[D] Apatornis celer [~/~]
Describer
Marsh, 1873
Time
Cretaceous Late Coniacian Santonian Campanian
Classification
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Ornithurae
Fossilsite
Niobrara Chalk Formation, Kansas, US
Info
Apatornis celer (Marsh, 1873) = Ichtyornis celer (Marsh, 1873)
Holotype: YPM 1451 is a sacrum lacking its proximal end (Marsh, 1880)
Locality and horizon: Marsh (1880: 192) specified that he collected the specimen in October 1872, from Butte Creek, a locality discussed with reference to Ichthyornis agilis. Butte Creek, also known as Twin Butte Creek, is a well-known locality of Logan County, Kansas, in the Smoky Hill Chalk Member, Niobrara Formation (Stewart et al., 1990), representing (Bennett, 1990) beds between Marker Units 15 and 19 of Hattin (1982). Stewart (1990) estimated that the interval containing Hattin’s (1982) Marker Units 8–10 was upper Santonian in age. Thus, Marker Units 15–19 must represent, at the earliest, a period of time later in the late Santonian through the early Campanian. The top of the Smoky Hill Chalk Member, close to Marker Unit 23 of Hattin (1982), is in the early Campanian (Stewart et al., 1990).
Discission: Marsh (1873a) originally differentiated the holotype (YPM 1451) from Ichthyornis dispar by more slender proportions and a more deeply concave posterior articular surface on the last vertebra of the sacrum. Neither of these characters was confirmed to differ discernibly in the holotype of Ichthyornis dispar. Marsh (1880), however, noted three additional differences from Ichthyornis dispar that are confirmed here.
First, while both holotypes preserve 10 remnants of transverse processes, indicating the presence of 10 fused sacral vertebrae, that of Apatornis celer is incomplete anteriorly (Marsh, 1880: 162). The sacrum of Apatornis celer had a least one more fused sacral vertebrae than Ichthyornis dispar.
Spinal nerve openings visible on the anterior edge of the first preserved vertebra indicate that the series continued anteriorly. Marsh also commented on a difference in the number of vertebrae with a particular morphology that occurs in approximately the middle of the sacral series (Marsh, 1880: 162–163). There is a difference in the number of sacral vertebrae appearing to lack transverse processes in the middle of the series. The transverse processes appear to be oriented directly dorsally (appendix 1, character 62).
Apatornis celer has four such sacrals between a short, blunt, morphologically distinct transverse process anteriorly and a costal strut at the level of the acetabulum, while Ichthyornis (YPM 1450; YPM 1732) has three. In a preliminary survey of extant birds, this count appeared even more conservative than total sacral number.
Finally, Marsh (1880: 163) noted that the Apatornis celer holotype does not have coossified tendons expanding ‘‘posteriorly over as broad a region [of the dorsal surface of the sacrum] as in Ichthyornis.’’ The last noted difference could also be described as the absence of a fan of ossified tendons in Apatornis celer that covers the posterior half of the dorsal surface of the Ichthyornis dispar holotype sacrum. Based on these three characters from Marsh (1880), Apatornis celer can be differentiated from Ichthyornis dispar. However, the few characters preserved, rather than suggesting that Apatornis celer is particularly close to Ichthyornis dispar as originally proposed (Marsh, 1873b), place Apatornis celer more closely to Aves than to Ichthyornis. (Clarke, 2004)
Marsh, 1873
Time
Cretaceous Late Coniacian Santonian Campanian
Classification
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Ornithurae
Fossilsite
Niobrara Chalk Formation, Kansas, US
Info
Apatornis celer (Marsh, 1873) = Ichtyornis celer (Marsh, 1873)
Holotype: YPM 1451 is a sacrum lacking its proximal end (Marsh, 1880)
Locality and horizon: Marsh (1880: 192) specified that he collected the specimen in October 1872, from Butte Creek, a locality discussed with reference to Ichthyornis agilis. Butte Creek, also known as Twin Butte Creek, is a well-known locality of Logan County, Kansas, in the Smoky Hill Chalk Member, Niobrara Formation (Stewart et al., 1990), representing (Bennett, 1990) beds between Marker Units 15 and 19 of Hattin (1982). Stewart (1990) estimated that the interval containing Hattin’s (1982) Marker Units 8–10 was upper Santonian in age. Thus, Marker Units 15–19 must represent, at the earliest, a period of time later in the late Santonian through the early Campanian. The top of the Smoky Hill Chalk Member, close to Marker Unit 23 of Hattin (1982), is in the early Campanian (Stewart et al., 1990).
Discission: Marsh (1873a) originally differentiated the holotype (YPM 1451) from Ichthyornis dispar by more slender proportions and a more deeply concave posterior articular surface on the last vertebra of the sacrum. Neither of these characters was confirmed to differ discernibly in the holotype of Ichthyornis dispar. Marsh (1880), however, noted three additional differences from Ichthyornis dispar that are confirmed here.
First, while both holotypes preserve 10 remnants of transverse processes, indicating the presence of 10 fused sacral vertebrae, that of Apatornis celer is incomplete anteriorly (Marsh, 1880: 162). The sacrum of Apatornis celer had a least one more fused sacral vertebrae than Ichthyornis dispar.
Spinal nerve openings visible on the anterior edge of the first preserved vertebra indicate that the series continued anteriorly. Marsh also commented on a difference in the number of vertebrae with a particular morphology that occurs in approximately the middle of the sacral series (Marsh, 1880: 162–163). There is a difference in the number of sacral vertebrae appearing to lack transverse processes in the middle of the series. The transverse processes appear to be oriented directly dorsally (appendix 1, character 62).
Apatornis celer has four such sacrals between a short, blunt, morphologically distinct transverse process anteriorly and a costal strut at the level of the acetabulum, while Ichthyornis (YPM 1450; YPM 1732) has three. In a preliminary survey of extant birds, this count appeared even more conservative than total sacral number.
Finally, Marsh (1880: 163) noted that the Apatornis celer holotype does not have coossified tendons expanding ‘‘posteriorly over as broad a region [of the dorsal surface of the sacrum] as in Ichthyornis.’’ The last noted difference could also be described as the absence of a fan of ossified tendons in Apatornis celer that covers the posterior half of the dorsal surface of the Ichthyornis dispar holotype sacrum. Based on these three characters from Marsh (1880), Apatornis celer can be differentiated from Ichthyornis dispar. However, the few characters preserved, rather than suggesting that Apatornis celer is particularly close to Ichthyornis dispar as originally proposed (Marsh, 1873b), place Apatornis celer more closely to Aves than to Ichthyornis. (Clarke, 2004)