Groenlandaspis pennsylvanica, 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: 48-51
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Holotype ( Fig. 2B View Fig ). ANSP 20755 View Materials , median dorsal plate ( MD) in right lateral view and partially exposed in left lateral view. Complete except for missing tip of the anteroventral projection. Ventral margin slightly crushed.
Other material. All of the material of this species consists of isolated trunk plates: 9 ALs (3 complete and 6 partial); 1 ADL (complete); 10 PDLs (5 complete and 5 partial); 4 MDs (2 complete and 2 partial); 1 AVL (partial); 1 PVL (partial). Specimen catalogue numbers and the measurements of complete elements are listed in Appendix 1.
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 View Fig ). Catskill Formation, Duncannon Member GoogleMaps .
Age. Late Devonian, late Famennian Stage (Fa2c substage); Grandispora cornata / Rugispora flexuosa palynomorph zone (Traverse, in press).
Etymology. Named for the State of Pennsylvania where this taxon was discovered and studied.
Diagnosis. The MD has a low profile (average H/L = 0.48) with ornament of coarse tubercles along the anterodorsal margin and covering the lateral surface of the posteroventral region with the largest tubercles on the posterodorsal margin. Ornament of fine ridges on the MD radiate from apex to the ventral margin. The dorsal margin of the ADL is steeply inclined with a narrow anterodorsal overlap zone for the MD. The lateral surface of the PDL is ornamented with fine tubercles that form ridges radiating from the bend in the sensory canal.
Remarks. The specimens referred to G. pennsylvanica occur as isolated plates within the 2-meter-thick fossiliferous zone at Red Hill. Although no articulated trunk armour has been discovered, the material is believed to be from a single taxon because the elements “fit together” both in terms of size and the shapes of plate margins and overlap zones. When compared to other groenlandaspidid material from the same fossiliferous zone at Red Hill, the G. pennsylvanica specimens are significantly larger and the trunk shield shape is much lower and longer. There is no overlap in the size or shape characteristics of most trunk elements between the two groenlandaspidid taxa at Red Hill.
ADL (anterior dorsolateral plate) ( Figs. 2C View Fig , 4C View Fig ). The ADL is known from a single specimen, ANSP 20743 ( Fig. 2C View Fig ), which is exposed in lateral view. This specimen is missing narrow parts of the caudal and ventral margins, although the natural mold of the overall shape of the element is preserved in the matrix. The dorsal and anteroventral margins are steeply inclined giving the entire element a tall, rhomboidal shape. It is difficult to quantify the shape of the ADL in a way that allows comparison to other taxa but for this paper we have derived a measure of the angle of the dorsal margin relative to the dorsolateral sensory canal on theADL (see Fig. 2 View Fig C'). OnANSP 20743, the angle formed by lines tracing the dorsal margin and the general trend of the dorsolateral sensory canal is 55°. The overlap zone for the median dorsal plate on the dorsal margin of the ADL is narrow along the anterodorsal margin and wider posteriorly. The ornament on the ADL consists of fine tubercles distributed evenly across the dermal surface. Height and length measurements for the single ADL are presented in Appendix 1.
PDL (posterior dorsolateral plate) ( Figs. 2D,E View Fig ; 4B View Fig ). The PDL is known from ten specimens (5 complete and 5 partial). The overlap area for the MD reaches the caudal margin at the tip of a pronounced posterodorsal projection. The dorsolateral sensory canal comes to a rounded apex slightly caudal to the centre of the element where its path bends at 90°. The ornamentation on the lateral surface of the PDL consists of rows of fine tubercles radiating from the bend in the sensory canal toward the edges. The medial surface of the PDL ( Fig. 2E View Fig ) has a central, thickened zone extending from the centre of the ventral margin to the symphysial area where the left and right sides of the trunk shield met beneath the median dorsal plate. The angle formed by this symphysial surface and the body of the PDL is about 35° suggesting that the two sides of the trunk diverged at an angle of 70° from beneath the MD ( Fig. 3 View Fig ). The height of the symphysial surface is about 30% of the total height of the PDL. Height and length measurements for the five complete PDL plates are presented in Appendix 1.
AL (anterior lateral plate) ( Figs. 2F,G View Fig ; 4D View Fig ). The AL is known from nine specimens (4 complete and 5 partial). The shape is rhomboidal with the length about twice the height. The caudal margin is concave, and the cranial margin slopes to a point ventrally. The lateral surface has a raised crest that slopes upward from the anteroventral tip and bifurcates slightly cranial to the centre of the element, forming two raised crests that diverge toward the caudal margin ( Fig. 2F View Fig ). The ornamentation on the lateral surface consists of fine tubercles randomly distributed and fine ridges near the ventral margin. The medial surface is smooth with contours mimicking the lateral surface ( Fig. 2G View Fig ). Height and length measurements for the three complete AL plates are presented in Appendix 1.
MD (median dorsal plate) ( Figs. 2A,B View Fig ; 4A View Fig ). The median dorsal plate of G. pennsylvanica is known from five specimens (3 complete and 2 partial). The general shape of the MD is long and low with a height to length ratio (using the measuring method shown in Fig. 10) of about 0.5. The blunt apex of the MD makes an 80° to 90° angle midway along the total length of the element. The anteroventral projection of the MD, where it overlaps the ADL, is long and narrow. Ornamentation of coarse tubercles is found along the anterodorsal margin and covers the posteroventral area of the element, with the largest elements of ornament along the posterodorsal margin. Ornamentation of fine striations radiating from the apex to the ventral margin covers the rest of the MD. Height and length measurements for the 3 complete MD plates are presented in Appendix 1.
Ventral Trunk. It is worth mentioning two incomplete ventral trunk elements here because they are the only examples of such material encountered. These specimens are judged to belong to G. pennsylvanica because they are significantly larger and morphologically distinct from material described later in this paper as Turrisaspis elektor . ANSP 21324 is a partial left AVL (anterior ventrolateral plate) with a length of at least 60 mm and a width of 38mm. The dermal surface is smooth except for a few scattered small tubercles near the centre of the plate, fine ridges along the posterolateral margin, and finely-reticulate ornament in the V-shaped depression in the posterolateral quadrant. ANSP 21288 is a partial right PVL (posterior ventrolateral plate) with a total length of at least 53 mm and a width of at least 42mm. Along the posterior half of the medial margin is a distinct overlap zone for the left PVL. Ornamentation of tubercles is well developed posteriorly, with the largest tubercles along the ridge where the lateral flange of the PVL angles dorsally. The tubercular ornament thins anteriorly to a smooth surface with only scattered tubercles. The aforementioned specimens,ANSP 21324 (AVL) and ANSP 21288 (PVL), occur in close proximity (3 mm) to one another on the same matrix block, but their association is uncertain.
Comparison to other species of Groenlandaspis
The elements of the trunk shield of Groenlandaspis pennsylvanica ( Figs. 2 View Fig , 4 View Fig ) are similar to those of the welldocumented taxon Groenlandaspis antarctica ( Ritchie, 1975, fig. 2; Denison, 1978, fig. 44A–C).There are, however, some morphological features of Groenlandaspis pennsylvanica that justify its status as a distinct species. The ADL of Groenlandaspis pennsylvanica has a more narrow and tall shape with a steeper dorsal margin than Groenlandaspis antarctica . The angle of the dorsal margin of Groenlandaspis antarctica , measured from published illustrations with the method shown in Fig. 2 View Fig C', is 42°, as opposed to 55° in Groenlandaspis pennsylvanica . This comparison is only tentative because of very limited sample size (n=1) for each taxon. The ornament on the lateral trunk elements of Groenlandaspis pennsylvanica consists of fine tubercles that are evenly distributed or in some areas, particularly the PDL, arranged in rows, forming fine ridges. The MD of Groenlandaspis pennsylvanica has the same general shape as Groenlandaspis antarctica but details of the ornament such as fine ridges radiating from the apex, and a narrow anteroventral projection, are distinct. Aside from the morphological differences, the early Frasnian age and Gondwanan occurrence of Groenlandaspis antarctica also suggest that the two taxa are not conspecific.
Comparisons to the Late Devonian Euramerican forms G. disjectus and G. mirabilis are confounded by the incomplete nature of those taxa. The PDL is known from many groenlandaspidids, however, and can provide informative comparisons among most of the groenlandaspidid taxa. Figure 5 View Fig provides a comparison of the size and shape of groenlandaspidid PDLs re-drawn from published accounts. As mentioned above, the PDL of G. pennsylvanica is very similar in shape to G. antarctica ( Fig. 5B View Fig ), but distinctive ornament characterizes each. The PDL of G. disjectus ( Fig. 5C View Fig ) and G. mirabilis ( Fig. 5D View Fig ) have different shapes than G. pennsylvanica ( Fig. 5A View Fig ), particularly in the dorsal margin and posterodorsal projection. The PDLs of other groenlandaspidid taxa ( Fig. 5E–J View Fig ) are very different in shape from G. pennsylvanica . Such a comparison does not illustrate the variation found within any of the taxa.
Groenlandaspis pennsylvanica is rare at the Red Hill site, known only from the aforementioned 27 isolated trunk plates. Smaller groenlandaspidid material with morphologies distinct from G. pennsylvanica is found in the same 2- meter-thick fossiliferous zone and is much more abundant. This smaller material is described below from 81 complete isolated trunk plates, 10 measurable head shields, and five partially articulated ventral trunk shields.
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