Morgula donosochagrense, Gracia & Correa-Metrio & Carvalho & Velez-Juarbe & P̆rik & & Kri, 2022

Gracia, Carlos De, Correa-Metrio, Alex, Carvalho, Monica, Velez-Juarbe, Jorge, P ̆ rik, Tom ́ a ̆ s, , Carlos Jarami & Kri, nd Jurgen, 2022, Towards a unifying systematic scheme of fossil and living billfishes (Teleostei, Istiophoridae), Journal of Systematic Palaeontology (2091959) 20 (1), pp. 1-36 : 11-13

† Morgula donosochagrense sp. nov.

( Figs 5 A – C, G, H, 6 A, B, 7 C)

Diagnosis. The new species is characterized by the following combination of morphological traits (autapomorphic characters are indicated by an asterisk): slender rostrum along its entire length combined with a circular cross-section at 0.25 L ( Fig. 6B); premaxillae divided into two separate bones for about 60% of the total rostral length$; the internal canals circular ( Table 3 ) in ventral position ( Fig. 6A, B); rostrum elongated triangular in dorsal view$; nasals angularly extended laterally ( Figs 2 A, 5 C)$.

Derivation of name. The species name ‘ donosochagrense ’ is coined from a combination of the words ‘Donoso’ and ‘Chagres’, the names of the two districts in which the outcrops of the Chagres Formation are located including parts of the natural reserve of Donoso.

Holotype. MUPAN–STRI31293, a nearly complete rostrum ( Fig. 5 A – C) with associated lower jaw lacking the predentary ( Fig. 5G–I).

Occurrence. The specimen was collected in 2011 from a sea cliff exposure on the beach of the town of Pina ( Fig. 1), about 20 metres west of 9 ǫ 9 Ɩ 16 ʺ N; 80 ǫ 2 Ɩ 71 ʺ W. The fossiliferous unit is ~ 5 m above a massive bioturbated layer that is exposed at tide level.

Remarks. The specimen was found in a weathered block above the tide zone. The surface of the fossil consequently is worn ( Fig. 2 A – C) and alveoli for denticles were not recognized. The specimen includes an associated lower jaw ( Fig. 2G–I) that was found in the same block during preparation.

The rostrum of † Morgula donosochagrense gen. et. sp. nov. is nearly complete and consists of prenasals, premaxillaries and a complete left nasal ( Fig. 5 A – C, G – I). The posterior edge of the left nasal is in the same transverse plane as the lateral ethmoid ( Fig. 2), which allows a comparison with other taxa ( Fierstine 1999a). The overall preserved length of the rostrum is 611 mm and only c. 13 mm of the distal tip are missing. The estimated total bill length therefore is 624 mm (Supplemental material 1). The rostrum’ s cross-section is oval at 0.5 L (307 mm from the tip) but circular at 0.25 L (155 mm from the tip [ Fig. 6A, B]; see the outrange ratio D2/W 2 in Table 1 ). The distal tip has a slender outline ( Figs 5 A – C, Supplemental material S1B), is acute and dorsoventrally compressed. The premaxillaries start to separate at 248 mm ( VSPM) measured from the distal tip ( Fig. 5 C). The VSPM distance represents approximately 30% of the total rostrum length and coincides with the point where the ventral canal starts to be visible in cross-section at 0.5 L ( Fig. 6A). The ventral aperture ( L – VSPM) represents 60% of the rostral length ( Fig. 6A). In dorsal view, the nasals are angularly oriented posterolaterally ( Fig. 7C). The internal canals are large, ventrally located, centrally aligned, and with a circular cross-section ( Table 3 ; Fig. 6A, B). The rounded internal canals that are ventrally placed are autapomorphies of † Morgula donosochagrense gen. et sp. nov. ( Fig. 6A, B), as is the triangular shape of the rostrum and the nasal bones that are angularly extended laterally in dorsal view ( Figs 5 A – C, 7 C). Other istiophorids have oval internal canals in a central or dorsal position (see Fig. 9 for examples and Supplemental material Fig. S2 for definitions).

Lower jaw. The associated lower jaw is represented by partial right and left dentaries ( Fig. 5G–I). In the articulation of the dentaries, two variables (DAD and DJL) were studied and converted into a ratio ( Fig. 3C). The ratio DAD/DJL for † Morgula donosochagrense gen. et. sp. nov. is 0.52 ( Table 1 ) and this value represents a shape that is more rectangular than quadratic distally where both dentaries join ( Figs 3C, 5I). The general appearance of the dentaries is slender with a height at the interdentary joint of 21 mm ( DAD distance) and of 38 mm at the level of the articular socket for the head of the quadrate. The right dentary is nearly complete with its proximal part missing ventrally for half of its length ( Fig. 5G), while its dorsal surface is completely preserved ( Fig. 5H). The left dentary is partially preserved but broken in its middle part at the location of the articular joint ( Fig. 5G, H). In dorsal view, the dentary is covered by denticles from its most distal part to about the level of the articular socket ( Fig. 5H). There is no further evidence of denticles from this point to the most proximal end. The surface that is covered with denticles presents circular structures of 3–4 mm that are possibly alveoli for larger teeth ( Fig. 5H). There are one and three of these structures visible on the right and left dentary, respectively ( Fig. 5H).