Turrisaspis elektor, Daeschler & Frumes & Mullison, 2003

Daeschler, Edward B., Frumes, Anna C. & Mullison, C. Frederick, 2003, Groenlandaspidid Placoderm Fishes from the Late Devonian of North America, Records of the Australian Museum 55 (1), pp. 45-60 : 51-58

Turrisaspis elektor n.sp.

Figs. 6–9 , 11

Holotype ( Fig. 6 ). ANSP 20961 A, articulated specimen on matrix block with right lateral view of trunk shield and dorsolateral view of head shield ( Fig. 6A ). The counterpart ( ANSP 20961 B) was etched in 10% hydrochloric acid to make a clean external mold of the specimen. Figure 6B is a whitened latex peel of the counterpart block. The counterpart lacks areas with impressions of the AL and SP. Figure 6C is a labelled drawing of ANSP 20961 A/B showing plate boundaries and sensory canals.

Other material. Turrisaspis elektor is known from the following 81 isolated, measurable elements: MD, 29; PDL, 5; ADL, 7; AL, 11; PL, 6; AVL, 12; PMV, 7; PVL, 1; SP, 3. Additionally there are 10 measurable head shields and 5 articulated ventral trunk shields. See Figs. 10 and 12 for methods used to take measurements and Appendix 2 for specimen catalogue numbers and measurements.

Locality and horizon. USA, Pennsylvania, Clinton County, Red Hill site, north side of Route 120, 1 km east of the village of North Bend (41°20.633'N 77°40.814'W) ( Fig. 1 ). Catskill Formation, Duncannon Member .

Age. Late Devonian, late Famennian Stage (Fa2c substage); Grandispora cornata / Rugispora flexuosa palynomorph zone (Traverse, in press).

Etymology. Named for the truest friend of a supporter of our research.

Diagnosis. A small groenlandaspidid with a short head shield and tall and narrow dorsal trunk elements. The head shield is almost as wide as long (average W/L equals 0.96). The pineal is divided into two distinct plates in every specimen and the postmarginal makes contact with the postorbital plate, excluding the marginal plate from the lateral margin of the head shield. A continuation of the supraorbital sensory line reaches the caudal margin of the paranuchal plate. The PDL has an average H/L ratio of 2.42. The posterodorsal projection of the PDL is short and angled sharply upward so that the overlap zone for the MD does not reach the caudal margin of the element. The height of the symphysial area on the visceral surface of the PDL is 40% of the total height of the element. The MD is also high and narrow and approximately equal in height to the PDL. The MD has numerous coarse serrations along the caudal margin and tubercles on the lateral surfaces that form rows parallel to the ventral margin.

Description

Head Shield (Figs. 6–8, 11F). The pattern of sensory lines and bones in the head shield of Turrisaspis elektor is similar to other Late Devonian groenlandaspidids in which head shields are known (Tiaraspis, G. riniensis, and G. antarctica). The supraorbital sensory line begins in the centre of the rostral margin of each preorbital plate and

terminates within the central plate. Unreported from other taxa, the supraorbital sensory line on some Turrisaspis specimens, including the holotype, re-appears just caudal to the middle of the central plate and continue across the paranuchal plate to the caudal margin of paranuchal where it terminates near the end of the main lateral line. The infraorbital sensory line starts where the orbit meets the postorbital plate and divides in the centre of the postorbital plate. The central sensory line, the medial branch of the infraorbital sensory line, extends onto the central plate and terminates there. The main lateral line, the lateral branch of the infraorbital sensory line, terminates at the caudal margin of the paranuchal plate.

ANSP 20705 (Fig. 7) preserves the head shield in 3-dimensions with parts of the lateral (AL) and ventral trunk shield (IL, AMV, AVL, and partial SP). The head shield of Turrisaspis elektor has a pineal plate that is divided into two separate elements, a character also seen in some undescribed Australian forms (A. Ritchie, pers. comm.). The APi is roughly square and has a distinct indentation in the centre of the visceral surface (see Fig. 8A,B). A similar indentation is seen on the Pi of Groenlandaspis disjectus (Ritchie, 1974) and Tiaraspis subtilis (Schultze, 1984). The PPi of Turrisaspis elektor is roughly triangular in ANSP 20705, but it is smaller and approaching oval-shaped in other specimens.

The relative size and shape of the two pineal

elements is variable among the sample of

Turrisaspis head shields but two pineal

elements are present in every specimen

examined. Schultze (1984) illustrates

a small single pineal in Tiaraspis

subtilis and a space not covered

by bone between the pineal and

the rostral. It seems possible

that two pineal elements may have been present in Tiaraspis subtilis but that the APi was missing from the specimen that Schultze (1984) described. This contention is perhaps contradicted by the presence of an indentation on the visceral surface of the APi of Turrisaspis and on the caudally placed Pi of Tiaraspis .

The pattern of head shield elements in Turrisaspis is quite like Groenlandaspis antarctica as illustrated by Ritchie (1975). Turrisaspis has a V-shaped caudal margin of the head shield unlike the relatively flat caudal margins of Tiaraspis and G. riniensis . The nuchal element in Tiaraspis narrows dramatically toward the caudal margin, unlike the others in this discussion. Unique to Turrisaspis among the known groenlandaspidid head shields is contact between the postmarginal and postorbital plates, excluding the marginal plate from the lateral margin of the head shield. In ANSP 20705 ( Fig. 7 ) the marginal plate appears to reach the lateral margin on the left side of the head shield but that is an artifact of crushing in that area. Ornamentation of evenly-spaced tubercles cover the entire head shield of Turrisaspis elektor .

Two specimens that were prepared by acid etching show details of the feeding apparatus on the dermal bones of the palate and rostrum. ANSP 20748 ( Fig. 8A ) includes a mold of the palatal surface showing four elongate, denticulated surfaces ( Fig. 8A,B ). The raised surface in the centre of the palatal area is interpreted as the ventral surface of the parasphenoid and the surfaces on either side of the parasphenoid are interpreted as supragnathals. The crescentshaped, denticulated surface on the anterior-most margin of the mouth is interpreted as the anteroventral margin of the rostral plate ( Fig. 8C ). ANSP 21137 (not figured) shows the same morphology of feeding apparatus, although the denticulated surface on the ventral surface of the parasphenoid is broader.

Appendix 2 provides numerical data for the 10 measurable head shields of Turrisaspis elektor in the Red Hill sample. The specimens are usually flattened and thus distorted to varying degrees. The head shields in this sample show a range of lengths from 2.2 to 4.7 cm (measured from the anterior-most point on the rostral plate to the caudal-most point on the nuchal). The width of the head shields range from 2.2 to 4.3 cm (measured between the lateral-most points of the postmarginal plates). The head shield of Turrisaspis is almost as wide as long (average W/ L equals 0.96). Other groenlandaspidid head shields (known from Tiaraspis , G. riniensis , and G. antarctica ) have relatively longer skulls, but no statistical comparisons were done for this study.

Lateral Trunk Shield

ADL ( Figs. 9G–I , 11C ). The ADL of Turrisaspis elektor has a rhomboidal shape. The sample of seven measurable ADLs (listed in Appendix 2) shows variation in size and proportions of this element. The height of the ADL plates varies from 1.5 to 3.8 cm, and the average height/length ratio is 1.95 (measured as shown in Fig. 12 ). The dorsal and anteroventral margins are steep and the anteroventral and posterodorsal corners form acute angles. The anterodorsal margin of the ADL forms an angle of between 50° and 60° with the sensory canal. The caudal margin is concave where it overlaps the PDL. The ornamentation on the lateral surface consists of evenly distributed tubercles. There is a raised crest running from the condyle to the centre of the caudal margin (see Fig. 11C ). This character is used by Long et al. (1997) to separate Africanaspis from Tiaraspis . A comparison of these high-spired taxa with Turrisaspis is presented below.

PDL ( Figs. 9E,F ; 11B ). The PDL is tall and narrow, with an average height/length of 2.42. The sample of five measurable PDLs is listed in Appendix 2. The caudal margin of the PDL does not have the pronounced posterodorsal projection seen in the genus Groenlandaspis ( Fig. 5 ). Instead, the caudal margin of the Turrisaspis PDL angles sharply upward at the posterodorsal corner of the element, creating a relatively vertical caudal margin and excluding the overlap zone of the MD from reaching the caudal margin. The posterior and anterior segments of the dorsolateral canal meet at an angle of 70°. Ornamentation of small tubercles are evenly distributed on the dermal surface of the element. In visceral view there is a tall, narrow symphysial area in the dorsal region of the PDL. The angle formed by this symphysial surface and the body of the PDL is about 30°, suggesting that the two sides of the trunk diverged at an angle of 60° from beneath the MD. The height of the symphysial surface is 40% of the height of the entire PDL.

AL ( Figs. 9L–N , 11E ). All specimens of the anterior lateral plate are longer than they are high. The sample of 11 measurable ALs is listed in Appendix 2. Height to length ratios vary from 0.55 to 0.83, with the higher ratios occurring in the smaller individuals such as ANSP 20947 ( Fig. 9L ). Shape change of the AL during ontogeny may indicate a change in the angle of the head shield relative to the trunk. The caudal margin is concave, and the cranial margin slopes steeply to a ventral point. The dermal surface has a raised crest that slopes upward from the anteroventral tip and changes angle cranial to the centre of the element, continuing to the posterodorsal corner of the element. The ornamentation on the dermal surface consists of tubercles that are evenly distributed or sometimes arranged in roughly concentric rows.

PL ( Figs. 9J,K ; 11D ). The PL of Turrisaspis elektor is a small, triangular element with a large overlap zone ventrally for the dorsal lamina of the PVL and the AL. On the visceral surface there is a pronounced thickened rim along the ventral margin. The sample of seven measurable PLs is listed in Appendix 2. The ornament on the lateral surface consists of tubercles and crests.

MD ( Figs. 9A–D , 11A ). The MD of Turrisaspis elektor is high and narrow when compared to the genus Groenlandaspis . The apex of the MD makes an angle of 30° to 40°. Numerous coarse serrations line the caudal margin. The caudal margin of the MD is nearly vertical and sometimes actually slopes back so that the apex of the MD is the most posterior part of the element ( Fig. 9D ). In specimens with associated trunk shield elements, the height of the MD is roughly equal to the height of the PDL. The ornamentation on the MD consists of tubercles that are evenly distributed across most of the element or arranged in sinuous rows that are roughly parallel to the ventral margin.

The large sample size of MDs from Turrisaspis elektor (29 measurable specimens listed in Appendix 2) exhibits variation in size and shape. The large sample size provides an opportunity to confirm continuous variation within the sample and to examine intraspecific variation and ontogenetic change in this taxon. Figure 10 plots the height and length for the sample of measurable median dorsal plates of Turrisaspis elektor from Red Hill. Height was measured as the distance from the ventral margin of the MD (at the apex of the notch for the PDL) to the apex of the element. Length of the MD was measured perpendicular to the height measurement at the level of the apex of the notch for the PDL (see inset of Fig. 10). The distribution of points in Fig. 10 suggests that the size variation is continuous, arguing against an interpretation of more than one taxon within the sample, based on size. The variation around the best-fit line in Fig. 10 illustrates intraspecific and ontogenic variation in the shape of the MD, as captured by the simple height and length measurements described above. Individual variation is also evident in the MD sample with respect to details of the serrations along the caudal margin, the development of ornament, and the slopes of the caudal and cranial margins (see Fig. 9A–D). Figure 10 also plots the three measurable specimens of the MD of Groenland- aspis pennsylvanicus. The distribution of these points clearly falls outside the range of variation in Turrisaspis.

Ventral Trunk Shield (Fig. 9T). The ventral trunk shield of Turrisaspis elektor is known from five articulated specimens and 23 measurable isolated elements. ANSP 20688A (Fig. 9T) is a well preserved example of an articulated ventral trunk shield. The pattern and relative proportions of the elements are like the ventral trunk shield of Groenlandaspis antarctica (Ritchie, 1975, fig. 2b),

although Turrisaspis elektor is significantly smaller.

AVL ( Fig. 9 O,P ). TheAVL of Turrisaspis elektor is represented by 12 isolated specimens (list and measurements in Appendix 2). Within the sample of AVLs there is variation in the proportions of the element (width to length ratio varies from 0.62 to 1.0). The AVL has a straight lateral margin and a Vshaped depression that opens to the caudal margin. Ornamentation of fine tubercles are evenly distributed or arranged in rows following the margins of the plate.

PMV ( Fig. 9Q ). The PMV of Turrisaspis elektor is represented by seven isolated specimens (list and measurements in Appendix 2). The PMV is coffin-shaped, with a wide margin where it is overlapped from all sides. The PMV is ornamented with fine tubercles on the dermal surface.

PVL ( Fig. 9R ). The PVL of Turrisaspis elektor is represented by one isolated specimen (list and measurement in Appendix 2). On the cranial margin there is a distinct, crescent-shaped overlap area where the AVL overlaps the PVL. It appears that the left PVL overlaps the right PVL along part of the midline, although no isolated right PVL was present to observe the nature of the overlap area. The lateral flange of the PVL is angled at about 130° relative to the ventral portion of the plate. Ornamentation of fine tubercles are evenly distributed or arranged in rows following the margins of the plate.

SP ( Fig. 9S ). The SP of Turrisaspis elektor is represented by three isolated specimens. The spinal plate of Turrisaspis elektor is covered with tubercles that also cover the lateral margin, giving it a serrated appearance. Coarse, slightly forward-pointing serrations are particularly well developed along the medial margin at the caudal end of the SP. Data for the three measurable SPs are listed in Appendix 2. Width of the spinal plate was taken at the midpoint along its length.

Comparison to other groenlandaspidids

The large sample size for Turrisaspis elektor allows for recognition of variation in size and shape of the dermal elements. The authors feel justified in interpreting this material as a single taxon because the variation within each element is continuous as was demonstrated for the MD in Fig. 10. Although other elements have smaller sample size, they also exhibit continuous variation within different sized specimens. Importantly, this continuous variation in Turrisaspis does not extend to specimens that have been described as Groenlandaspis pennsylvanica and the adults of the two taxa are very different in size. Some elements such as the AL have similar shapes in the two taxa found at Red Hill and in these cases we may be unable to distinguish a juvenile of G. pennsylvanica from T. elektor . In other cases, elements such as the MD and PDL have very different shapes and ornament in each Red Hill taxon and identification of the two taxa is unambiguous.

The tall median dorsal plate of Turrisaspis elektor is similar to the tall MD plate seen in Tiaraspis sp. ( Gross, 1965), Tiaraspis subtilis Gross, 1933 , and Africanaspis doryssa Long et al., 1997 . The isolated, high-spired MDs of Tiaraspis sp. that were reported from the Late Devonian (Famennian) of Belgium by Gross (1965) have ornamentation of tubercles arranged in wavy rows and coarse serrations along the caudal margin, remarkably like the MD of Turrisaspis . Gross referred these Belgian specimens to Tiaraspis sp. but was skeptical about their provenance since the Famennian was beyond the known temporal range for Tiaraspis . The presence of the high-spired Turrisaspis from the Catskill Formation in Pennsylvania confirms that such forms were present in Euramerica during the Famennian and may remove doubt about the provenance of the Belgian specimens. In Turrisaspis and Tiaraspis subtilis the MD is approximately the same height as the PDL. Many features of Tiaraspis subtilis , however, such as the shape of lateral trunk elements and the nature of the head shield elements are quite unlike Turrisaspis elektor . A close relationship between these two taxa is also improbable due to the long temporal gap between their occurrences. In Africanaspis doryssa the MD is about twice the height of the PDL and the symphysial surface on the visceral side of the PDL is nearly half of the height of the entire element ( Long et al., 1997). Although a raised crest running from the condyle of the ADL to the centre of the caudal margin is seen in both Africanaspis and Turrisaspis , the ADL and PDL of Africanaspis are not shortened cranially-caudally to the dramatic extent seen in Turrisaspis . Therefore, although Turrisaspis elektor , Tiaraspis subtilis and Africanaspis doryssa share the character of a tall MD with a H/L ratio greater than one, other aspects of their morphology suggest that they are not closely related. With the present knowledge of the phylogenetic relationships within groenlandaspidids it is not clear if a tall MD has evolved several times in groenlandaspidids or only once.

Turrisaspis elektor differs from all species of Groenlandaspis and from Boomeraspis by the high-spired MD, and the tall, narrow plates of the lateral trunk shield. Despite the fact that the dorsolateral plates of the trunk shield are quite unique in Turrisaspis elektor , the head shield and ventral trunk shield are similar to Groenlandaspis antarctica . This may indicate that Turrisaspis is more closely related to Groenlandaspis than to the high-spired, but otherwise distinctive, Tiaraspis and Africanaspis .

Figures

Fig. 6. Turrisaspis elektor, holotype,ANSP 20961, articulated specimen in right lateral view(headshield in dorsolateral view).A, specimen on matrix block; B, whitened latex peel made from counterpart block;C, line drawingof specimen using part and counterpart.Scale bar =2 cm.

Fig. 7. Turrisaspis elektor, ANSP 20705. A, line drawing of head shield in dorsal view; B, dorsal view of head shield and anterior lateral plate; C, ventral view of head and partial trunk shield. Scale bar = 1 cm.

Fig. 8.ANSP 20748,Turrisaspis elektor. A, whitened,acid-prepared mold of the ventral surface of the dorsal head shield and medial surfaces of the lateral trunk shields; B, close-up of whitened latex peel of denticulated surfaces on the dermal bones of the palate; C, interpretive drawing of denticulated surfaces. Scale bar = 1 cm.

Fig. 9. Trunk plates of Turrisaspis elektor. Median dorsal plates (MD): A, ANSP 20687; B, ANSP 20714; C, ANSP 20707; D, ANSP 20703. Posterior dorsolateral plates(PDL):E,ANSP 20742,lateral aspect;F,ANSP 20741,lateral aspect.Anterior dorsolateral plates (ADL):G,ANSP 20946,lateral aspect;H,ANSP 20766,medial aspect;I,ANSP 20754,medial aspect.Posterior lateral plates(PL):J,ANSP 20952,lateral aspect; K, ANSP 20953, lateral aspect.Anterior lateral plates (AL): L, ANSP 20947, lateral aspect; M, ANSP 20726, lateral aspect; N,ANSP 21328, lateral aspect.Anterior ventrolateralplates(AVL):O,ANSP 20768,ventral aspect;P,ANSP 20681,ventral aspect.Posterior medianventral plate (PMV):Q,ANSP20695,ventral aspect.Posterior ventrolateral plate (PVL):R,ANSP20684, ventralaspect.Spinalplate (SP):S,ANSP20770. Articulated ventral trunk shield:T,ANSP 20688A.All specimens whitened with ammonium chloride.Scale bar =1 cm.

Fig. 11. Reconstructed headshield, lateral plates

Fig. 12. Methods for deriving measurements of isolated trunk elements and head shields presented in Appendix 1 and 2.

Fig. 1. Location and stratigraphic framework of the Red Hill site, Clinton County, Pennsylvania, United States of America. Twometer-thick fossiliferous zone marked with an asterisk.

Fig. 5. Comparison of posterior dorsolateral plates (PDL) from groenlandaspidids. A, Groenlandaspis pennsylvanica n.sp.; B, G. antarctica; C, G. disjectus; D, G. mirabilis; E, G. seni; F, G. riniensis; G, Boomeraspis goujeti; H, Tiaraspis subtilis; I, Africanaspis doryssa; J, Turrisaspis elektor n.gen. and n.sp. Scale bar = 2 cm.