Aletopelta (; meaning 'wanderer shield') is a monospecific genus of basal ankylosauridae dinosaur from Southern California that lived during the Late Cretaceous (upper Campanian stage, 75.5 Ma) in what is now the Point Loma Formation. The type and only species, Aletopelta coombsi, is known from a partial skeleton preserving osteoderms. It was originally described in 1996 by W. P. Coombs, Jr. and T.A. Deméré before being named in 2001 by Tracy Ford and James Kirkland. Aletopelta has an estimated size of 5 metres (16 feet) and weight of 2 tonnes (4,409 lbs). The holotype formed a miniature reef and was scavenged upon by invertebrates and sharks.
Discovery and naming
In 1987, construction work was done on the College Boulevard near Carlsbad at the Californian coast. While paleontologically surveying the work, Bradford Riney noted that a skeleton had been uncovered by a ditch dug for a sewage pipe. Within days, the specimen was secured by the San Diego Natural History Museum. It was dubbed the "Carlsbad Ankylosaur".
[W.P. Coombs, Jr. and T.A. Deméré, 1996, "A Late Cretaceous nodosaurid ankylosaur (Dinosauria: Ornithischia) from marine sediments of coastal California", Journal of Paleontology 70(2): 311-326] The skeleton was later described, but not named, in 1996 by Thomas Deméré and Walter Preston Coombs before being named in 2001 by Tracy Lee Ford and James Kirkland.
[T.L. Ford and J.I. Kirkland, 2001, "Carlsbad ankylosaur (Ornithischia, Ankylosauria): an ankylosaurid and not a nodosaurid", In: The Armored Dinosaurs. Indiana University Press, Bloomington pp 239-260] The skeleton originated from a layer of the marine Point Loma Formation which dates to the upper
Campanian stage, 75.5 Ma. The formation has yielded specimens pertaining to calcareous nannoplankton, foraminifers, scaphopods, pelecypods, gastropods, cephalopods, ostracods, decapods, echinoids, elasmobranchs, and actinopterygians, with the addition of a
femur, right
dentary containing
teeth and cervical
vertebrae of a
hadrosauridae.
[Hilton ( Dinosaurs and Other Mesozoic Reptiles of California, 2003), pp. 233-36.] The type and only known specimen of
Aletopelta was once a bloated carcass that had been washed out to sea, likely by a stream, which sank to the bottom with its underside facing upwards and became a miniature
reef, as evidence by Pelecypoda such as
Ostrea sp. and
Spondylus sp. being attached to the bones and the presence of
ammonites and gastropods found in association with the skeleton. The carcass was also scavenged upon by marine invertebrates and sharks such as
Squalicorax and
Scapanorhynchus, which resulted with most of the long bones being hollow and many shallow pits on the osteoderms and ribs.
The
holotype specimen,
SDNHM 33909, consists of
teeth, fragmentary
, partial
humerus, partial
ulna, possible fragment of right radius,
ulna, partial left and possibly right
ischium,
femur,
,
, four or five partial
vertebrae, dorsal neural arch, neural arches of the
sacrum, fragmentary
,
osteoderms including
pelvis shield and cervical half ring. The type specimen may represent an immature individual based on the unfused
Talus bone, partly fused scutes and unfused neural spines.
The generic name, Aletopelta, is composed of the Ancient Greek words " aletes" (wanderer) and " pelte" (small shield), in reference to its armour and the fact that the tectonic plate containing the Peninsular Ranges Terrane was somewhere opposite the middle of Mexico in the Cretaceous, only moving northward due to plate tectonics, carrying the holotype specimen with it. The specific name, coombsi, honours the vertebrate palaeontologist Walter Preston Coombs, Jr., for their work and years of research.
Description
Size and distinguishing traits
Ford & Kirkland (2001) originally gave
Aletopelta an estimated length of 6 metres (19.7 feet).
However, Gregory S. Paul gave a lower estimate of 5 metres (16 feet) long and weight of 2 tonnes (4,409 lbs) in 2010.
[Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 233]
Ford and Kirkland (2001) originally diagnosed Aletopelta based on the teeth being wider than tall; the femur being much longer than both the tibia and fibula; the presence of three metatarsals; a pelvic shield composed of polygonal, low-peaked osteoderms; a large short-pointed spike in the shoulder region; the presence of hollow cap-like osteoderms across dorsum; the presence of hollow pup-tent-like osteoderms over neck and shoulders; triangular, dorsally compressed caudal osteoderms that are highly asymmetrical from top to bottom; and hollow and thin osteoderms.
Classification
Deméré & Coombs (1996) originally interpreted SDNHM 33909 as an indeterminate
nodosauridae that showed similarities with
Edmontonia,
Panoplosaurus and
Stegopelta but refrained from naming it as they considered it to be undiagnostic.
Ford (2000) considered that SDNHM 33909 formed the clade Stegopeltinae, along with
Stegopelta and
Glyptodontopelta, within
Ankylosauridae based on the presence of a pelvic shield composed of co-ossified hexagonal osteoderms.
[T.L. Ford. (2000). "A review of ankylosaur osteoderms from New Mexico and a preliminary review of ankylosaur armor", In: S. G. Lucas and A. B. Heckert (eds.), Dinosaurs of New Mexico. New Mexico Museum of Natural History and Science Bulletin 17: 157-176] Ford & Kirkland (2001) also reinterpreted it as an ankylosaurid and considered it to be taxonomically distinct enough to be warranted as a new genus and species.
Vickaryous et al. (2004) considered
Aletopelta as a
nomen dubium due to the lack of diagnostic characteristics.
Arbour & Currie (2015), however, considered it as a valid genus based on new characteristics and found Stegopeltinae to not be a valid clade. Arbour & Currie (2015) additionally recovered
Aletopelta within a
polytomy containing
Liaoningosaurus,
Cedarpelta,
Chuanqilong and more derived ankylosaurids.
A similar position was recovered by Arbour & Evans (2017) and Rivera-Sylva et al. (2018) but also placed
Hylaeosaurus,
Crichtonpelta,
Gobisaurus and
Shamosaurus within the polytomy.
Park et al. (2019) also placed
Aletopelta within a polytomy but with the inclusion of
Jinyunpelta,
Nodosaurus and
Kunbarrasaurus.
Although most often being recovered as a basal ankylosaurid, Frauenfelder et al. (2022) found it to be the basalmost nodosaurid.
The results of a phylogenetic analysis conducted by Arbour & Currie (2015) is reproduced below.
See also
-
Timeline of ankylosaur research
External links
