Hadrosaurus minor Marsh, 1870

Prieto-Márquez, Albert, Weishampel, David B. & Horner, John R., 2006, The dinosaur Hadrosaurus foulkii, from the Campanian of the East Coast of North America, with a reevaluation of the genus, Acta Palaeontologica Polonica 51 (1), pp. 77-98 : 91-93

Hadrosaurus minor Marsh, 1870 .

Holotype: YPM 1600 , an undetermined number of presacral vertebrae.

Referred specimens: YPM 1587, fragment of femur; YPM 1593, dorsal centrum; YPM 745, fragment of femoral shaft, distal half of a tibia, and other fragments; YPM 7896, numerous unidentified fragments; YPM 7898, a left coracoid; ANSP 10007, distal half of a femur; ANSP 15202, right pubis, left and right femora, fragment of left fibula, three caudal vertebrae, a chevron, a cranial dorsal, and three rib and other unidentified fragments.

Type locality and horizon: YPM 1600, YPM 745, and ANSP 10007: Barnsboro, Gloucester County; New Egypt (Navesink) Formation, Upper Cretaceous (Maastrichtian). YPM 1593: Mullica Hill, Gloucester County, New Jersey; New Egypt Formation, Upper Cretaceous. YPM 1587: Swedesboro, New Jersey; Upper Cretaceous, unrecorded strata, probably New Egypt Formation. YPM 153: Barnsboro, New Jersey; unrecorded horizon. YPM 7896 and 7898: West Jersey Marl County, Barnsboro, New Jersey; New Egypt Formation, Maastrichtian. ANSP 15202: Sewell, New Jersey; Navesink Formation, Upper Cretaceous.

Comments.—When Marsh named H. minor in 1870 he based his new taxon on non−diagnostic dorsal vertebral centra ( YPM 1600). Other specimens catalogued as H. minor at the Yale Peabody Museum collection are fragmentary, except for a left coracoid ( YPM 7898). The proportions and size of YPM 7898 indicate that it probably pertains to a juvenile individual. As in most hadrosaurid coracoids, the cranial and ventral processes are broken and the element is hook−like cranioventrally. Everything about this element conforms to the typical hadrosaurid coracoid and YPM specimens do not contain diagnostic characters.

In 1948, Colbert described ANSP 15202 and assigned this material to H. minor and not to H. foulkii due to the smaller size of the former and its much higher stratigraphic occurrence (the Maastrichtian Navesink Formation underlying the Cenozoic along the eastern regions of the sequence; Colbert 1948). He referred ANSP 15202 to Marsh’s taxon on the basis of the size of the elements and the same stratigraphic and close geographic occurrence of YPM 1600 and ANSP 15202. Colbert’s diagnosis consists of the small size of the elements relative to H. foulkii , the elongation of the proximal constriction of the prepubic process of the pubis ( Fig. 6B), and the pointed distal end of the ischium. We were unable to find the ischium in the ANSP collection.

The small size of H. minor should not be used for diagnosis because the remains may have belonged to a subadult individual. Muscle scars, such as those on the femoral fourth trochanter of ANSP 15202, are weakly developed compared to those observed in larger (possibly adult) hadrosaurids (viz., MOR 794, B. canadensis ). It is remarkable that the vertebrae with fused neural arches are caudals, whereas isolated centra lacking neural arches correspond to dorsals (one specimen from ANSP 15202 has its neural arch attached to the centrum with plaster). Among archosaurs, crocodiles have a neurocentral closure sequence that progresses during ontogeny in a caudal to cranial direction during ontogeny ( Brochu 1996). This sequence might also be present in hadrosaurids. Likewise, Colbert (1948) noted that the pubis of ANSP 15202 ( Fig. 6B) shows a strong resemblance with that of Edmontosaurus and, later, Baird and Horner (1977) later assigned H. minor to this hadrosaurine. While it is true that E. annectens (as exemplified by AMNH 5370, 5886, and MOR 985) has an elongated and narrow proximal prepubic process, E. regalis (e.g., CMN 2289) has a different, much thicker proximal prepubic process ( Fig. 7 ). Although the pubis of ANSP 15202 appears similar to those known from E. annectens , a quantification of the proportions of the length and breadth of the proximal and distal regions of the prepubic process in several hadrosaurids shows that ANSP 15202 referral to Edmontosaurus is equivocal, due to the variability among the measurements of this genus ( Fig. 7 ).

Conclusion.— H. minor is a nomen dubium: YPM 1587, 1593, 745, 7896, 7898, and ANSP 10007 and 15202 cannot be diagnosed to any taxonomic level lower than Hadrosauridae . This conclusion is in agreement with Lull and Wright (1942).

Figures

Fig. 7. Relationship between two ratios in the pubis of several hadrosaurid dinosaurs. P, total length of the pubis, from the acetabular edge to the cranial edge of the prepubic process; p, length of the proximal constriction of the prepubic process, from the iliac peduncle to the most proximal and highest point of the dorsal border of the pubic blade; w, minimum breadth of the proximal constriction of the prepubic process; and B, maximum breadth of the pubic blade between the end of “p” and the ventral end of a perpendicular line taken from the end of “p”. The star symbol represents Hadrosaurus minor (ANSP 15202). Letters within squares represent the following taxa: B = Brachylophosaurus canadensis; C = Corythosaurus casuarius; E = Edmontosaurus annectens; E’= E. regalis; H = Hypacrosaurus altispinus; H’= H. stebingeri; K = Kritosaurus latidens; M = Maiasaura peeblesorum; P = Prosaurolophus blackfeetensis; S = Saurolophus osborni.