Fauna

Sacabambaspis es un género extinto de peces agnatos (sin mandíbulas) que vivió enPeríodo Ordovícico Superior. Está relacionado con Astraspis.Su tamaño era de aproximadamente 30 cm. Su cabeza era ancha, con los ojos muy juntos en la parte delantera.^2 3 Las partes superior e inferior de la coraza de la cabeza estaban demarcados por unas 20 placas más pequeñas a cada lado, entre las cuales se escondían las agallas. El cuerpo, ahusado, acababa en una única aleta caudal que se extendía más allá de otras dos aletas, dorsal y ventral, y en una extensión del notocordio con una pequeña aleta al final. Al carecer de aletas direccionales, es probable que no fuera un buen nadador.

The tail of the earliest known articulated fully skeletonized vertebrate, the arandaspidSacabambaspis from the Ordovician of Bolivia, is redescribed on the basis of further preparation of the only specimen in which it is most extensively preserved. The first, but soon discarded, reconstruction, which assumed the presence of a long horizontal notochordal lobe separating equal sized dorsal and ventral fin webs, appears to have considerable merit. Although the ventral web is significantly smaller than the dorsal one, the presence of a very long notochordal lobe bearing a small terminal web is confirmed. The discrepancy in the size of the ventral and dorsal webs rather suggests that the tail was hypocercal, a condition that would better accord with the caudal morphology of the living agnathans and the other jawless stem gnathostomes.

Keywords:

1. Introduction

The anatomy of the earliest known articulated vertebrate possessing an extensive dermal skeleton, Sacabambaspis janvieri (Gagnier, Blieck & Rodrigo, 1986), from the Ordovician (Llanvirn and Caradoc) of South America, has been described in detail by Gagnier (1993a), on the basis of several specimens from the locality of Sacabambilla, Cochabamba area, Bolivia. Although the head armour, body scales and histology of this ‘ostracoderm’ (armoured jawless vertebrate) are now relatively well known (Gagnier 1993a,b; Sansom et al. 2005), the morphology of its caudal fin remains a puzzle and has been interpreted in a number of different ways. Further preparation of the only specimen that displays the caudal fin web now allows its reconstruction, which lends support to Gagnier's (1989) long debated reconstruction although with some modification, and provides clear evidence for the structure of the oldest recorded ostracoderm tail fin.

2. Material and methods

The material considered comes from the Ordovician (Caradoc) Anzaldo Formation of Bolivia. The articulated Sacabambaspis material from Sacabambilla consists of a number of three-dimensional specimens preserved in a very large concretion and, at least, six dorsoventrally flattened specimens preserved in a large sandstone slab. The specimens are housed in the Museo de Historia Natural Alcide d'Orbigny (MHNC), Cochabamba and (as a temporary deposit) in the Museum National d'Histoire Naturelle, Paris. The specimen MHNC 1182 (figure 1a), which displays the caudal fin, comes from the sandstone slab and has been further prepared by removing a small part of the overlying head shield of another, neighbouring, articulated specimen (MHNC 1180). The dermoskeleton of the caudal region has been removed with dilute hydrochloric acid, an elastomer cast of the resulting external mould made, whitened with magnesium and photographed.

• Download figure
 
• Open in new tab
 
• Download powerpoint

Figure 1

Sacabambaspis janvieri Gagnier et al. 1986, Ordovician (Caradoc), Anzaldo Formation, Sacabambilla Bolivia. (a) Specimens MHNC 1182 and 1180 in ventral view, caudal region outlined (thick white line) and median ventral ridge indicated (thin white line). (b–f, h) Elastomer cast of a portion of the counterpart of MHNC 1182 and 1180. (b) General view of the caudal region of MHNC 1182 and part of the ventral shield of MHNC 1180 (partly removed to expose the tip of the notochordal lobe). (c) Explanatory drawing of the part of the caudal region in (b). (d) Ventral and dorsal fin webs. (e) Detail view of the parallel scale series in the presumed dorsal web. (f) Detail view of the proximal part of the notochordal lobe; compare with the epibranchial plate of MHNC 1011 illustrated in (g). (h) Posterior end of the notochordal lobe and adjacent fin ray-like scale rows of the terminal fin web. Scale bars, (a) 100 mm, (b–h) 10 mm. Abbreviations: brpl, branchial plates; bsc, body scales; ?dfw, probable dorsal fin web or epichordal lobe; epl, epibranchial plate;le, leading edge of the fin web; mvr, median ventral ridge; ncl, notochordal lobe; tfw, terminal fin web; ?vfw, probable ventral fin web or anal fin.

Los radiodontos (Radiodonta) son un taxón propuesto para contener tanto los anomalocáridos (como Anomalocaris ySchinderhannes) como los Opabinidae (siendo Opabinia el único conocido). Otro género es Kerygmachela, que aunque no es un anomalocárido, se considera miembro de los radiodontos.

(Radiodonta), in the arthropod stem group.

FromAcute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes

• John R. Paterson,
• Diego C. García-Bellido,
• Michael S. Y. Lee,
• Glenn A. Brock,
• James B. Jago
• & Gregory D. Edgecombe

Nature

 

480,

 

237–240

 

(08 December 2011)

 

doi:10.1038/nature10689

Numbers refer to the inclusiveness of the monophyletic group that can be confidently inferred to possess compound eyes: (1) based on extant taxa alone; (2) based on discovery of Schinderhannes¹⁹; and (3) based on new data herein. Phylogeny after ref. 19.