optic notches, and up to three

lateral longitudinal ridges.

Remarks. — This diagnosis is quite general and is based on

features that essentially characterize the order. Sixteen species of

Concavicaris, including the two described here, exhibit a wide

array of sizes, shapes, and decorations (Briggs and Rolfe, 1983).

Clearly the genus is a candidate for revision. However,

Concavicaris is not unique in this regard since very few of the 25

genera of Thylacocephala currently recognized ( Arduini and Pinna,

1989; Arduini, 1990) are clearly diagnosed with a concise set of

apomorphic characters,

Concavicaris gcoraeoriim n. sp. (Figs. 1-7)

Diagnosis. — The carapace is suboval in outline with a short

rostrum, an optic notch prominently occupying about half of the

anterior aspect, a ventral margin marked by a notch anterior of its

midpoint, a pointed postero-dorsal aspect, and a single, dorsally

situated, longitudinal ridge.

Stratum. — Francis Creek Shale, Desmoinesian. Middle Penn-

sylvanian.

Holotxpe.— SDSNH 36777: Peabody Coal Company Pit 11,

Will, Grundy, and Kankakee Counties. Illinois (Fig. I).

Etxmologv. — Named in honor of the George family, Calvin.

Harriet, and Steven, well-known collectors of Mazon Creek fossils

and donors of the holotype.

Description. — The carapace is roughly oval in outline, marked

with thickened free margins, and from 9.4 to 17.0 mm long

(Table 1). The anterior aspect of the carapace features a small

Table 1 . Representative measurements of Concavicaris

georgeorum (mm). Body length measured from tip of

rostrum to posterior of carapace; carapace length mea-

sured from optic notch to posterior of carapace.

Figure 1. Concavicaris georgeorum. Holotype. SDSNH 36777, with

compound eye (e), carapace (c), subchelate mouthparts (m), ventral cara-

pace notch (n), 4,4x.

rostrum (SDSNH 36768; PE 24591, Fig. 2A). a well-differentiated

optic notch that occupies its dorsal half (SDSNH 36759; PE 23146,

Fig. 2B), and closely spaced alternating grooves and ridges on the

margin of the ventral half (MCP 596, F^ 2C; PE 15346, Fig. 2D).

The carapace appears to have been relatively thin and not particu-

larly well mineralized, since it is commonly bent and distorted by

underlying structures, but it does bear a slight dorsal longitudinal

ridge extending from the optic notch to just below the pointed

dorso- posterior aspect (PE 10853. Fig. 2E).

The compound eyes are large, slightly more than twice as long

as they are wide (PE 24590, Fig. 3A). They are broader dorsally

than ventrally such that they do not interrupt in lateral silhouette the

oval outline of the body. In occasional specimens, the carapace has

separated from the body and slipped posteriad during diagenesis to

reveal the part of the head that bears the eyes (PE 23146, Fig. 2B;

PE 31049). In these instances, the eyes are clearly seen to be sessile.

Although the compound eyes are often preserved as a unit, fine

detail is rarely seen. Only a few specimens (e.g., SDSNH 36762,

Figs. 3B, C; PE 23144) preserve any facets on the eye surface.

These facets are extremely small, a few tenths of a millimeter in

diameter.

Mandibles are preserved on several specimens, and are typically

in close association with the mouth just dorsal and anterior to the

ventral, marginal notch of the carapace (SDSNH 36764, Fig. 3D;

PE 51937: PE .30575; PE 15346, Fig. 3E). Each mandible consists

of a well-developed molar process with four or five ridges that

interface nicely with ridges on the opposite member of the pair (PE

24592, Fig. 3F). This interfacing is not always easy to discern, but

one specimen (PE 24529, Fig. 3G) clearly preserves one mandible

in natural position near the mouth while the other mandible is

dislocated from this at a 90° angle.

The three pairs of raptorial subchelate limbs posterior to the

mandibles are large (Fig. 4A-C). The animals, however, are so

narrow that compaction of the specimens during diagenesis often

superimposes right and left members of the limb pairs onto each

other, complicating their study and interpretation. The most anterior

set of these three is the smallest and is rarely preserved completely

(PE 30556, Fig. 5A), while the most posterior pair is the largest.

These subchelate limbs are often completely folded under the

Mazon Creek Thylacocephala

Figure 2. Comavicaris georgeorum. A. PE 2457 1 , dorso-oblique preservation showing left optic notch (o) and rostrum (r), 4.3x . B, PE 23 147, close-up

with optic notch (ol, remnants of compound eye (e). exposed portion of cephalon (ce), 7x. C and D, MCP 596 and PE 15346 respectively, with notched

"tongue-in-groove" carapace margins (arrow) ventral to the eye (e), 6x. E, PE 10853. displaying longitudinal carapace ridge (arrow) extending postenorly

from the optic notch (o) and tightly flexed raptorial mouthparts (m) ventral to the body, 4x .

carapace (e.g., SDSNH 36759, PE 10853, Fig. 2E). I inteipret them

as a set of highly specialized mouthparts: from front to back, the

ma.xillules, maxillae, and maxillipedes, respectively (see Discus-

sion below).

The exact number of articles in these limbs is difficult to discern.

The most distal portions clearly have three podomeres, and these

appear to have been held fairly rigidly with respect to each other.

The preservation of these podomeres ranges from virtually straight

(SDSNH 36777, Fig. 1; PE 30556; PE 32957; PE 45696) to some-

what arcuate (PE 1 1020, Fig. 4A). The margins of these articles are

not marked by any teeth or spines. A few specimens preserve the

pronounced flexure in these limbs (PE 10916, Fig. 4D; PE 11020.

Fig. 4A; PE 14105, Fig. 4C; PE 29466), and at least four or five

podomeres lie proximal of those flexures (Fig. 4C). The exact

number is not known because these structures are generally poorly

preserved and because the articles on the ventral side of the body

are crowded under the margins of the carapace. However, some

specimens do preserve individual articles very clearly (most nota-

bly PE 45696, Fig. 4D). These have a punctate surface and at least

the most basal one appears to have been broader proximally than

distally, like an inverted paddle (PE 23144; PE 45696, Fig. 4D). In

ventral view (SDSNH 36774; PE 23145, Fig. 4E; PE 40107, Fig.

4F). these proximal segments floor the space lying ventral to the

trunk proper between the ventral margins of the carapace.

As in almost all thylacocephalan fossils, the trunk (or body or

abdomen; all terms have been used in the literature) is usually

enclosed completely within the great carapace chamber However,

through the vagaries of preservation, a few specimens available to

nie do reveal something of this region. The most dorsal part of the

trunk is discemable as a thick rod. Sometimes it can be seen in place

under the carapace along the dorsal margin (e.g., PE 31752). but

more often it becomes very noticeable when it arcs ventrad to

extend toward the postero-ventral aspect of the carapace (PE 15356:

PE 23144; PE 30556, Fig. 5A). In these cases, the rod often is

associated with an extruded trunk.

The trunk segments are seldom clearly preserved (e.g., SDSNH

36769), and usually are visible only when their ventral portions

extend beyond the margin of the carapace. The maximum number

Frederick R. Schram

Figure 3. Concavicaris genrgeomm. A, PE 24590. with outline of eye lobe in relation to carapace, ?x. B and C. SDSNH 36762, overall view of antero-

dorsal region of body (B, llx) showingpositionof fragment of eye surface (arrow) enlarged (C. 40x ) preserving facets. D, SDSNH 36764, mandible located

posterior to the carapace anterior margin (arrow). lOx. E. PE 15346. mandible. 6x. F, PE 24592. pair of mandibles (arrows) in position interfacing with each

other, 8.5x. G, PE 24529, with two halves of a mandible pair at right angles to each other (arrows), 7x.

of segments seen is 8 (MCP .5Q1, Fig. 5B). These segments are

associated distally with small qtiadrangular structures to which in

turn the diaphanous remains of the trunk limbs are attached (MCP

591; PE 29469, Fig. 5D). The limbs are never well preserved, and

the quadrangular structures are probably the limb bases or

protopods.

The gut seems a fairly simple structure. The anterior part of the

foregut is narrow (PE 3057.5. Fig. 6A) and is associated with the

mandibles (as noted above). This part of the foregut extends dor-

sally to a large region that seems to be a posterior foregut or

stomach, located posterior to the optic notch (SDSNH 36763. Fig.

6B; PE 30575, PE 45696). The mid- and hindguts extend posteriorly

(SDSNH 36778, Fig. 6C: MCP 595. Fig. 6D) to an anus located

near the postero-dorsal comer of the carapace.

As noted above, some specimens preserve the ventral aspect of

the body (SDSNH 36774; PE 23145. Fig. 4E; PE 24589; PE 40107.

Figure 4. Conciivicdils neort-i'iinini. A. PE 1 1020. displaying a slightly arcuate positioning of the three articles distal to the elbow (el) of the second (2)

and third (3) raptorial limbs, 6x. B. PE 10916. clearly preserving the elbows (el) of the raptorial mouthparts, 4.8x. C. PE 14105. tlexcd maxillipede with

partial preservation of proximal limbs segments (1-4), elbow (el), and distal segments (5-7). 6x. D, PE45696, close-up of carapace margin near the ventral

carapace notch (n) with punctate, proximal segments of the mouthparts (s), 8x. E and F, PE 23145 (7x) and PE 40107 (6x), respectively, ventral preserva-

tions showing right and left carapace valves (c), conjoined ventro-anterior margins (a), ventral opening to the ventral space beneath the body and between the

ventral carapace margins (v). and flexed segments of various mouthparts (m).

Mazon Creek Thylacocephala

Frederick R. Schram

Figure 5. Concavicaris georgeonim. A. PE 30556, note the dorsally positioned thick rod (rd) and the maxillules (I), maxillae (2), maxinipedes(3), 5x.

B. MCP 591, with eight trunk segments (1-8) extruded from under the carapace (c). 6x. C, MCP 590, under alcohol, note posterior carapace (c), trunk

segments (t), and limb bases (p), 6x. D, PE 29469, under alcohol, note the carapace (c), trunk segments (t), and remnants of the trunk limbs (1), 5.2x.

Fig. 4F). Though the distortion of these fossils that arises from

the compaction of the animals' deepest plane onto a thin surface at

right angles to that plane makes them difficult to interpret, some

tentative conclusions can be advanced. The right and left portions

of the ventral margin of the carapace anterior of the ventral notch

seem to lie in close proximity, if not being actually linked. This

corresponds to the area of the carapace with the marginal ridges and

grooves, and it is tempting to speculate that this region is held rigid

by these marginal decorations not unlike the "velcro pads" that hold

a jacket or pocket flap closed. The region posterior to the ventral

carapace notch appears to have been more widely spaced than the

anterior area, and this is the region typically occupied by the flexed

rami of the subchclate inouthparts.

Remarks. — Coiuavicaris i^ciiri;ei'niin is the most abundant of

the Mazon Creek fauna thylacocephalans, outnumbering

Cnncavicaris rcmipes by at least ,3.3: 1 and Convc.xicaris niazoiwnsis

by at least 12:1. These are undoubtedly conservative figures since

of all the Mazon Creek thylacocephalans C('/i<(;\7C(j)7,9,t;c()ri;('()/j(m

is more likely to be poorly preserved and therefore most likely to

have been discarded in the field as not "worth the effort to collect"

than either of the other two species. A reconstruction is offered in

Figure 7.

The tiny eye facets discerned on specimens of C. ^eorgcorum

are similar to those on other thylacocephalans, although none ex-

ceed in detail of preservation that found on Ckiusocaris (Polz, per-

sonal communication). Both Secretan (198.5) and Rolfe (1985)

commented on the small size of the individual oniniatidial units in

thylacocephalans. and they felt that the close packing of these facets

indicated a very acute sense of sight in this groupof arthropods. The

finely structured omniatidia of C f;eivi>e()nii>t (Fig. 3B) would ap-

pear to second those conclusions.

Secretan (1985) observed that the superimposition of the

Mazon Creek Thylacoeephala

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Frederick R. Schram

Figure 7. Concavicaiis georgeonim. Reconstruction, 5 mm scale.

subchelate limbs in Dollocaris made their study difficult. This is

certainly true for C. georgeonim. but the detection of the punctate,

plate-like, basal segments of these limbs suggests a possible e,\-

planation for some peculiar features of the Dollocaris fossils.

Secretan and Riou (1983) noted structures on their fossils that

suggested to them the remnants of the most anterior cephalic limbs

(see their plate 1, figs. 1 and 4); this interpretation (see their fig. 3)

was incorporated into their reconstruction (see their fig. 16). In a

subsequent paper. Secretan (1983) again noted these structures,

confessed problems with deciding exactly what they represent other

than anterior cephalic limbs, but deleted them from her updated

reconstruction of Dollocaris (see her fig. 3). A comparison of

Secretan's photographs with the punctate, basal plates seen on some

specimens of C. georgeonim suggests that the plate-like structures

of Dollocaris are not the remnants of antennae and other cephalic

structures but are the most basal segments of the raptorial,

subchelate mouthparts.

Rolfe(1985) homologized the "rods" noted by Briggs and Rolfe

(1983) with the concavicarid trunk, similar to that seen in the

Devonian Concavicaris material originally described by Bbhm

(1935), and maintained that the segmental structures ventral to it

represent fused limb protopods. However, Briggs ( 1985) interpreted

the dorsal rod and associated segments as a trunk. These ""rods"

clearly correspond to the thickened dorsal aspect of the C.

georgeonim trunk (Fig. 3A). The segments ventral to the rod are the

well-developed pleurites of the trunk; similar structures in living

arthropods are often associated with an internal skeleton of

apodemes to accommodate the origins of the trunk leg muscles.

Careful study of other thylacocephalans may show that specializa-

tions of this region are a diagnostic feature of the genus

Concavicaris and thus useful in some future revision of the

Concavicarida.

The gut fillings of C. georgeonim (Fig. 6) differ from those of

Ostemnans {V\nni el al ., 1985; Rolfe, 1985), which has an intlated

"stomach" region that extends into the cephalic sac. It is not clear

whether this is an inllated foregut, as Pinna el al. ( 1985) suggested,

or an anteriorly directed caecum, as Rolfe (1985) suggested. How-

ever, the less developed gut of C. georgeonim seems more primi-

tive.

Concavicaris remipes n. sp. (Figs. 8-10)

Diagnosis. — A small concavicarid with an oval carapace, a

large deep ventrally deflected rostrum, and a very small optic notch.

The body terminates (?) in a large paddle-shaped foot, or abreptor.

Siralum. — Francis Creek Shale, Desmoinesian. Middle Penn-

sylvanian.

Holoiype.— PE 21418; Peabody Coal Company Pit 11, Will,

Grundy, and Kankakee Counties, Illinois. (Fig. 8)

Elymology. — The name refers to the paddle-shaped foot or

abreptor.

Description. — This small concavicarid Is almost twice as long

as it is deep (Table 2). Of the seven specimens known, the carapace

length averages 9.4 mm and the depth averages 5.6 mm. The almost

perfectly oval carapace is marked externally by only a very small

optic notch (PE 30579, Fig. 9). The rostrum is very deep and

directed ventrally so that the anterior aspects of the optic notches

are partly occluded.

Two limbs types are noted. PE 46330 (Fig. 9C, D) preserves

part of a characteristic raptorial mouthpart. Little can be discemed

concerning this appendage other than that the segments preserved

distal to the principal flexure are well-developed. Neither the most

proximal or the most distal articles of this limb are preserved.

A prominent spinose paddle extends from under the carapace at

its postero-ventral aspect. The posterior margin of this organ carries

a row of spines or robust setae. The anterior margin is reinforced by

thickened cuticle, which temiinates distally in a pair of large spines

(PE 21418, Fig. 8: PE 39371, Fig. 9B).

Remarks. — A reconstruction of C. remipes is presented in Fig-

ure 10. One distinctive feature of this species is the paddle, but its

exact nature is unknown. It may be a limb. One specimen, PE 39371

(Fig. 9), seems to preserve two short segments that lie proximal to

the large spiny distal article. This suggests that the paddle is a

specialized distal segment of some longer structure.

Although this organ is preserved on several of the specimens at

hand, it occurs only singly and is not a part of some series as would

Figure 8. Concavicaris remipes. Holotype, PE 21418. note the postero-

ventral paddle (arrow), 4.6x.

Mazon Creek Thylacocephala

Table 2. Representative measurements of Concavi-

caris remlpes (mm).

Feature

A

Range

Body length 7 9.4 8.6-10,3

Carapace depth 7 5.6 5.0-6.5

Body length/carapace depth 7 1.69 1.46-1.91

be expected if it were an appendage. So the paddle may be a single,

median, terminal element akin to similar organs seen in ostracode.

conchostracan. and cladoceran Crustacea (e.g.. see Schram. 1986).

In the Conchostraca. the last or anal segment of the trunk is directed

ventrally as a paddle-like abreptor posteriorly bearing a row of

spines and tenninating in a set of large spines and spinose caudal

rami. In the Cladocera. the posterior part of the trunk loses any trace

of segmentation to become a postabdomen, curves ventrally. and is

decorated with posterior setae and lemiinal spines like the abreptor.

The cladoceran arrangement is similar to that of ostracodes. whose

ventrally directed terminus of the body bears an abreptor-like cau-

dal ramus. All of these structures in living crustaceans generally

assist in .some way with locomotion.

The C. remipes paddle, while not necessarily homologous to

terminal units of living crustaceans, may have had a similar func-

tion. Locomotory thrust can be generated in an aquatic organism by

either uniformly coordinating limbs that beat in a single power

stroke or reflexing a single rigid structure that acts as a rapidly

moving, spring-loaded lever. Which of these alternatives is used

depends on the fluid physics applicable around a particular animal.

Larger-bodied forms of Thylacocephala would have lived in a fluid

medium characterized by turbulent How and could have relied on a

single power stroke of all their trunk limbs to put them in a position

to capture prey. On the other hand, the hydrodynamics of quick

Figure 9. Cnncavicaris remipes. A. PE 30579. note the deep rostrum (r) and small optic notch (on). 7.7x. B. PE 39371. with well-preserved paddle and

an apparently, small segment proximal to it (arrow ), 7x , C and D. PE 46330. part and counterpan. note the segments of a raptorial limb (arrows ), and rostrum

(r).8.5x.

10

Frederick R. Schrani

Table 3. Representative measurements of Conve.xicaris

mazDiicnsis (mm). Specimens were oriented Into assumed living

position with eyes directed anteriad. The body length was then

measured along the antero-posterior axis of the body from points

tangent to the antero-dorsal to postero-ventral comers of the cara-

pace, while the carapace length was measured along an axis parallel

to the longitudinal carapace ridges from those same points. The

measurements of the V'!)Conve.\icai-is sp. specimen (see text) are

included for comparison.

I 1

Figure 10. Cinuuviciiris remi/vi. Reconstruction, 5 mm scale.

attack in small forms, such as C. icmipes. a species which because

of its size may have lived in a viscous medium characterized by

laminar (low, could have required the use of a lever to generate the

speed necessary to capture prey. Consequently, the nature of the

body terminus may be another important feature to consider in any

future revisions of the Concavicarida.

ORDER CONCHYLIOCARIDA

Secretan, 1983 (emended herein)

Diagnosis. — These are thylacocephalans with a carapace lack-

ing a clearly delineated optic notch, and often lacking a rostrum.

The eyes are situated on the surface of a large protruding cephalic

sac.

Remarks. — Rolfe ( 1985:391 ) anticipated that thylacocephalans

might one day prove to be more diverse than could be accommo-

dated in a single order. Indeed, it appears to me that some of the

controversy over the interpretation of the Jurassic thylacocephalans

from Italy (see Rolfe. 1985, for a summary) may be related to this

possibility. Certainly, my examination of some of the material Prof.

Pinna and his colleagues have been studying convinces me that

Ostenocaris and related forms are quite different from Coiicavicaiis

and related taxa. The most obvious difference is the presence in the

Italian material of a large cephalic sac almost completely covered

by ommatidia versus a more typical head in the concavicarids sensK

sthclo bearing discrete compound eyes. The presence or apparent

absence of the optic notch may then merely be a reflection on the

degree of development of the head as a cephalic sac.

Genera that belong in this order include Conve.xicaris from the

Carboniferous, Yant^zicaris from the Triassic, and Alropicaris.

Austriocaris. Claiisocaris. Kilianocaris. Ostenocaris. and Para-

osfenia from the Jurassic. Other genera that might belong in this

order include Iso.xrs. Prohosicaris. and Silesicaris from the Cam-

brian and Psenderichlluis from the Cretaceous.

GENUS CONVEXICARIS n. gen.

Diai;nosis. — Conchyliocaridan with a sinuously shaped cara-

pace displaying a rounded antero-dorsal aspect, a convex antero-

ventral margin, and a blunt postero-ventral aspect. The subchelate

raptorial limbs are very elongate, composed of narrow articles. The

body terminates in long caudal rami.

Type of genus. — Conve.xicaris mazonensis n. sp.

Conve.xicaris mazonensis n. sp. (Figs. 11-13)

Diagnosis. — Since there is only one species, the diagnosis is

the same as that of the genus.

Stratum. — Francis Creek Shale, Desmoinesian. Middle Penn-

sylvanian.

Holotype.— PE 32958; Peabody Coal Company Pit II, Will,

Grundy, and Kankakee Counties, Illinois. (Figs. 1 lA, B)

Description. — Convexicaris mazonensis is relatively large

(Table 3). The carapace is sinuous in outline, with the anterior

aspect more dorsal and the posterior more ventral (SDSNH 36781;

PE 11255; PE 23525; PE 32958. Figs. 1 1 A.B). A pair of ridges runs

parallel to the sinuous axis of the carapace ( MCP 594. Fig. 11 C; PE

23525); the dorso-posterior ridge is more prominent than the fainter

median one. These ridges extend from just posterior of the eye

toward the postero-ventral region of the carapace. The carapace is

rounded at its antero-dorsal aspect, not quite as developed as a full-

fledged rostrum (SDSNH 36781, Fig. UD) The antero-ventral

aspect of the carapace is broadly convex (PE 38169; SDSNH

36781; MCP 594, Fig. IIC). while the postero-ventral comer is

blunted (MCP 594).

The compound eyes are large, in fact enomious relative to the

size of the body ( SDSNH 3678 f. Fig. 1 1 D; PE 45695 ). The eyes do

not occupy any well-defined optic notch but rather appear to be part

of the cephalon that protmdes anteriorly from the carapace in that

area.

The thin raptorial limbs are relatively enomious (PE 45692. Fig.

12A,B). Those of the first set, or maxillules, are the shortest; the

second, or maxillae, are longer; and the third set. the maxillipedes,

are the longest of all. The unamied distal three articles of each of

these limbs are fairly rigid relative to each other with a slight arc

noted on some specimens (PE 45695. Fig. 12C). The proximal and

intemiediate segments of these limbs generally are not well pre-

served, though PE 45695 has at least four articles proximal of the

elbow, and PE 39350 (Fig. 12D) has possibly seven or eight seg-

ments that extend virtually from what appears to be the limb base to

the elbow.

The trunk is only faintly preserved on a few specimens at hand,

but there appear to be about eight segments in this area (PE 1 1255;

PE 32958, Figs. 1 1 A. B ). A series of limbs is often seen attached to

these segments, but the preservation is always as a color difference

in the concretion and no structural details are discemable (PE

Mazon Creek Thylacocephala

11

Figure 1 1 . Convexicahs mazonensis. A and B, PE 32958, holotype. part and counterpart. 2x . C, MCP 594. displaying the concave form of the carapace

antero-ventral margin (mr). the double longitudinal ridges (Ir). and the blunt posterior end of the carapace (arrow). 3.6x . D. SDSNH 3678 1 . close-up view

of the head region, note the rounded anterior end of carapace (arrow), carapace margin (c). the cephalic sac somewhat disrupted during preservation (cs). and

the compound eye (e), 5.7x.

32958, Fig. 11 B ). Posterior to the trunk, the body terminus appears

to bear a set of long caudal rami (PE 32958, Fig. 1 IB), but these

rami too are preserved, unfortunately, only as color differences in

the rock.

Some soft anatomy is evident on the specimens at hand. An

anterior portion of the foregut just inside the mouth is evident as a

narrow tube (PE 40076, Fig. 12E). Remnants of gills appear to be

preserved on one specimen as blackened lobe-like structures under

the carapace just dorsal to the bases of the raptorial limbs (PE

38169. Fig. 13F), but their preservation is not very good.

Remarks. — A reconstruction of Coinexicaris mazonensis is

presented in Figure 13. Though Cnnvexicaris mazonensis was

larger than Concavicaris geoigeoium. specimens of the former are

not preserved nearly as well as those of the latter. More often than

not the structures of Convexicaris occur as color differences in the

rock, molds or casts in the concretion, or badly preserved pieces of

cuticle, patterns of preservation like those seen among fossil shells

in the Mazon Creek faunas. It is possible, therefore, that the differ-

ences in preservation noted between these two thylacocephalans

may reflect qualitative differences in the nature of their cuticles in

life. Concavicaris might have possessed a rather organic exoskel-

eton, perhaps with some sclerotization, but little mineralization.

Convexicaris, on the other hand, may have had more mineral than

organics in its exoskeleton. The acidic conditions that apparently

prevailed during diagenesis of Pennsylvanian concretion faunas

generally mitigated against good preservation of animals with much

mineral content in their cuticle, whereas animals with a high organic

content in their e.xoskeletons were relatively unaffected.

One specimen of a carapace (MCP 589, Fig. 14) is distinctly

larger than any specimen of Convexicaris mazonensis, though its

general outline resembles that species'. This specimen, however,

does not preserve the characteristic features of C. mazonensis such

as the carapace ridges, any portion of the eyes or cephalic sac, or

any of the raptorial limbs. Whether this specimen represents an

aberrant preservation of C. mazonensis or is actually some other

species of thylacocephalan or other arthropod can not be determined

at this time.

12

Frederick R. Schram

Figure 1 2. Convexicaris mazonensis, A and B. PE 45692. part and counterpart, preserving the eyes (e). the raptorial mouthparts, maxillules ( 1 ). maxillae

(2). maxillipedes (3). and a caudal ramus (cr), 1.5x. C. PE4.')695, close-up under alcohol of a maxilla (2) and maxillipcde (3). the latter preserving the three

segments distal to the elbow (arrow). 4x. D. PE39350. close-up of a raptorial inouthpart (probably the maxillipcde) that preserves limb segments (numbers)

proximal to the elbow (arrow). 4x. E. PE 4CK)76. with the anterior portion of the foregut. (f). 3.2x. F, PE 38169. under alcohol, with carbonized remains of

gills (arrows) under the carapace above the raptorial mouthparts, 4x.

Ma/.on Creek Thylacocephala

13

Figure 13. Convexicaris mazonensis. Reconstruction, 5 mm scale.

DISCUSSION

Mode of Life. — There now seems to be a consensus among

students of the Thylacocephala that these animals were carnivores.

Pinna el at. (1984). influenced by their postulated affinities of

Ostenocaris with cirripedes, originally felt Ostenocaris was a filter

feeder. However, the raptorial limbs, often well armed with spines,

as in Dollocaris (Secretan, 1985 ) and Ostenocaris (e.g.. Pinna et al..

1982), bespeak a carnivore with limbs like those of stomatopod

malacostracan and remipedean crustaceans. Secretan (1985) sug-

gested that these animals were scavengers, but certainly the stomach

residues found in Ostenocaris (Pinna et al.. 1985) imply active

camivory.

There is little agreement, however, over just how thylacoceph-

Figure 14. ("!)Com-e.\icaris sp.. MCP 589, thylacocephalan(?) carapace.

1.3x.

Figure 15. Concavicaris sinuata. Scanning electron micrograph of ter-

race structures on the surface of the carapace, developed from negative 1561

in ihe photo collection of the Field Museum Mazon Creek Project. -75x.

alans lived. Briggs and Rolfe ( 1983) examined the micro-ornamen-

tal terracing on the Concavicaris carapace and, influenced by

Seilacher (1973), concluded that the Gogo thylacocephalans could

have been burrowers, but they refused to rule out other options. The

Gogo concavicarids are small to moderate in size, but terracing is

clearly visible even on large Concavicaris sinuata from the Penn-

sylvanian black shalesof North America (Fig. 15). If we postulate a

carnivorous habit for thylacocephalans, then a burrowing or lurking

life style is not unreasonable. As Secretan (1985) observed, the

stomatopods are active, predaceous carnivores that routinely build

burrows, bury themselves in sediment, or lurk in cracks and crevices

while waiting for prey to pass by, whereupon they thrust forward

from hiding and strike at their victim.

Rolfe (1985), however, felt that there are physiological prob-

lems with this interpretation. If thylacocephalans were denizens of

low-oxygen benthic habitats, given that they almost always occur

as fossils in rocks deposited under such conditions, he questioned

how their physiology could have allowed them to be such active

carnivores, unless there were tluctuating oxygen layers near the

bottom that were responsible for mass kills. Moreover, Rolfe felt

that benthic creatures would do away with the lower aspect of their

huge compound eyes and limit their field of view to be directed

upward only. However, the analogy to stomatopods should not be

overlooked in this regard. The benthic mantis shrimps have among

the most acute senses of sight of any crustacean. Stomatopods bear

large, often multilobed eyes with overlapping fields of view (e.g.,

see Schrani, 1986:52), and have recently been noted to possess

acute color vision (Marshall, 1988). Eye structure in thylacoceph-

alans is more likely governed by the requirements of their feeding

habits than by the location of their resting places.

A pelagic habit for thylacocephalans remains an alternative, but

serious problems might intervene here. Though streamlined, many

thylacocephalans were apparently rather massive. Some, like

Concavicaris sinuata and Ostenocaris cypriformis. are "huge" by

14

Frederick R. Sthram

Figure 16. Concavicaris sinuala. A. PE 51939. from the black shale just above the coal in Peabody Coal Company Pit 1 5, Will Co., Illinois, displaying

the outline of individual gills (arrows) preserved beneath the surface of the carapace, 1.8x. B, PES 1940, from the Mecca Quarry Shale. Quarry Q, Park Co..

Indiana, with gill area indicated as a bulge in the carapace (arrow) apparently formed by a gas bubble as the gills decayed. 2x .

any standards, and it is difficult to conceive of such large arthropods

as being neutrally buoyant. Even if they were so, they would have

required a well-developed series of limbs to maintain their position

in the water column and, more importantly, to engage in sustained

active pursuit of prey. E.xcept for nectiopodan remipedes (Schram,

1986:40), no large predatory aquatic arthropods seem capable of

that kind of behavior, and the relatively small and presumably

delicate thylacocephalan trunk limbs do not seem to have been

capable of sustaining an active, massive, pelagic predator.

However, Rolfe (1983) found an analogy of the thylacocepha-

lans in hyperiid amphipods, based mainly on the size of their eyes

and bulkiness of form. Since hyperiids are mesopelagic, Rolfe

suggests thylacocephalans may have been so as well. This sugges-

tion should not be lightly discarded, but problems of scale may

intervene. Hyperiids are often small animals that generally subsist

in the water column by hitching rides on or in other creatures, such

as tunicates. Although some of the smaller thylacocephalans may

have been able to do this, it is difficult to envision just how the

many larger forms of this group could have done so and thus by

necessity would have to have been free swimming.

I generally favor a benthic habit for these animals. Arthropod

predators more often than not exhibit lurking behaviors, waiting for

their prey to come to them rather than actively seeking victims.

Oxygen requirements may not have been critical for thylaco-

cephalans. One group of active crustacean carnivores, the

nectiopodan remipedes. live under almost oxygen-free conditions

(see Schram. 1986). However, the diversity of size and form that is

slowly becoming evident for this group may indicate that

thylacocephalans occupied a variety of habitats and pursued a

variety of modes of seeking prey.

Thylacoccpluilcin affinities. — Despite growing interest in this

group, and given what we already know about their anatomy, we

still have no certain idea as to what thylacocephalans are other than

that they me Arthropoda.

Since the description of Oslenocaris by Arduini el al. ( 1980).

there has been a tendency to di.scuss thylacocephalans in terms of

their being some kind of crustacean. Nevertheless, the issue of the

higher taxonomic affinities of the Thylacocephala must remain

open!

Thylacocephalans do have a carapace, but that is the only

apomorphic feature currently known that could possibly ally them

with crustaceans. However, given the multiplicity of ways that a

carapace has independently evolved within Crustacea (see Schram,

1986), and the variety of carapaces found on all manner of non-

crustacean arthropods (e.g., see Briggs, 1983; Briggs and

Whittington. 1985). the carapace in thylacocephalans is not a suf-

ficient character in and of itself to place them categorically among

crustaceans.

On the other hand, a clearly defined head with two sets of

antennae, a set of mandibles, and two sets of maxillae constitute an

excellent suite of apomorphies that could place thylacocephalans

within the Crustacea. However, this is not the case at this time. We

still have no certain knowledge about thylacocephalan antennae;

we do not know if they even had any. As mentioned above, Secretan

and Riou (1983) observed structures on specimens o'i DoUocaris

that they interpreted as possibly being the antennae and nearby

mouthparts. However, Secretan (1985) was more cautious in this

regard, and an alternative interpretation of these features as the

most basal segments of the raptorial appendages presents itself

here.

We have evidence of thylacocephalan mandibles in

Concavicaris geoigeoiiim as well as DoUocaris iiif;ens (Secretan,

1985). Their location near the mouth and foregut is to be expected,

but their association with three pairs of distinct grappling append-

ages, the inferred raptorial mouthparts. is interesting. This pattern

recalls that of many crustaceans, suggesting that these limbs are

maxillules. maxillae, and a set of maxillipedes. Indeed, the combi-

nation of mandibles with large molar processes and three sets of

subchelate multiarticulated "mouthparts"" is identical to that seen in

reniipede crustaceans (e.g., see Schram el a!.. 1986). The interpre-

tation of thylacocephalan raptorial limbs as mouthparts lends some

credence to possible crustacean affinities for flea shrimp.

Nevertheless, the definitive identity of the postoral raptorial

limbs remains a problem. Arduini el al. (1980) originally inter-

preted these limbs as antennules, antennae, and mandibles (pre-

sumably as a large palp). Subsequently, Pinna et. al (1982) desig-

nated these limbs as the antennules, antennae, and maxillipedes.

Quite in contrast, Secretan ( 1985) regarded these subchelipedes as

Mazon Creek Thylacocephala

15

thoracic limbs, perhaps in keeping with her analogies with stoniuto-

pods, which have a series of subchelate limbs on the anterior

segments of the thorax. The association of the massive gills with

these limbs might support her argument. Certainly these gills are

hccoiiiing one of the diagnostic features of the thylacocephalans

and when preserved always occur in this position (Fig. 16). How-

c\er. the gills could equally be considered "cephalic" gills, i.e..

aulapomorphic features of thylacocephalans, rather than "thoracic"

gills, and thus synapoinorphic features with other crustaceans. Fi-

nally, Rolfe (198.'i) suggested that the raptorial limbs of

th\ lacocephalans are maxiliules. maxillae, and maxillipedes. Rolfe

conceded, however, that if these limbs really are posterior to a

possible carapace adductor muscle as Secrelan (1985) observed,

then they could be thoracopods. All these limb designations are

predicated on the premise that the thylacocephalans are crustaceans.

Concerning the carapace adductor muscle. 1 recently was dis-

cussing thylacocephalans with Dr. Richard Brusca, an authority on

In ing isopods. In the course of explaining the complex anatomy of

these fossils, he chanced to see the cover illustration of Oslcnocari.s

used by Arduini and Pinna ( 1989). "Oh, they had parasitic bopyrid

isopods under their carapaces," he remarked without prompting. 1

pninted out that his "isopod" was usually interpreted as a muscle,

hut when we examined some illustrations in the literature of the

supposed carapace adductor muscles he remained unconvinced.

W hen he saw that the disputed muscles were close to the region

under the carapace containing the gills, he suggested that the "ad-

ductor muscles" be reexamined to determine if they were, if not

hiipyrids. some kind of ectoparasite. No "carapace adductor

muscles" were seen on any of the Mazon Creek flea shrimps.

The interpretation of Pinna el al. (1985) is worth returning to

brictly. Recently, Prof. Pinna and his colleagues allowed me to

examine some newly collected material belonging to an as yet

undescribed thylacocephalan. The cephalic sac clearly bore tiny

hexagonal facets, but on this fossil "microsclerites," much smaller

than those seen in Ostenocavis (Pinna et. al, 1985), occurred al the

uitersection of the hexagons and were not centered below them.

J. This arrangement is similar to that seen in larval ascothoracidan

Maxillopoda (Grygier, 1988: fig. 2), where the carapace, a structure

which, as in thylacocephalans, envelops almost all of the parasite's

body and is marked by hexagonal sculpturing with fine setae

merging from the intersections of the hexagons. In addition,

ascothoracidans are often characterized by chelate antennules. So,

despite the comments attributed to Newman that reject OsleiuHuris

from consideration as a maxillopodan crustacean (Briggs and Rolfe,

1983:272), it is possible that the first and second subchelate limbs

of flea shrimp are modified antennae, albeit uniquely so. If this limb

identification is proven, thylacocephalans might be allied in some

way to ascothoracidans, and the original concept of the Italian

workers of thylacocephalan/cirripede affinities might be partially

vindicated.

Finally, the nature of the posterior part of the body of

thylacocephalans remains a problem in interpreting the affinities of

this group. Though Briggs and Rolfe ( 1983) did note some varia-

tions in size of segments along the trunk, this region for the most

part seems to be homonomous. The segment numbers, however,

vary from taxon to taxon and create some problems. Eight such

segments are noted in the Mazon Creek thylacocephalans, as well

as in the Italian material and some of the Gogo specimens (though

as many as 14 were recorded in the Australian material). Secretan

(1985) reconstructed Doltocaris and indicated up to 16 trunk seg-

ments. A possible explanation for the higher numbers was advanced

by Briggs and Rolfe (1985), who observed that slippage could

occur between right and left halves of the body. I have seen this not

only in the Mazon Creek specimens but also in Coiicavicaiis

sinuata from the Pennsylvanian black shales. It is possible that the

extra segments at the end of the trunk could be artifacts of right

side/left side slippage. That is, Doltocaris might actually have only

eight trunk segments instead of 16. However, this explanation

should not be forced. The Gogo specimens with more than eight

trunk segments clearly do not seem to be explained by slippage, and

Mikulic ('/ al. (1985) reported a thylacocephalan from the Silurian

of Wisconsin with more than 20 trunk segments. It is possible that

total trunk number may have varied in thylacocephalans.

So, what are the affinities of the Thylacocephala'.' One common

theme seems to emerge from the literature. All attempts to under-

stand this group return to comparisons with crustaceans. Certainly

nothing among the uniramians and little among the cheliceriforms

has anything to offer toward understanding thylacocephalans. It is

conceivable that their affinities lie outside any of the "mainline"

arthropod groups, but nothing among Cambrian "trilobitoids"

seems relevant.

Secretan (1985) mentioned in passing that the conchostracans

might be a sister group to thylacocephalans. Though the paddle of

Cdiuavicaris remipes recalls structures seen among some

phyllopodans, the similarity is possibly only analogous, not neces-

sarily homologous. The large eyes and subchelate limbs of

thylacocephalans are not features seen among branchiopods or

other phyllopodans.

The maxillopodan connection remains an option, if only be-

cause of the persistence of the Italian interpretation of thylaco-

cephalans as cirripedes (Pinna el al.. 1985). However. 1 agree with

Rolfe ( 1985) that the affinities of thylacocephalans do not lie with

barnacles, and only by means of some liberal assumptions about

possible ascothoracid affinities might we be able to draw this group

into the thecostracan Maxillopoda.

Among the Malacostraca, many features of the life style of

thylacocephalans are closely paralleled in stomatopods: the large

eyes, raptorial limbs, and possible lurking behaviors to stalk prey.

However, there is no similarity to anything seen among malacost-

racans that compares in this regard to the cephalon or the posterior

body regions of thylacocephalans.

Some thylacocephalan features are similar to those seen among

remipedes: large sessile compound eyes (at least in the fossil

enantiopodans), mandibles with large molar processes, three sets of

subchelate postoral grappling or raptorial limbs with many

podomeres, and a homonomously structured trunk. The last feature

is considered the primitive condition for arthropods and thus not

particularly useful in terms of deciding thylacocephalan affinities.

In addition, remipedes, as currently understood, lack gills (let alone

on the cephalon) and a carapace, and generally have long trunks.

However, short bodies repeatedly evolved within many lines of

crustaceans, carapaces independently evolved many times among

arthropods (especially crustaceans), and gills of many different

forms and locations evolved many times among articulate phyla. It

is not inconceivable that thylacocephalans could be highly derived

remipedes.

Thus, the question of thylacocephalan relationships is unre-

solved. 1 can do no better than echo the conclusion of Briggs and

Rolfe (1983:273): "...the affinities. ..remain obscure, but probably

lie within the Crustacea."

ACKNOWLEDGMENTS

This research has been carried out over a period of 10 years with

the assistance of many people and financial support from several

sources. The late E. S. Richardson Jr. was for many years an

inspiration for those of us who worked on the Mazon Creek biotas,

and acted as a conduit of information on and specimens of "flea

shrimp." Drs. W. D. I. Rolfe, S. Secretan, and G. Pinna and his

colleagues have been the source of information and ideas, and

16

Frederick R. Schram

without their pioneering research on thylacocephalans my efforts

would have been impossible. Early work on these fossils was

funded in pan by NSF grant DEB 82-12335 and a travel grant from

the Royal Society of Edinburgh in 1984. The work was completed

with a'visiting Scholars grant from the Field Museum of Natural

History. Drs. D. E. G. Briggs and W. D. 1. Rolfe reviewed the

manuscripfand offered much appreciated criticism.

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