Ontogeny, structure and moulting of Parabolina frequens argentina (Kayser) (Trilobita, Olenidae) from the Furongian of northwestern Argentina
M. Franco Tortello1 and Euan N.K. Clarkson2
1Departamento Paleozoología Invertebrados, Museo de Ciencias
Naturales, Paseo del Bosque s/n°, 1900 La Plata, Argentina.
tortello@museo.fcnym.unlp.edu.ar
2Grant Institute, School of Geosciences, University of Edinburgh,
West Mains Road, Edinburgh EH9 3JW, United Kingdom.
Euan.Clarkson@glg.ed.ac.uk
Abstract. Parabolina frequens argentina (Kayser) (Trilobita, Olenidae) is a guide fossil occurring in different assemblages and lithologies of the upper Cambrian (Furongian) of northwestern Argentina. The black and dark grey shales of the Lampazar Formation at sierra de Cajas and the Santa Rosita Formation (lower part) at Iruya and Alfarcito (Jujuy and Salta provinces) contain numerous well-preserved specimens at different stages of development. The juveniles represent anaprotaspis, metaprotaspis and degrees 0-11 meraspis. The latter exhibit a high variability related with the pattern of glabellar furrows and lobes, the sagittal length of the thoracic segments, the splay and length of the genal and thoracic spines, the relative width of the axis, and the relative development of the axial tubercles. As in other species of Parabolina, the meraspid stages of P. frequens argentina do not form well-defined instar groupings. Besides, in the adult specimens the length of the anterior cephalic border and the preglabellar field are also variable characters. The hypostome is characterized by having almost completely reduced anterior wings; it lies in close association with the ventral doublure (conterminant condition). Although some authors suggested that P. frequens argentina and P. frequens frequens (Barrande) could be synonyms, these taxa differ in having distinctive pygidia and hypostomes. Many exoskeletal configurations can be interpreted as exuviae. From a functional point of view, it is likely that the genal and macropleural spines on the 8th thoracic segment supported the body of the trilobite when resting on the sea floor. The exoskeleton can be reconstructed in two alternative postures, the "alert" and "relaxed" attitudes.
Resumen. Ontogenia, Estructura Y Muda De Parabolina Frequens Argentina (Kayser) (Trilobita, Olenidae) Del Furongiano Del Noroeste Argentino. Parabolina frequens argentina (Kayser) (Trilobita, Olenidae) se encuentra ampliamente representado en el Cámbrico superior (Furongiano) del noroeste argentino, en distintas asociaciones y litologías, constituyendo un elemento valioso para datar afloramientos y establecer correlaciones. Las lutitas oscuras de la Formación Lampazar en la sierra de Cajas y de la Formación Santa Rosita (parte inferior) en Iruya y Alfarcito (provincias de Jujuy y Salta, Argentina) contienen numerosos ejemplares bien preservados, en diferentes fases de desarrollo. Los especímenes juveniles representan estadios anaprotaspis, metaprotaspis y meráspidos 0-11. Estos últimos exhiben una alta variabilidad relacionada con el grado de expresión de los surcos laterales de la glabela y lóbulos glabelares, la longitud exsagital de los segmentos torácicos, la curvatura y longitud de las espinas genales y torácicas, la anchura del axis y el desarrollo relativo de los nodos axiales. Como se ha documentado en otras especies de Parabolina, los estadios meráspidos de P. frequens argentina no conforman grupos de tamaño (clusters) bien definidos. Por su parte, los ejemplares adultos presentan un borde cefálico anterior y un campo preglabelar de longitudes algo variables. El hipostoma se dispone en contacto con el doblez cefálico anterior (condición coincidente) y posee alas anteriores muy reducidas. Aunque algunos autores consideran a P. frequens argentina y P. frequens frequens (Barrande) como sinónimos, estos taxones se distinguen por presentar pigidios e hipostomas distintivos. Numerosos especímenes con configuraciones particulares representan exuvias generadas durante el proceso de muda. Con relación al hábito de vida, las espinas genales y las macroespinas del octavo segmento torácico habrían constituido los principales puntos de apoyo del organismo sobre el sustrato. El exoesqueleto puede ser reconstruido en dos posturas alternativas, activa y de reposo.
Key words. Parabolina; Trilobita; Olenidae; Furongian; Argentina; Ontogeny; Morphology; Ecdysis.
Palabras clave. Parabolina; Trilobita; Olenidae; Furongiano; Argentina; Ontogenia; Morfología; Ecdisis.
Introduction
The first conspicuous lower Paleozoic fossils in
South America were collected in Tincuya (southern
Bolivia) and described by Kayser in 1876 ( "Agnostus"
tilcuyensis Kayser (= Trilobagnostus tilcuyensis), "Arionellus"
lorentzi Kayser and "A". hyeronimi Kayser
(=Angelina hyeronimi), and "Olenus" argentinus
Kayser (=Parabolina frequens argentina)). On the basis
of their affinities with species from Scandinavia and
England, Kayser (1876) originally assigned these
trilobites to the Late Cambrian, indicating that P. frequens
argentina was the commonest taxon in the assemblage.
During the 1910's, 1930's and 1940's, P. frequens argentina was successively documented in other localities of southern Bolivia and northwestern Argentina
(Salitre, Tilcara, río Volcancito; Steinmann and Hoek,
1912; Kobayashi, 1937; Harrington, 1938; Harrington
and Leanza, 1943). Finally, Harrington and Leanza
(1957) provided a complete review on the morphology
and synonymy of this taxon, showing that it is
widely distributed in the Cordillera Oriental (Jujuy
and Salta Provinces), as well as in the Famatina
System (La Rioja Province). Since P. frequens argentina
is a common fossil occurring in different assemblages
and lithologies, it has proved to have great
biostratigraphic value for establishing correlations
within the latest Cambrian of the "Baltic Province"
(Shergold, 1988) (e.g., Harrington and Leanza, 1957;
Frederickson, 1958; Aceñolaza, 1992).
Sierra de Cajas (Aceñolaza, 1968), Iruya (Vilela,
1960) and Alfarcito (Harrington and Leanza, 1957)
are representative Furongian localities of Salta and
Jujuy, which contain numerous well-preserved specimens
of P. frequens argentina at different stages of development.
The aim of this paper is to describe the
ontogeny of this subspecies, including some aspects
of its holaspid morphology which, as originally indicated
by Harrington and Leanza (1957), is highly
variable. In addition, the record of different exoskeletal
configurations provides interesting paleobiological
data, related to possible techniques of ecdysis
and mode of life of the taxon.
Stratigraphy and localities
Harrington and Leanza (1957) erected the
Parabolina frequens argentina Biozone on the basis of
numerous trilobites dominated by olenids and agnostoids.
The unit is represented in many successions
in the Cordillera Oriental, comprising the Lampazar,
Casa Colorada and Santa Rosita (lower part)
formations and equivalents. Though Harrington and
Leanza (1957) originally referred the whole biozone
to the early Tremadoc, subsequent studies demonstrated
that its lower part is assignable to the Furongian
(e.g., Rushton, 1982, p. 46; Ludvigsen, 1982, p.
150; Aceñolaza, 1983; Salfity et al., 1984; Tortello,
2003). Parabolina frequens argentina is particularly
abundant in the lower part of the unit, below the first
appearance of the lowest Ordovician Jujuyaspis keideli
Kobayashi (Trilobita, Olenidae) and Rhabdinopora
Eichwald (Graptolithina) (see Benedetto, 1977; Aceñolaza,
1983; Aceñolaza and Aceñolaza, 1992; Ortega
and Rao, 1995; Tortello and Esteban, 1998, 1999;
Tortello et al., 1999; Tortello, 2003).
The P. frequens argentina Biozone is mainly composed
of shales, sandstones and rare interspersed
marls and limestone layers, representing a wide
range of sedimentary environments, including wavedominated
and open marine conditions. Those sections
representing deep outer shelf settings, with
conditions of low energy, deposition of fine sediments
and abundant organic matter, generally contain
well-preserved fossils. In these settings P. frequens
argentina commonly occurs associated with agnostoids
(Lotagnostus Whitehouse, Gymnagnostus Robison
and Pantoja-Alor, Pseudorhaptagnostus (Machairagnostus)
Harrington and Leanza, Micragnostus Howell)
and olenids (e.g., Parabolinella Kobayashi, Plicatolina
Shaw, Beltella Lake, Angelina Salter), which
comprise benthic species especially adapted to live in
environments low in oxygen (Parabolinella, Plicatolina).
Tortello (2003) described the Pseudorhaptagnostus
(Machairagnostus)-Gymnagnostus Subzone (latest Furongian)
in such a facies (="Olenid facies" of Fortey,
1975), and designated a composite stratotype that encloses
the Quebrada Azul (Lampazar Formation, sierra
de Cajas, Jujuy) (Aceñolaza, 1968; Tortello and Esteban,
2003).
Most specimens studied herein come from the
Lampazar Formation outcropping at the Quebrada
Azul, 5 km East of El Molino-Mina Aguilar (Humahuaca
Department, Jujuy Province) (figure 1). This
180 m thick section represents an outer shelf setting
with some transitions to shallower environments.
Oxygen and energy levels were not uniform during
deposition: fine grained sediments vary from laminated
black and dark grey shales in the lower part of
the formation to laminated greenish shales with intercalations
of siltstones and sandstones in the middle
and upper part (Tortello and Esteban, 2003). It
lies on the quartzites of the Padrioc Formation (Late
Cambrian) and is overlain by the sandstones, quartzites
and interbedded shales and limestones of the
Cardonal Formation (Aceñolaza, 1968). The columnar
section showing the stratigraphic range of the
trilobite faunas ( Gymnagnostus perinflatus (Harrington
and Leanza), G. bolivianus (Hoek), Pseudorhaptagnostus
(Machairagnostus) tmetus Harrington and
Leanza, P. (M.) corrugatus (Suárez-Soruco), Micragnostus
vilonii Harrington and Leanza, Parabolina frequens
argentina (Kayser), Parabolinella coelatifrons Harrington
and Leanza, Angelina hyeronimi (Kayser),
Beltella ulrichi (Kayser), Plicatolina scalpta Harrington
and Leanza, Akoldinioidia sp. and Asaphellus cf. aspinus
Robison and Pantoja-Alor]
is shown in figure 2.
Parabolina frequens argentina largely dominates the assemblages,
representing more than 80 per cent of the
total fauna.
Figure 1. Location map / mapa de ubicación.
Figure 2. Columnar section of the Lampazar Formation
(Furongian) at Quebrada Azul (Cajas Range), showing fossiliferous
levels (A-G) and distributions of Parabolina frequens argentina and associated trilobites (after Tortello and Esteban, 2003) / columna estratigráfica de la Formación Lampazar (Furongiano) en la quebrada Azul (Sierra de Cajas), mostrando los niveles fosilíferos (A-G) y las distribuciones de Parabolina frequens argentina y trilobites asociados (basado en Tortello y Esteban, 2003).
Additional material was collected from the lower San Isidro River, 3 km north-east of Iruya Town, Salta Province (figure 1). This outcrop is mainly composed of dark shales assigned to the Santa Rosita Formation (lower part), interposed between the quartzites of the Mesón Group (Cambrian) and the sandstones of the Salta Group (Cretaceous-Paleogene) (Vilela, 1960; Turner, 1964; Turner and Mon, 1979). In this locality Parabolina frequens argentina is documented in association with the trilobites Angelina hyeronimi (Kayser), Plicatolina sp., Lotagnostus (Semagnostus) sp. and Micragnostus cf. vilonii Harrington and Leanza, and the hyolithid Tajinella? iruyensis Pagani, Sabattini and Tortello (see Pagani et al., 2005). Finally, some specimens come from the Alfarcito region, about 8 km east of Tilcara Town, Jujuy Province. In this locality the latest Cambrian is represented by the Casa Colorada Formation, which mainly consists of shales containing P. frequens argentina associated with Parabolinella, Plicatolina, and Onychopyge Harrington. The unit is interposed between the Mesón Group and the sandstones and siltstones of the Alfarcito Fm. (lower Tremadoc). Detailed information on this locality was provided by Harrington and Leanza (1957, p. 6-7) and Zeballo and Tortello (2005).
Description of the ontogeny of Parabolina frequens argentina
The descriptive methods used in this work are
similar to those applied by Berard et al. (2000) and
Tortello and Clarkson (2003) for specimens preserved
in shales. Many latex replicas of external and
internal moulds were prepared. For light macrophotography,
the late meraspid and holaspid specimens
were coated with magnesium oxide, whereas latex
replicas of the early meraspides with gold-palladium
coating were examined under SEM. Based on techniques
by Clarkson and Taylor (1995a), Berard et al.
(2000) and Clarkson and Ahlberg (2002), the reconstructions
of the smaller stages were made from the
SEM photographs by drawing a squared grid on each
print and transposing this on to millimetre-squared
graph paper to give a constant scale.
The material is housed in the Museo de Ciencias
Naturales de La Plata (MLP), and the Facultad de
Ciencias Naturales e Instituto Miguel Lillo, Universidad
Nacional de Tucumán (PIL), Argentina.
Order PTYCHOPARIIDA Swinnerton, 1915
Suborder OLENINA Burmeister, 1843
Family OLENIDAE Burmeister, 1843
Subfamily Oleninae Burmeister, 1843Genus Parabolina Salter, 1849
Subgenus P. (Neoparabolina) Nikolaisen and Henningsmoen, 1985
Type species. Parabolina frequens (Barrande, 1868), original designation.
Parabolina (Neoparabolina) frequens (Barrande, 1868)
Remarks. Following Pribyl and Vanek (1980), Parabolina argentina (Kayser, 1876) is regarded as a subspecies of the earlier described species Parabolina frequens (Barrande, 1868) (see below, Systematic position).
Parabolina (Neoparabolina) frequens argentina (Kayser, 1876) Figures 3-11
Figure 3. Parabolina frequens argentina (Kayser), from Quebrada Azul (Cajas Range), bed A / de la quebrada Azul (sierra de Cajas), nivel A. 1, ?Anaprotaspid stage / ?anaprotaspis, MLP 31577, x176. 2, Metaprotaspid stage / metaprotaspis, MLP 31580, x121. 3-4, Degree 0
meraspid specimens lacking librigenae / meráspidos 0 (sin librígenas); 3, MLP 31579a, x153; 4, MLP 31578a, x134. 5-8, Degree 1 meraspides
/ meráspidos 1; 5, Axial shield / escudo axial, MLP 31581, x80; 6, MLP 31582, axial shield slightly flattened / escudo axial suavemente deformado, x85; 7, Cranidium / cranidio, MLP 31583, x112; 8, Cranidium / cranidio, MLP 31584, x108. 9, Degree 2 meraspis, almost complete
specimen damaged anteriorly / meráspido 2, ejemplar casi completo, mal preservado anteriormente, MLP 31579b, x70. 10, Probable degree
3-5 meraspis, cranidium / probable meráspido 3-5, cranidio, MLP 31585, x82.
1876. Olenus argentinus sp. nov. Kayser: 6, pl. 1, figs. 1-3.
1957. Parabolina argentina (Kayser). Harrington and Leanza: 81-85,
figs. 25-26 (see for synonymy).
1957. Parabolina argentina (Kayser). Henningsmoen: 116-117 (see
for synonymy).
1958. Parabolina argentina (Kayser). Frederickson: 541-543, pl. 80.
1965. Parabolina argentina (Kayser). Branisa: pl. 1, fig. 5.
1965. Parabolina sp. Branisa: pl. 1, fig. 9.
?1968. Parabolina cf. P. argentina (Kayser). Robison and Pantoja-
Alor: 788, pl. 101, figs. 25-26.
1980. Parabolina frequens argentina (Kayser). Pribyl and Vanek: 14-
15, pls. 2-5 (see for synonymy).
2000. Parabolina (Neoparabolina) frequens argentina (Kayser). Tortello
and Rao: 69, figs. 3.D-I.
2003. Parabolina (Neoparabolina) frequens argentina (Kayser). Tortello
and Esteban: 338-340, figs. 5.G-I.
2005. Parabolina (Neoparabolina) frequens argentina (Kayser). Zeballo
and Tortello: 134-135, figs. 4.D-E, K; 6.A.
Protaspid stages
We follow here Størmer's (1942, p. 56) division of
the protaspid period of growth into anaprotaspid
and metaprotaspid stages as used in Clarkson and
Taylor (1995a) and Clarkson et al. (1997).
Two protaspids are present in our material. None
of these specimens, however, is especially well preserved
and some inferences have had to be made in
the reconstruction. There is a rather large size gap between
the first and second protaspid stages described
below, so an intermediate instar is probably
missing.
An ?anaprotaspis is a cambered disc, ovoid in
form, 0.25 mm long (sag.) and 0.2 mm wide, slightly
tapering posteriorly (figures 3.1, 4.1). A short intergenal
spine is present on the right side. The axis is
raised above the genal regions; it is long and narrow,
less than 0.1 mm across, tapering anteriorly and posteriorly,
with no clear indication of segmentation.
Figure 4. Parabolina frequens argentina (Kayser), protaspid and early meraspid stages, reconstructed from specimens in figure 3 / protáspidos y meráspidos tempranos; reconstrucciones basadas en los especímenes ilustrados en la figura 3. 1, Anaprotaspis. 2, Metaprotaspis. 3, Degree 0 meraspis / meráspido 0. 4, Degree 1 meraspis / meráspido 1. 5, Degree 2 meraspis / meráspido 2. All specimens x76 / todos los especímenes x76.
In addition, a slightly distorted metaprotaspis, 0.38 mm long (sag.) and 0.30 mm wide, is illustrated in figures 3.2, 4.2. It is somewhat pyriform in shape, distinctly tapering posteriorly. At 0.26 mm from the anterior border there is a distinctive transverse furrow, indicating the boundary between the cephalon from the protopygidium; the latter is not yet separated nor released. The protoglabella is elongated-barrel shaped, with indistinct traces of furrowing, and there is some trace of eye ridges and palpebral lobes at the antero-lateral margins. The transitory pygidium has a node on the first axial ring, and a smooth ring behind. The posterior margin is rounded and, in this specimen, no traces of spines are visible.
Meraspid stages
The specimens studied include axial shields with
0, 1, 2, 4… thoracic segments, as well as isolated tagmata.
The traditional descriptive methodology by
Raw (1925), consisting of the division of the
meraspid period into degrees (1, 2, 3, etc.), marked by
the addition of successive segments to the thorax, can
be applied here.
Degree 0 meraspis. The best preserved complete individual
representing this stage is an external mould
illustrated in figure 3.3 (see also figure 4.3). As reconstructed,
it is 0.44 mm in length and 0.40 mm at its
widest point. The cephalon is 0.26 mm long (sag.),
subcircular in outline, with a distinct flattened border
and straight intergenal spines. Glabella occupying central third of the cranidium, slightly wider at
its mid point, reaching anterior border, well defined
by deep and narrow axial furrows. In this specimen
S0 is deeply impressed, transglabellar, curved backward.
S1 impressed laterally and oblique backward,
fainter medially. S2 appears to be transglabellar but
indistinct. S3 poorly defined. Occipital ring slightly
narrower (tr.) than L0, smooth. Fixigenae quite inflated,
with faintly defined palpebral lobes.
Transitory pygidium 0.15 mm long (sag.) and 0.30
mm wide, curled down posteriorly, semicircular in
outline. Axis 0.08 mm across at its widest point, convex,
tapering posteriorly, surrounded by narrow axial
furrows; it consists of three segments lacking central
nodes. Pygidial margin with three pairs of short spines.
A second, distorted specimen (figure 3.4) is of
similar size and form. Likewise specimen illustrated
in figure 3.3, it can be used to confirm the slightly
barrel-shaped glabella and distinct intergenal spines,
as well as the overall proportions described above.
Degree 1 meraspis. Two complete specimens and
several other disarticulated or distorted individuals
are assignable to this growth stage. The reconstruction
(figure 4.4) is based on the specimen illustrated
in figure 3.5, but some details have been added from
other specimens (figures 3.6-3.8). As reconstructed
this degree is 0.82 mm long (excluding spines) and
0.72 mm across at its widest point. Cephalic contour
originally semicircular with a transverse anterior
border. Glabella 0. 20 mm wide, long, rounded anteriorly,
with subparallel lateral margins, well defined
by deep and narrow axial furrows. Lateral furrows
(S0, S1, S2, S3) subequally spaced, transverse to
slightly curved backwards, each swollen medially,
defining lobes L1-L4. S0 deeper and wider (sag.) than
S1-S3. Occipital ring of similar length (sag.) and appearance
to glabellar lobes, extending backwards
over the anterior part of the single thoracic segment
and provided with a medial node. Posterior border
furrow deep and well defined. Fixigenae quite inflated,
with a pair of ocular ridges curving anteriorly,
and faintly defined palpebral lobes. Intergenal spines
illustrated in the reconstruction are hypothetical.
Librigenae and hypostome unknown.
Thoracic segment with a short (sag.) axial ring,
which is partially covered by the projecting occipital
ring, with a conspicuous median node. Pleura long
(exsag.), about two thirds the total width of the thorax,
with obliquely truncated outer terminations,
each with a narrow spine. Pleural furrow slightly
oblique, reaching the outer margin.
Transitory pygidium semicircular, 0.52 mm long
(sag.) and 0.55 mm wide (excluding spines). It is
slightly curled down posteriorly. Axis very convex,
raised above the pleural fields, less than two thirds
the total width of the pygidium, tapering backward;
it is composed of six axial rings, each with a median
node. In this specimen the rear edge of the anterior
axial ring is slightly separated from the articulating
half-ring of the succeeding segment, interpreted as a
stage preceding the release of the second thoracic
segment. Pleural furrows shallow, increasingly oblique
posteriorly. Pygidial margin with at least three
pairs of marginal spines, the rearmost pair prolonged
posteriorly.
Degree 2 meraspis. Only a single specimen (figure 3.9),
almost complete but damaged anteriorly was available.
The reconstruction (figure 4.5) is accordingly tentative.
The individual is 0.92 mm long and 0.80 mm
across at its widest point (excluding spines). Cranidium
0.40 mm long (sag.) and 0.75 mm wide (tr.).
Glabella occupying central third of cranidium, slightly
barrel-shaped, rounded anteriorly. S0, S1 and S2 conspicuous,
dividing the glabella into distinct lobes L1
and L2 of similar size. The anterior lobe is traversed by
a faint S3. Occipital ring of similar dimensions and
form to L1 and L2, projecting posteriorly. The eyes are
set further back than in degree 1. The course of the posterior
facial suture can be faintly discerned on the left
side of this specimen; it is almost straight, making an
angle of some 20° to the sagittal plane, thus defining a
trapezoidal cranidium. Intergenal spines still present.
The right side of the specimen shows a narrow librigenae,
with a genal spine extended to about the rear of
the first thoracic segment.
Thorax consisting of two similar segments, 0.20
mm long (sag.), the first being somewhat longer (sag.)
than the second, terminating in recurved spines.
Pygidium elongate, semicircular, axis tapering
posteriorly with five to six axial rings, reaching almost
to the posterior margin. Median nodes poorly
developed. Margin with five pairs of spines, laterally
splayed, becoming inwardly curved posteriorly.
Degree ?3 meraspis. Small juvenile cranidia were
available to us, but to which degree they pertain is
not easy to define. The specimen illustrated in figure
3.10 may be a degree 3 meraspid judged from its aspect,
but it could possibly pertain to a degree 4 or
even 5 meraspid because of its size. It is a slightly distorted
cranidium 0.65 mm long and 1.1 mm broad,
trapezoidal in form. The glabella, occupying the central
third of the cephalic width, tapers forward almost
reaching the anterior border, and is rounded
quadrate anteriorly. Transglabellar furrows S0, S1, S2
and S3 are subequally spaced, transverse to slightly
curved backward, defining lobes of almost equal
length (sag.). S0 is deeper than S1-S3. The occipital
ring of which the rear margin curves posteriorly
bears a prominent central node. There is a narrow
(sag.), straight anterior border. The eye ridges are
strong, and the palpebral lobes prominent and
curved. Intergenal spines apparently are absent.
Degrees 4-5 meraspis. The slightly distorted specimen
figured in 5.1 is probably a degree 4 or 5
meraspis (see also figure 6). Since the posterior region
is not particularly well-preserved it is not possible
to state with certainty which it actually is. The
rear part is thus reconstructed on the basis of complementary
material (see below). The total length is
about 1.5 mm. Cranidium trapezoidal, 0.60 mm long
(sag.) and 1 mm broad. Facial suture sinuous, posterior
branch making an angle of some 40° to the sagittal
plane. Axial and posterior border furrows deeply
impressed, the former defining a glabella which
though broken seems to have possessed distinct S0,
S1, S2 and probably S3 furrows, delimiting subequal
rectangular lobes. Occipital ring large, curving posteriorly.
Eye ridges strong, curving, laterally joining
the large palpebral lobes subopposite S3. Librigenae
distinct, still relatively narrow and bearing stout
genal spines. Intergenal spines absent. Thorax of four
or five segments, axis tapering backward. Pleurae
bearing terminal spines which become progressively
longer and more inclined posteriorly toward the rear.
Pygidium unclear, tentatively reconstructed with reference
to other specimens.
Figure 5. Parabolina frequens argentina (Kayser), from Quebrada Azul (Cajas Range) / de la quebrada Azul (sierra de Cajas). 1, Degree 4-
5? meraspis, complete specimen with glabella imperfectly preserved, bed A / meráspido 4-5?, ejemplar completo con la glabela preservada en forma imperfecta, nivel A, MLP 31578b, x35; note a degree 0 meraspid specimen on the left side / nótese un ejemplar meráspido 0 sobre el sector izquierdo. 2, Degree 4 meraspis, axial shield, bed F / meráspido 4, escudo axial, nivel F, MLP 31586, x40. 3, Degree 6 meraspis, axial shield,
bed A / meráspido 6, escudo axial, nivel A, MLP 31588, x28. 4, Probable degree 4 meraspis, cranidium, bed A / probable meráspido 4, cranidio, nivel A, MLP 31587, x62. 5-7, Degree 6 meraspis / meráspidos 6; 5, cranidium, bed A / cranidio, nivel A, MLP 31589, x35; 6, axial shield,
bed F / escudo axial, nivel F, MLP 31590, x25; 7, cephalon-thorax, bed D / céfalo-tórax, nivel D, MLP 31591, x27. 8, Degree 9 meraspis, complete
exoskeleton, bed A / meráspido 9, exoesqueleto completo, nivel A, MLP 31593, x18. 9, Degree 8 meraspis, thorax-pygidium and displaced
cephalon, bed B / meráspido 8, tórax-pigidio y céfalo desplazado, nivel B, MLP 31592, x22. 10, Degree 9 meraspis, complete exoskeleton,
bed A / meráspido 9, exoesqueleto completo, nivel A, MLP 31594, x19.
The complete specimen illustrated in figure 5.2 has four thoracic segments, and is of length 1.5 mm. In the reconstruction (figure 6), pygidial spines and librigena-facial sutures are conjectural. The cranidium 0.70 mm long of figure 5.4 seems to be that of a degree 4 meraspis.
Figure 6. Parabolina frequens argentina (Kayser). Reconstruction
of meraspid degree 5 / reconstrucción de un meráspido 5, x50.
Degree 6 meraspis. The primary description is mainly
based on specimens illustrated in figures 5.3, 5.5-
5.7, all of which are of similar size and form. Total exoskeleton
length (estimated) is 2.1 mm (without
spines).
Cranidium trapezoidal, 0.87 mm long (sag.) and
1.6 mm broad. Glabella quite inflated, almost reaching
anterior border, occupying slightly more than the
central third. S0 deep, S1 faintly defined medially
and deeply impressed laterally, oblique backward;
S2 represented by a lateral pair of shallow excavations
opposite centre of the large palpebral lobes.
Occipital ring quite similar in form to the anterior
thoracic axial ring; the axis is broadest here.
Fixigenae are not especially broad in this stage.
Course of facial suture sinuous, broadly inclined at
about 45° to the sagittal plane.
Thorax 0.85 mm long, consisting of 6 segments.
Axis markedly tapering posteriorly, the spinose tips
of the thoracic spines increase in length and posterior
inclination rearward. This backward swing continues
into the pygidium, which has five pairs of
spines so that the macropleural spines of the 8th segment
are subparallel with the sagittal plane and the
shorter posterior spines curve inwards. The total
length of the pygidium (without spines) at this stage
is 0.45 mm and the macropleural spines are already
defined.
Later stages in ontogeny. Specimens assigned to these
degrees are rather variable. Those representing degrees
7 and 8 in most cases resemble miniature adults
although having fewer segments. That reconstructed
here is 2.8 mm long (figure 5.9), whereas degree 9
meraspis (figures 5.8, 5.10) is 3.5 mm long. The
smallest holaspides (figures 7.15, 10.12) in our material
have attained a length of 4 mm. The time of origin
and growth of the axial spine on segment 12 still
remains unclear. There are indications of a short axial
spine in some specimens but otherwise it seems to
have grown at a later stage of development.
Figure 7. Holaspid specimens of Parabolina frequens argentina (Kayser). 1, Cranidium / cranidio, Alfarcito, MLP 31620b, x7.7. 2, Cranidium / cranidio, Quebrada Azul, bed B / nivel B, PIL 14270, x4. 3, Cranidium / cranidio, Quebrada Azul, bed D /nivel D, MLP 31608,
x4.7. 4, Cranidium, latex mould / cranidio, molde de látex, Quebrada Azul, bed F / nivel F, MLP 31610, x8.7. 5, Cranidium, latex mould / cranidio, molde de látex, Alfarcito, MLP 31613, x6.5. 6, Hypostome / hipostoma, Quebrada Azul, bed A / nivel A, MLP 31627, x7. 7, Hypostome, latex mould / hipostoma, molde de látex, Quebrada Azul, bed A / nivel A, MLP 31612, x7.7. 8, Cranidium / cranidio, Quebrada
Azul, bed A /nivel A, MLP 31596b, x8. 9, Cranidium / cranidio, Alfarcito, MLP 31604, x7.2. 10, Hypostome / hipostoma, Quebrada Azul,
bed A/ nivel A, MLP 31618, x6.5. 11, Cranidium / cranidio, Iruya, MLP 31605, x5.1. 12, Cranidium, latex mould / cranidio, molde de látex,
Quebrada Azul, bed D / nivel D, MLP 31616, x7.2. 13, Hypostome / hipostoma, Quebrada Azul, bed D / nivel D, PIL 13843, x10. 14, Cephalon and thorax, latex mould / céfalo y tórax, molde de látex, Alfarcito, MLP 31615, x4. 15, Three complete early holaspides, latex
mould / tres especímenes completos, holáspidos tempranos, molde de látex, Alfarcito, PIL 12536, x6. 16, Thorax-pygidium, latex mould / tóraxpigidio, molde de látex, Quebrada Azul, bed C / nivel C, MLP 31611, x2.8.
The late holaspis morphology of P. frequens argentina is well known since Harrington and Leanza
(1957, p. 81-85) provided an accurate description of
the taxon, including a discussion on its synonymy
and intraspecific variability. Thus, only a few complementary
aspects will be pointed out here. In addition,
some variable features of the adult are discussed
below (see Ontogenetic Variability).
According to Nikolaisen and Henningsmoen
(1985, p. 4), the genus Parabolina is characterized by a
hypostome with almost completely reduced anterior
wings. We describe some small holaspides and several
adults of P. frequens argentina retaining their hypostomes
in close association with the ventral doublure
(conterminant condition) (figures 7.10, 7.13, 8.7,
8.10). On moulting it is shed along with the librigenae
and may remain in its original position (figures
10.7, 10.9) or may become displaced (figures 7.6, 7.7)
(see below). Originally briefly described by
Kobayashi (1937, p. 477, pl. 4, fig. 13) and illustrated
by Zeballo and Tortello (2005, figs. 4.K, 6.A), the hypostome
is subtrapezoidal in outline, longer than
wide. The contour of the anterior margin is a variable
feature: in some specimens it is forwardly curved
(figure 7.13), whereas in others it is virtually planar
(figure 7.10), providing a more efficient surface of
contact to the rear edge of the cephalic doublure. The
middle body is prominent, ovoid, convex, with the
anterior edge poorly defined, highest posteriorly.
Anteriorly, the middle body virtually merges with
the frontal margin, whereas it is separated from the
posterior lobe by a deep, narrow and uniformly impressed
middle furrow which is strongly bowed
backward. The surface is smooth. Anterior wings
small, very narrow (tr.). A thin external rim is present
on each side of the hypostome. Posterior margin
semicircular, surrounded by a very narrow (sag.),
convex posterior border.
Figure 8. Holaspid specimens of Parabolina frequens argentina (Kayser). Most specimens from Quebrada Azul (Cajas Range) excepting
9 (Alfarcito) / la mayoría de los especímenes de la Quebrada Azul (sierra de Cajas) excepto 9 (Alfarcito). 1, Cranidium, thorax and displaced, inverted
pygidium, bed A / cranidio, tórax, y pigidio desplazado e invertido, nivel A, PIL 13835, x2.4. 2, Cranidium, bed F / cranidio, nivel F,
PIL 14263, x5.5. 3, Incomplete thorax, bed F / tórax incompleto, nivel F, MLP 31597, x4.3. 4, Cranidium and displaced fragmentary thorax,
bed D / cranidio y tórax fragmentario, desplazado, nivel D, MLP 31623, x4.4. 5, Fragmentary thorax and pygidium, bed D / tórax fragmentario y pigidio, nivel D, MLP 31619, x4.2. 6, Fragmentary thorax and displaced, inverted pygdium, bed A / tórax fragmentario y pigidio desplazado e invertido, nivel A, MLP 31600, x4.2. 7, Thorax-pygidium + librigenae and hypostome, bed A / tórax-pigidio + librígenas e hipostoma, nivel A, MLP 31622, x3.5. 8, Fragmentary thorax, latex mould, bed D / tórax fragmentario, molde de látex, nivel D, MLP 31632, x6.2. 9, Cephalon and thorax / céfalo y tórax, MLP 31603, x8. 10, Thorax-pygidium + librigenae and hypostome, bed D / tórax-pigidio + librígenas e hipostoma, nivel D, PIL 13843, x3.6. 11, Cephalon and displaced thorax-pygidium, latex mould, bed F / céfalo y tórax-pigidio desplazado, molde de látex, nivel F, MLP 31614, x8.3. 12, Pygidium, latex mould, bed D / pigidio, molde de látex, nivel D, MLP 31617, x8. 13, Pygidium,
bed A / pigidio, nivel A, MLP 31606, x6.2.
Some well-preserved fully adult specimens of P. frequens argentina show a delicate rounded protuberance in front of the occipital node, in the centre of the occipital ring (e.g., figures 7.1, 7.4, 7.5, 7.11, 9.6). In whitened specimens it is possible to note indications of its internal structure, which seems to consist of fine pits. This protuberance resembles the median occipital organ described from the upper Cambrian olenid Olenus wahlenbergi Westergård by Clarkson and Taylor (1995a). It is similar to dorsal organs in larval decapod crustaceans, which are combined glandular and sensory organs (Barrientos and Laverack, 1986). An occipital ring with "two tubercles" was also described in P. (Neoparabolina) frequens finmarchica Nikolaisen and Henningsmoen (1985, figs. 4, 11.C-J, 5.A-Ea), from the Lower Tremadocian of Norway.
Figure 9. Holaspides of Parabolina frequens argentina (Kayser). Most specimens from Quebrada Azul (Cajas Range) excepting 2 and 6
(Alfarcito) / la mayoría de los especímenes de la quebrada Azul (sierra de Cajas) excepto 2 y 6 (Alfarcito). 1, Axial shield, bed C / escudo axial, nivel C, MLP 31607, x6.8. 2, Complete specimen / ejemplar completo, MLP 31601, x6.6. 3, Librigenae and thorax, bed D / mejilla libre y tórax, nivel D, MLP 31628, x5.4. 4, Thorax-pygidium, bed D / tórax-pigidio, nivel D, MLP 31598, x3.5. 5, Thorax-pygidium, bed F / tórax-pigidio nivel F, MLP 31599, x4.3. 6, Complete specimen, latex mould / ejemplar completo, molde de látex, MLP 31609, x4.5. 7, Librigenae, thoraxpygidium
and hypostome, bed D / mejilla libre, tórax-pigidio e hipostoma, nivel D, MLP 31595, x4.3. 8, Complete specimen, bed D / ejemplar completo, nivel D, MLP 31596a, x5.4. 9, 12th thoracic segment showing axial spine, bed F / último segmento torácico, con espina axial, nivel F, MLP 31602, x5. 10, Librigenae and thorax-pygidium, bed A / librígenas y tórax-pigidio, nivel A, PIL 14271, x5.3.
Figure 10. Holaspides of Parabolina frequens argentina (Kayser). 1, Thorax, latex mould / tórax, molde de látex, Quebrada Azul, bed F / nivel F, MLP 31634, x5.2. 2, Thorax-pygidium / tórax-pigidio, Quebrada Azul, bed B / nivel B, MLP 31626, x7.8. 3, Two early holaspides
/ dos holáspidos tempranos, Alfarcito, MLP 31621, x7.5. 4, Disarticulated cranidium, librigenae and thorax, latex mould / cranidio, librígenas y tórax desarticulados, molde de látex, Quebrada Azul, bed A / nivel A, MLP 31630, x6.3. 5, Thorax-pygidium / tórax-pigidio, Quebrada
Azul, bed A / nivel A, PIL 13088, x2. 6, Thorax-pygidium, latex mould / tórax-pigidio, molde de látex, Quebrada Azul, bed F / nivel F, MLP
31633, x2.6. 7, Thorax-pygidia and displaced librigena with associated hypostome, latex mould / tórax-pigidios y librígena con hipostoma asociado, molde de látex, Alfarcito, PIL 12536, x2.5. 8, Displaced cranidium, librigenae and thorax / cranidio, librígenas y tórax, desplazados,
Quebrada Azul, bed D / nivel D, MLP 31629, x3.3. 9, Thorax and displaced librigenae + hypostome / tórax y librígenas + hipostoma desplazados,
Quebrada Azul, bed C / nivel C, MLP 31625, x5. 10, Cephalon and fragmentary thorax, showing Harrington's configuration / céfalo y parte del tórax, con las librígenas desplazadas, dispuestas debajo del cranidio, Quebrada Azul, bed D / nivel D, MLP 31624, x3.6. 11, Fragmentary thorax, latex mould / tórax fragmentario, molde de látex, Quebrada Azul, bed D / nivel D, MLP 31631, x5.7. 12, Axial shield
with displaced cranidium, and fragmentary thorax / escudo axial con cranidio desplazado, y tórax fragmentario, Alfarcito, MLP 31620a, x6.
Figure 11. Parabolina frequens argentina (Kayser), reconstruction in lateral view in (1) active and (2) relaxed posture / reconstrucción en vista lateral de las posturas activa (1) y de reposo (2).
The present study has revealed no further details of the structure of the eye, and the nature of the lentiferous surface remains unknown.
Ontogenetic variability
In the majority of trilobites and, certainly in most
olenids, morphology and size are closely coupled
throughout development. In some cases it has proved
possible, by measuring length/width ratios, to establish
instar groupings which plot out on a graph as a sequence
of clusters (Clarkson and Ahlberg, 2002;
Clarkson et al., 2003, 2004). These clusters correspond
to protaspid and meraspid degrees, and sometimes
can be traced into the early holaspid stages. Within
each degree, morphology and size are tightly constrained,
and all the individuals forming a cluster are
very similar in appearance. An isolated cranidium or
pygidium can thus be confidently referred to its correct
degree by comparison with intact specimens.
Even if an ontogeny has been worked out on the basis
of disarticulated sclerites alone, it may still be possible
to determine, with a fair degree of certainty, to which
degree an individual cranidium or pygidium belongs.
In some olenids, however, clusters are more open,
or less evident (Tortello and Clarkson, 2003). Some
instances have been described where, although most
individuals follow a "normal" growth trajectory some
others do not fall within the expected size range.
Thus in Ctenopyge (Ctenopyge) gracilis Henningsmoen,
from the Furongian of Sweden, degree 7 meraspides
have been described which are the size of degree
6 or even degree 5 individuals (Clarkson et al., 2004).
They have achieved a more mature form when still
quite small.
The highest degree of variability in olenids described
up until now is in Parabolina spinulosa (Wahlenberg),
from the Furongian of Sweden, both in the
juvenile stages and the adult. The ontogeny of P. spinulosa
resembles that of P. frequens argentina, as would
be expected, though the early stages of development
are more clearly represented in the latter, and the
macropleural spines are lacking in the former. But direct
comparisons are difficult because of this variability,
which affects both size and proportions as well as
basic morphology (Clarkson et al., 1997, p. 83). The
same kind of variability is true on Parabolina frequens
argentina, but to an even greater degree. The problem
of assigning disarticulated meraspid/young holaspid
sclerites to their correct degree, noted by these authors
in P. spinulosa, is more acute and has led to some
problems in working out successive stages in development.
As in P. spinulosa both size and shape are
highly variable. Thus, specimen illustrated in figure
5.2 is an undoubted degree 4 meraspis 1.4 mm in
length, while that of figure 5.1 is a degree 5 meraspis
of very similar size. The degree 6 meraspid is considerably
larger (specimen in figure 5.3 is nearly twice
the size of that of figure 5.1, being 2.2 mm in length).
Degree 7 meraspid is not much larger.
Shape and proportions are also greatly variable.
This is particularly evident in the glabellar furrows.
In "normal" early olenid development there are four
subequal lobes in the glabella, defined by furrows S0,
S1, S2 and S3. These become modified later in development,
reducing to two or three. In P. frequens argentina
the range of variation in juvenile specimens is
formidable. Thus the small degree 5 meraspid has
three transglabellar furrows, retaining a rather juvenile
form while in a larger degree 6 meraspis (figure
5.3) only S1 is present, impressed laterally but hardly
transglabellar. Because of this great range of variation
it is not usually possible to assign disarticulated
sclerites to their correct degree. Thus, a well-preserved
cranidium 0.65 mm in length and 1.1 mm in
width has four subequal glabellar lobes as in an early
meraspid (figure 3.10). From its size it could represent
any stage of development between a degree
3 and a degree 5 meraspid and it is just not possible
to be more definite.
Other proportions of the developing exoskeleton
are also singularly variable; the sagittal length of the
thoracic segments, the splay and length of the genal
and thoracic spines, the relative width of the axis,
and the relative development of the axial tubercules
all vary greatly. Thus, the evident differences between
the reconstructions of degree 1 and degree 2
meraspis are actual, and not just the result of imperfect
preservation.
In addition, Harrington and Leanza (1957) cited
several variable features in the holaspid morphology
of P. frequens argentina. For instance, no less than five
gradational patterns in glabellar furrows have been
defined for the adult: one, two or three furrows may
be present and of variable form. Similarly, the
cephalic anterior border can be, according to
Harrington and Leanza (1957, p. 83), 1.7 to 3 times
wider (sag.) than preglabellar field, a fact clearly exemplified
in figures 7.1-7.5, 7.8-7.9, 7.11-7.12. The anterior
cephalic border of many specimens from Alfarcito
seems to be narrower (sag.) than that of individuals
from sierra de Cajas, though the border of a
few individuals from the former locality is proportionately
wide (see figure 7.1). The width (sag.) of the
preglabellar field is also a rather variable character in
the material studied, which intergrades between 2%
and 9% the length (sag.) of the cranidium.
As documented in other olenids, the nodes on the
axis of the thorax and the pygidium are better presented
in juveniles and small adults of P. frequens argentina,
whereas the full-grown individuals can bear
distinct, partially developed, or smooth axial nodes
(e.g., compare figures 8.1, 9.1-9.8, 9.10). Clarkson et al.
(1997) suggested that the axial nodes originated as juvenile
structures, which disappeared in the later
stages of development of many Ordovician olenids
(e.g., Jujuyaspis keideli (Kobayashi); see Tortello and
Clarkson, 2003), but were paedomorphically retained
in the adults of Parabolina.
Pribyl and Vanek (1980) postulated a variability
affecting number of marginal spines in the holaspid
pygidium of P. frequens argentina, stating that some
specimens can bear two pairs of spines instead of
three as it is usual. Nevertheless, we could not find
such variation in the material examined. All wellpreserved
pygidia from Cajas, Iruya and Alfarcito
show three pairs of marginal spines, suggesting that
it is a stable condition (cf. Harrington and Leanza,
1957).
Systematic position
After Kayser (1876), Kobayashi (1937), Harrington
(1938), and Harrington and Leanza (1957), Pribyl and
Vanek (1980) illustrated additional material of P. argentina
from Bolivia and pointed out its great morphological
similarity with P. frequens (Barrande,
1868), a species previously described from the Lower
Ordovician of Germany and Bohemia (Barrande,
1868; Sdzuy, 1955, pl. 3, figs. 58-70; text-fig. 10). Pribyl
and Vanek (1980) noted that argentina hardly distinguishes
from frequens in having "2-3 pygidial
spines" and a pygidial axis reaching slightly nearer to
the posterior margin of pygidium. Therefore, they regarded
the two as subspecies of P. frequens.
Rushton (1982, pl. 2, fig. 15) illustrated additional
material of P. frequens frequens from the Furongian of
Wales and, in view of the variability reported in P.
frequens argentina by Harrington and Leanza (1957),
suggested that both taxa could be synonyms (cf.
Nikolaisen and Henningsmoen, 1985). More recently,
Zylinska (2001, text.-fig. 11; pl. 5, figs. 1-14) described
new specimens from the upper Cambrian of Poland
and made a comparative analysis of the subspecies of
P. frequens ( P. frequens frequens, P. frequens argentina,
as well as P. frequens finmarchica Nikolaisen and
Henningsmoen (1985, figs. 4, 11.C-J, 12.A-Ea) from
the lower Tremadocian of northern Norway).
Zylinska (2001) evaluated the large morphologic
variation within these subspecies, concluding that
they fall into the variability of P. frequens and hence
cannot be maintained.
Parabolina frequens is evidently a variable taxon.
However, we consider that its described subspecies
have some stable features which justify their validity.
P. frequens argentina differs from P. frequens frequens
in having a more transversely expanded pygidium,
three pygidial marginal spines (2-4 may occur in P.
frequens frequens), and a proportionately narrower
(sag.) pygidial posterior border (cf. Pribyl and
Vanek, 1980). In addition, the hypostome of the former
has a more rounded middle body.
Parabolina frequens argentina and P. frequens finmarchica
share a relatively narrow pygidial border,
but the latter clearly differs in having forwardly located
genal spines, stronger ocular ridges, convergent
anterior branches of facial suture, and a hypostome
with a narrower middle body.
Moulting
The studied material consists of numerous specimens
representing both ecdysial remains and exoskeletons
of dead individuals. Since ecdysial units
are hardly attacked by scavengers, and each trilobite
normally produced several exuviae, the chances of
finding moulting configurations in low-energy environments
should be greater than finding remains of
dead animals (Henningsmoen, 1975). The latter are
generally represented by complete specimens showing
no fissures between exoskeletal plates, occasionally
found in life attitude or tightly enrolled (individuals
illustrated in figures 9.2 and 9.6 may represent
such condition). On the other hand, the material
at our disposal includes many exoskeletal remains
that can be interpreted as exuviae. Olenids moulted
by opening the facial suture and emerging forwards,
leaving the exuvial parts on various distinctive
arrangements (Henningsmoen, 1957, 1975; Chatterton
and Ludvigsen, 1998). It is common to find the
exoskeleton of P. frequens split in two main parts: axial
shield (cranidium + thorax + pygidium) (e.g., figure
9.1) and librigenae (e.g., figure 10.7). They may
lie close together or removed from each other.
Similar configurations were described by Chatterton
and Ludvigsen (1998) in the upper Steptoean trilobites
Wujiajiania sutherlandi Chatterton and
Ludvigsen (Olenidae), Labiostria Palmer (Aphelaspidae)
and Pterocephalia Roemer (Pterocephaliidae).
The librigenae are fused into a single piece in Parabolina
(Sdzuy, 1955; Henningsmoen, 1957; Clarkson
et al., 1997). Several displaced librigenae of Parabolina
frequens argentina retaining their hypostomes are illustrated
in figures 10.7 and 10.9. In some cases the
librigenae were displaced backwards below the axial
shield during the ecdysis ("Harrington's configuration"
Henningsmoen, 1975 - see figure 10.10-), a fact
also documented by Harrington and Leanza (1957) in
other olenids (e.g., Parabolinella Kobayashi) as well as
in some kainellids (e.g., Pseudokainella Harrington,
Apatokephalus Brogger) (Harrington and Leanza,
1957, figs. 38.8, 52.6, 58.3), and by Chatterton and
Ludvigsen (1998, fig. 19.5) in Labiostria.
Other specimens of P. frequens argentina can be interpreted
as typical exuviae. In some individuals the
cranidium is missing, whereas the librigenae are preserved
more or less in normal position (figures 8.7,
8.10, 9.3, 9.10). A similar ecdysial configuration has
been previously documented in Parabolina (Harrington
and Leanza, 1957, fig. 25.2) and many other olenids ( e.g., Cloacaspis Fortey (1974, pl. 12, fig. 1), Bienvillia
Clark (Harrington and Leanza, 1957, fig. 43.1f),
Saltaspis Harrington and Leanza (1957, fig. 32.1c),
Acerocare Angelin (Henningsmoen, 1957, pl. 30.5;
Henningsmoen, 1975, fig. 10.A)) and non-olenid
trilobites (see McNamara and Rudkin, 1984; Chatterton
and Ludvigsen, 1998). Similarly, the collections
studied include specimens consisting of a thorax
or a thoracopygon and a displaced cranidium
(figures 8.4, 10.4, 10.12).
It is also common to find disarticulated axial
shields and isolated fragmentary thoraxes (e.g., figures
8.4, 8.8, 9.4, 9.9, 10.1-10.4, 10.7-10.9, 10.11-
10.12), suggesting that in many cases moulting was a
difficult process. In many cases thoracic segments are
preserved displaced, separated, or telescoped under
each other. Although exuviation in P. frequens argentina
generally occurred by egression of the
postecdysial trilobite from its exuvia without inversion
of any exoskeletal elements, there are some examples
consisting of an inverted pygidium close to
its corresponding thorax (see figures 8.1, 8.6).
Figure 8.9 shows a specimen with "open" facial
sutures. According to Henningsmoen (1975), it may
represent either an exuvium or a dead trilobite with
sutures opened at its site of death or after postmorten
transport.
Most of these examples were collected from the
Lampazar Formation in Sierra de Cajas, from levels
representing a quiet-water, outer shelf setting
(Tortello and Esteban, 2003). Moulting of P. frequens
argentina took place on the sea floor. The preservation
of arrangements of exoskeletal elements which
have not suffered postecdysial disturbance, by bioturbation
or by physical proceses, is compatible with
a low-energy environment.
Life attitude
All the available material of P. frequens argentina is
flattened, at least to some extent, and the original
convexity of the exoskeleton remains unknown. Trilobites
of the subfamily Oleninae, however, are never
greatly convex, as exemplified by Olenus wahlenbergi
Westergård reconstructed from material retaining its
original form (Clarkson and Taylor, 1995a, fig. 16b).
Here we assume a similar convexity for P. frequens argentina.
From the functional point of view, this
species differs from Olenus in having long genal
spines, macropleural spines on the 8th thoracic segment,
and the strong axial spine on the 12th and final
thoracic segment. It is very likely that the genal and
macropleural spines supported the body of the trilobite
when resting on the sea floor. The exoskeleton
can be reconstructed in two alternative postures, the
"alert" and "relaxed" attitudes (figure 11). In the
"alert" posture the body would rest on the horizontal
genal and macropleural spines but the whole body
would be stretched out horizontally. The extensor
muscles would be contracted and all the apodemes
would lie in a single horizontal plane.
The difference between the "alert" and the "relaxed"
posture is that in the latter case the extensor
muscles are no longer contracted. Thus the rigid
frame provided by the genal and macropleural
spines would hold the anterior part of the body as in
the "alert" posture but the rear part posterior to the
macropleural spines of the 8th thoracic segment
would incline downward as the muscles relaxed.
The pleural extremities of the segments posterior
to the macropleural spines are quite long and graduated
in size toward the rear. In the "relaxed" attitude
they may well have touched the seafloor but most
importantly, strong support would be given to the
posterior part of the body by the axial spine of the final
thoracic segment. The angle which this spine
made, in life, to the horizontal plane cannot be
known with certainty since it is normally preserved
broken or crushed against the thorax. If, however,
the spine was parallel with the extended body or rose
at an angle of only a few degrees, it would support
the relaxed trilobite most effectively, as it is reconstructed.
In the "alert" attitude the trilobite would be ready
for action, whereas when fully relaxed it would have
been supported at all five points with minimal expenditure
of energy. Fortey (1985) suggested that the
morphology of Parabolina of P. frequens type (=
Neoparabolina) is comparable to that of the Furongian
genus Irvingella Ulrich and Resser, a taxon with possible
pelagic habits. The occurrence of P. frequens argentina
in different lithologies (Tortello and Esteban,
2003) is compatible with such a mode of life; however,
the size of the eyes of Parabolina seems to be similar
to that of most benthic olenids. As stated above,
P. frequens argentina was able to rest on the sea floor
supported by the genal and macropleural spines.
Most leptoplastines (Clarkson and Taylor, 1995b;
Clarkson et al., 2003, 2004) used their long genal
spines to support the outstretched body above the
water-sediment interfacies. In P. frequens argentina a
similar objective has been achieved in a somewhat
different way, testifying to the remarkable evolutionary
flexibility among trilobites of the Family Olenidae.
Acknowledgements
We thank S. Esteban, G. Aceñolaza and P. Payrola for assistance in the field. B. Waisfeld and E. Vaccari provided additional specimens for comparison. Figures 1-2 were produced by D. Ruiz Holgado and E. Gómez-Hasselrot. We also thank J. Rodríguez for assistance with photography. The paper was greatly improved by constructive comments from B. Chatterton and S. Peng. Financial support was provided by the Consejo Nacional de Investigaciones Científicas y Técnicas (Argentina) and the Carnegie Trust for the Universities of Scotland, which enabled E. N. K. Clarkson to visit La Plata in August 2005.
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Recibido: 6 de diciembre de 2006.
Aceptado: 18 de setiembre de 2007.