APPENDIX 1.

Terms, definition, and usage of pneutmatic morphology and histology used in this analysis.

Pneumatic Architecture (Wedel et al., 2000)

Camera - Round cavity, 5-150 mm in size, septal thickness of 2-10 mm, and a regular branching pattern.

Camella - Angular cavity, 2-20 mm in size, septal thickness of 1-3 mm, and an irregular branching pattern.

Acamerate - Pneumatic structures limited to fossae. Fossae do not significantly invade the centrum.

Procamerate - Deep fossae penetrate to median septum, but are not enclosed by osteal margins.

Camerate - Large, enclosed camerae with regular branching pattern; cameral generations usually limited to 3.

Camellate - Fine internal structures composed entirely of small scaled, thin-walled camellae; can produce a “honeycomb”-like network.

Histologic Terms (Francillon‐Vieillot et al., 1990; de Ricqlès et al., 1991; Castanet et al., 1992; Huttenlocker et al., 2013)

Line Of Arrested Growth (LAG) - Thin bands that represent temporary arrest of osteogenesis, and are considered osteological response to predictable environmental cues.

Annuli - Translucent to opaque bands, thicker than LAGs, represent a slowing (but not a cessation) of osteogenesis.

Woven Bone - Highly disorganized arrangement of collagen fibers, which reflects a high rate of osteogenesis.

Fibrolamellar Bone - Woven bone with intervening and randomly oriented primary osteons.

External Fundamental System (EFS) - Slowly deposited parallel-fibered or lamellar tissue along the outermost cortex (closely spaced outermost series of LAGs).

Haversian Bone - Bone that is completely remodeled by secondary osteons.

Primary Osteon - Central blood vessel and surrounding concentric bone tissue.

Secondary Osteon - Osteon formed by replacement of existing bone, surrounded by an outer cement sheath.

Cancellous Bone - Highly vascular bone that contains a higher surface area to mass ratio.

Trabeculae - Rod-shaped bone tissue in cancellous bone. Provides lightweight internal support.

Laminar Vascular Canal - Circumferentially oriented rows of vascular canals.

Longitudinal Vascular Canal - Canals oriented parallel to the long axis on the bone.

Reticular Vascular Canal - Obliquely oriented vascular canals.

APPENDIX 2.

Sauropod specimens examined in this analysis (H-MOS = Histo-Morph Ontogeny Scale [see Appendix 3]).

Specimen Number Material Examined Taxonomy H-MOS Estimation
AMNH 353 Femur Apatosaurus sp. (McIntosh, 1990) Stage 4
AMNH 435 Femur Apatosaurus sp. (this analysis) Stage 2 or 3
AMNH 606 Femur Apatosaurus sp. (this analysis) Stage 4
AMNH 613 Femur Diplodocidae (this analysis) Stage 1
AMNH 5855 Femur Diplodocus sp., Barosaurus (Mook, 1917; McIntosh, 2005) Stage 2
AMNH 6341 Vertebrae Barosaurus (Lull, 1919; Tschopp et al., 2015) Stage 3 or 4
AMNH 7530 Vertebrae Barosaurus, Kaatedocus (Michelis, 2004; Tschopp et al., 2015) Stage 2
AMNH 7535 Vertebrae Barosaurus (Michelis, 2004; Tschopp et al., 2015) Stage 2 or 3
AMNH 7539 Femur Diplodocus sp. (this analysis) Stage 2
ANS 21122 Vertebrae, Femur, Skull, Dorsal Ribs Suuwassea, Apatosaurinae, Dicraeosauridae (Harris, 2006a,b; Whitlock and Harris, 2010; Woodruff and Fowler, 2012; Tschopp et al., 2015) Stage 3
BYU 601-17103 Femur Apatosaurus sp. (Wilhite, 2003) Stage 4
BYU 725-4889 Femur Diplodocus sp. (Wilhite, 2003) Stage 2
BYU 725-9026 Femur Diplodocus sp. (Wilhite, 2003) Stage 2 or 3
BYU 725-11421 Femur Diplodocus sp. (Wilhite, 2003) Stage 2 or 3
BYU 725-12155 Femur Diplodocus sp. (Wilhite, 2003) Stage 3
BYU 725-13369 Femur Diplodocus sp. (Wilhite, 2003) Stage 3
BYU 725-13643 Femur Diplodocus sp. (Wilhite, 2003) Stage 4
BYU 725-13670 Femur Diplodocus sp. (Wilhite, 2003) Stage 1
BYU 725-16569 Femur Diplodocus sp. (Wilhite, 2003) Stage 2
BYU 725-16610 Femur Diplodocus sp. (Wilhite, 2003) Stage 2
BYU 17096 Skull Apatosaurus sp. (Balanoff et al., 2010) Stage 2
BYU Fe-4-DM197 Femur Diplodocus sp. (Wilhite, 2003) Stage 3
BYU Fe-5-DM172 Femur Diplodocus sp. (Wilhite, 2003) Stage 3
CM 84 Vertebrae, Femur Diplodocus carnegii (Hatcher, 1901; Tschopp et al., 2015) Stage 4
CM 85 Femur Apatosaurus sp. (McIntosh, 1981) Stage 4
CM 87 Femur Apatosaurus sp. (McIntosh, 1981) Stage 4
CM 94 Vertebrae, Femur, Dorsal Ribs Diplodocus carnegii (Hatcher, 1901; Tschopp et al., 2015) Stage 4
CM 555 Vertebrae Apatosaurus excelsus,Brontosaurus excelsus (McIntosh, 1981; Tschopp et al., 2015) Stage 3
CM 563 Vertebrae Apatosaurus excelsus, Brontosaurus parvus (Gilmore, 1936; Tschopp et al., 2015) Stage 4
CM 566 Femur Apatosaurus sp., Brontosaurus parvus (McIntosh, 1981; Tschopp et al., 2015) Stage 1
CM 572 Vertebrae Haplocanthosaurus priscus (Hatcher, 1903; Tschopp et al., 2015) Stage 3 or 4
CM 879 Vertebrae Haplocanthosaurus utterbacki (Hatcher, 1903; Tschopp et al., 2015) Stage 2 or 3
CM 3018 Vertebrae, Femur, Skull Apatosaurus louisae (Gilmore, 1936; Tschopp et al., 2015) Stage 4
CM 3390 Vertebrae Apatosaurus sp. (McIntosh, 1981) Stage 2
CM 11161 Skull Diplodocus longus; Diplodocinae Indeterminate (Berman and McIntosh, 1978; Tschopp et al. 2015) Stage 4
CM 11162 Skull Apatosaurus louisae (Berman and McIntosh, 1978; Tschopp et al., 2015) Stage 4
CM 11338 Vertebrae Camarasaurus lentus (Gilmore, 1925) Stage 2
CM 21785 Femur Apatosaurus sp. (McIntosh, 1981) Stage 4
CM 21788 Femur Diplodocus sp. (Wilhite, 2003) Stage 1 or 2
CM 30762 Femur Diplodocus sp. (Wilhite, 2003) Stage 2
CM 30766 Femur Apatosaurus sp., Apatosaurine (Wilhite, 2003; Tschopp et al., 2015) Stage 3
CM 33976 Femur Apatosaurus sp., Diplodocus sp. (Wilhite, 2003, this analysis) Stage 1
CM 33991 Femur Diplodocus sp. (McIntosh, 1981) Stage 1
CMC VP 7747 Femur Diplodocidae, Diplodocus sp. (Meyers, 2004; Woodruff and Fowler, 2004) Stage 2
CMC VP14128 Skull Diplodocus sp. (this analysis) Stage 2
CMNH 10039 Femur Apatosaurus sp. (Wilhite, 2003) Stage 2
CMNH 10380 Vertebrae Haplocanthosaurus delfsi (Wilhite, 2003) Stage 4
DNM 3781 Femur Diplodocus sp. (Wilhite, 2003) Stage 4
GPDM 220 Vertebrae Camarasaurus sp. (this analysis) Stage 4
HMNS 175 Femur Diplodocus hayi, Galeamopus (Holland, 1924; Tschopp et al., 2015) Stage 4
KUVP 1351 Femur Apatosaurus sp. (Wilhite, 2003) Stage 4
MOR 592 Vertebrae, Femur, Skull, Dorsal Ribs, Cervical Ribs Amphicoelias altus, Dicraeosauridae, Diplodocus sp. (Wilson and Smith, 1996; Whitlock and Harris, 2010; Woodruff and Fowler, 2012) Stage 3
MOR 700

(2 specimens)

Skull, Cervical Ribs Apatosaurus sp. (Woodruff and Fowler, 2014) Stage 2 and 4
MOR 714 Vertebrae diplodocid indeterminate (this analysis) Stage 1
MOR 790

(15 specimens)

Vertebrae, Femur, Dorsal Ribs, Cervical Ribs Diplodocinae; Diplodocus sp. (Myers, 2004; Woodruff and Fowler, 2012) Stage 2
MOR 957 Vertebrae, Femur Apatosaurus sp. (this analysis) Stage 4
MOR 7029 Vertebrae, Skull Diplodocus sp. (Woodruff and Fowler, 2014) Stage 3
MWC 5439 Femur Apatosaurus sp. (this analysis) Stage 1
MWC

“Moffit Co. Apato.”

Femur Apatosaurus sp. (this analysis) Stage 4
NSMT-PV 20375 Vertebrae, Femur Apatosaurus ajax, Apatosaurinae Indeterminate (Upchurch et al., 2004b; Tschopp et al., 2015) Stage 4
OMNH 1793 Femur Diplodocus sp. (Wilhite, 2003) Stage 1
OMNH 01667 Femur Apatosaurus sp. (Wilhite, 2003) Stage 4
SMA 0003 Vertebrae Diplodocidae, Diplodocus sp. (Schwarz et al., 2007a; this analysis) Stage 3
SMA 0004 Vertebrae, Skull Kaatedocus (Tschopp and Mateus, 2012; Tschopp et al., 2015) Stage 2 or 3
SMA 0009 Vertebrae, Femur, Cervical Ribs Diplodocidae; Barosaurus, Brachiosaurus (Schwarz et al., 2007a; Woodruff and Fowler, 2012; Carballido et al., 2012; Tschopp et al., 2015) Stage 1
SMA 0011 Vertebrae, Skull Diplodocidae, Galeamopus (Klein and Sander, 2008; Tschopp et al., 2015) Stage 3
SMA 0014 Femur Diplodocidae (this analysis) Stage 4
SMM P84.15.2 Femur Apatosaurus sp. (this analysis) Stage 1
TMM 993-1 Femur Diplodocus sp. (Wilhite, 2003) Stage 4
USNM 2672 Skull Diplodocus sp., Diplodocinae Indeterminate (Berman and McIntosh, 1978; Tschopp et al., 2015) Stage 4
USNM 2673 Skull Diplodocus sp.; Galeamopus (Berman and McIntosh, 1978; Tschopp et al., 2015) Stage 4
USNM 4797 Femur Apatosaurus sp. (Wilhite, 2003) Stage 4
USNM 10865 Femur Diplodocus sp. (Wilhite, 2003) Stage 4
USNM 11162 Skull Apatosaurus louisae (Berman and McIntosh, 1978) Stage 4
USNM 337871 Femur Diplodocus sp. (Wilhite, 2003) Stage 1 or 2
WDC BS-157 Femur Apatosaurus sp. (Wilhite, 2003) Stage 3
YPM 429 Vertebrae Barosaurus (Lull, 1919; McIntosh, 2005; Tschopp et al., 2015) Stage 4
YPM 1980 Vertebrae Apatosaurus excelsus, Brontosaurus excelsus (Ostrom and McIntosh, 1966; Tschopp et al., 2015) Stage 4
YPM 5862 Femur Apatosaurus sp. (Wilhite, 2003) Stage 1

APPENDIX 3.

The Histo-Morph Ontogeny Scale (H-MOS).

  Stage 1 Stage 2 Stage 3 Stage 4
HOS <4 5-7 8-10 >10
Postparietal aperture ? Present Present Absent
LAGs (minimum record via dorsal ribs) <2 2-6 7 - 15*
(15 is an estimated demarcation)
>15
Cervical pneumatic
architecture
Acamerate with shallow Fossae (4-8 mm) Procamerate to Camerate with deepening Fossae
(7-24 mm)
Camerate with increasing depth and abundance of Fossae and Foramina Camerate with extensive and numerous Fossae and Foramina
Cervical pneumaticity
(CT scan)
No internal structures Camerae & Camellae Thinning median septum with Camerae and Camellae ?
Dorsal pneumaticity
(CT scan)
? Thinning median septum with Camerae Thinning median septum with Camerae and Camellae Extensive Camerae in centrum and arch
Ant. cervical bifurcation No bifurcation No bifurcation Notched to weakly bifurcated Fully bifurcated
Post. cervical bifurcation No bifurcation Weakly bifurcated Narrow bifurcation Fully bifurcated
Ant. dorsal bifurcation No bifurcation Narrow bifurcation Narrow bifurcation Fully bifurcated
Post. dorsal bifurcation No bifurcation Narrow bifurcation Bifurcated Fully bifurcated
Ant. Caudal bifurcation No bifurcation No bifurcation No bifurcation Fully bifurcated
Neural spine Trabeculae
(mm)
? ≤ 1 ≥ 3 ?
Femoral head orientation ? 20° - 11° 10° - 5° <5°
4th Trochanter position Proximal Proximal Proximal to mid-diaphysis Mid-diaphysis
Medial Condyle Not pronounced Ventrally expanding Laterally epanding Greatly pronounced
Femur length (mm) <800 800 - 1,200 1,200 - 1,450 >1,450
Body mass (kg) <2,000 2,000 - 3,000 3,000 - 6,000 >6,000

APPENDIX 4.

Diplodocidae body mass table using the allometry based body mass formula of Mazzetta (2004) (log Body Mass = 2.955 x log Femur Circumference − 4.166).

Taxon Specimen Number Femur Circumference (mm) Body Mass (kg) Mass - Pneumaticity (kg)
Apatosaurus sp. MWC "Moffat Co. Apato." 908.05 49378.59631 44440.73667
Apatosaurus SMA 0014 810 26827.06928 24144.36235
Apatosaurus AMNH 613 805 26340.67055 23706.60349
Apatosaurus AMNH 606 795 25385.44993 22846.90494
Apatosaurus AMNH 7539 766 22745.52756 20470.9748
Apatosaurus AMNH 353 725 19332.98035 17399.68231
A. louisae CM 30766 701 17502.36514 15752.12863
Brontosaurus CM 21785 690 16703.17623 15032.8586
Apatosaurus CM 85 678 15859.29017 14273.36116
Brontosaurus CM 87 650 14000.9972 12600.89748
Apatosaurus CM 566 626 12527.86759 11275.08083
Apatosaurus CM 33976 600 11051.89995 9946.709959
Apatosaurus P25112 555 8777.801694 7900.021524
Apatosaurus OMNH 01667 513 6956.5965 6260.93685
Apatosaurus KUVP 1351 453 4816.928356 4335.235521
Apatosaurus BS 157 349 2228.687292 2005.818563
Apatosaurus CMNH 10039 310 1570.26809 1413.241281
Apatosaurus USNM 4797 288 1263.29507 1136.965563
Apatosaurus BYU 601-17103 216 539.896901 485.9072109
Apatosaurus YPM 5862 184.15 336.963175 303.2668575
Apatosaurus MWC 5439 148 176.6539566 158.9885609
diplodocid indeterminate SMM P84.15.2 120 95.05622917 85.55060625
Galeamopus HMNS 175 590 10516.4145 9464.77305
Diplodocus USMN 10865 564 9205.126313 8284.613681
Diplodocus TMM 993-1 551 8592.172245 7732.95502
D. carnegii CM 94 540 8095.12502 7285.612518
Diplodocus BYU 725-13643 536 7919.211712 7127.290541
Diplodocus BYU 725-13369 513 6956.5965 6260.93685
D. carnegii CM 84 510 6837.067544 6153.36079
Diplodocus BYU Fe-4-DM197 490 6074.764083 5467.287674
Diplodocus AMNH 435 458 4975.737808 4478.164027
Diplodocus BYU 725-11421 456 4911.804923 4420.624431
Diplodocus AMNH 7539 455 4880.043319 4392.038987
Diplodocus BYU 725-12155 455 4880.043319 4392.038987
Diplodocus BB 761 451 4754.355816 4278.920235
Diplodocus BYU Fe-5-DM172 435 4273.028154 3845.725339
Diplodocus AMNH 5855 410 3587.37103 3228.633927
Diplodocus MOR 592-35 409 3561.577304 3205.419574
D. longus CM 30762 387 3024.72504 2722.252536
Diplodocus BYU 725-16569 385 2978.766464 2680.889817
Diplodocus MOR 790 7-23-95-122 371.5 2680.577713 2412.519942
Diplodocus BYU 725-16610 365 2544.341575 2289.907418
Diplodocus MOR 790 7-5-95-7 362 2483.04063 2234.736567
Diplodocus BB 463 361 2462.826294 216.543665
Diplodocus BYU 725-4889 354 2324.36667 2091.930003
Diplodocus BYU 725-9026 350 2247.610597 2022.849537
Diplodocus USMN 337871 325 1805.574164 1625.016747
D. longus CM 21788 317 1677.374822 1509.63734
D. longus CM 33991 315 1646.295103 1481.665593
Diplodocus OMNH 1793 254 871.5325918 784.3793326
Diplodocus DNM 3781 243 764.6567709 688.1910938
Diplodocus BYU 725-13670 162 230.7368014 207.6631213
diplodocid indeterminate P.84.15.2 120 95.05622917 85.55060625

APPENDIX 5.

Supplemental Information.

The material contained herein serves to address some of the comments and questions raised by Wedel and Taylor (2013), along with some minor discussion on non-Apatosaurus and Diplodocus genera.

Anterior Cervical Bifurcation. Wedel and Taylor (2013) suggest that there is no evidence of bifurcated neural spines more anteriorly than C6 in any known North American diplodocid. While it is correct that the anterior most neural spines are damaged and reconstructed from Diplodocus carnegii (CM 84), Brontosaurus excelsus (YPM 1980), and Brontosaurus parvus (UWGM 15556), resulting in unknown spine morphology for these specimens, other cervical series from North American diplodocids do show anterior bifurcation (Appendix 6). The anterior-most cervical neural spines from CM 555 (Apatosaurus or Brontosaurus excelsus) are damaged, yet C4 distinctly has weak bifurcation. C5 has weakly expressed bifurcation; while C6 and the remaining vertebrae have fully expressed bifurcation. Even within Barosaurus lentus we see similar anterior-most bifurcation. In AMNH 7530 (alternatively identified as Kaatedocus by Tschopp et al., 2015), the neural spine of C2 is not bifurcated, yet the neural spine of C3 clearly is. After C3, the spines of C4 and C5 are broad, but not bifurcated. The remaining cervical vertebrae are on display, and removal to examine the anterior morphology was not possible at the time of visitation. A slightly larger specimen AMNH 7535 exhibits similar spine morphology. The neural spine of C2 is slightly damaged at the apex, but the neural spine would appear to be bifurcated (if so it would be the anterior most documentation of spine bifurcation). C3 and C5 are missing, and the neural spine of C4 is badly damaged. The neural spine of C6 is clearly bifurcated, yet C7-C13 are un-bifurcated. The final vertebra from this specimen, C14, does have a broad neural spine with incipient or weakly expressed bifurcation.

The claim that no North American diplodocids possessed bifurcated neural spines farther anteriorly than C6 is now shown to be incorrect. Through examination of several complete or nearly complete cervical series, we now see that in diplodocids incipient to weak neural spine bifurcation can occur between C3-C5 (Appendix 6). It is interesting to see that in the anterior most cervical vertebrae there is a small span of bifurcation preceded and followed by a lack of bifurcation. If the hypothesis that bifurcation is developed as a means to maintain and sustain horizontal mobility (Woodruff, 2014) is correct, the presence of select anterior-most bifurcation could be indicative of active cranial mobility. As demonstrated by Wedel and Taylor (2013), serial position is critical. If serial position was unknown in these specimens then there would be a valid argument to place these bifurcated anterior cervicals more posteriorly. From the examined specimens it appears that diplodocids (at least North American taxa) could have neural spine bifurcation prior to C6, and that these damaged historic specimens could be reconstructed with such.

The “End” Of Diplodocid Ontogeny. With the information attained from many of these 2/3 sized diplodocids (such as MOR 592 MOR 7029 [both Diplodocus sp.]), it would appear that once a diplodocid reaches roughly 2/3 its maximum length that proportionally (and potentially mechanically) it is more equivalent to the skeletally mature form. Simply applying force per unit area or a Ponderal Index (Thompson, 1942), the gravitational forces acting upon a 15 meter and 27 meter long animal are more similar (approximately 2 times an increase in both body mass and body length) than those acting upon a 2 meter and a 15 meter long animal (approximately 38 times heavier and 7.5 times longer). Once a diplodocid reaches that 2/3 sized threshold it has achieved the skeletal adaptations needed to support the large vertebral column (i.e., bifurcation of the neural spines). From the 2/3 size through the remainder of its life, the animal then modifies the existing structure to deal with the increasing stresses enacted upon it. This explains why the spine bifurcation changes are less dramatic, and why the centrum begins to expand along with other proportional changes. At the point of dramatic weight increase, it is far easier to modify a pre-existing structure than to suddenly develop a new feature. It would appear that from hatching, an immature diplodocid is in a dramatic ontogenetic race to develop the skeletal features needed to support its eventual gigantic girth.

Woodruff and Fowler (2012) Clarification. In the discussion section Woodruff and Fowler (2012) say, “Just as particularly large diplodocid specimens... have been more recently recognized as large and potentially older individuals of already recognized taxa,... taxa defined on small specimens... might represent immature forms of Diplodocus or Apatosaurus.” Clarifying the original phrasing, the meaning of this passage is that ontogenetic and statigraphic analysis of the characters that diagnose particular taxa within Dipodocoidea (particularly those described based on immature holotypes) may significantly alter the structure of the phylogenetic tree. Further, isolated specimens currently attributed to a given taxon may instead turn out to be ontogenetic stages of a different taxon (without sinking the original designated taxon), these mis-assignments due to heterochronic shifts are only recognizable with stratigraphic and ontogenetic analysis.

Barosaurus. One possible contentious specimen to the ongoing discussion of ontogenetic development of neural spine bifurcation is the immature Barosaurus sp. (DINO 2921) described by Melstrom et al. (2016). While documenting numerous important ontogenetic vertebral characters (ranging from Elongation Index, neurocentral fusion, to pneumatic architecture; and thus further verification of the allometric development of the sauropod skeleton), Melstrom et al. (2016) claim that the morphology of the spine bifurcation in this ~1/3 adult sized individual is indistinguishable from that of a mature animal. The precise spinal morphologies and details are hopefully forthcoming (and respectfully such is not demonstrated nor documented in Melstrom et al. [2016]), but perhaps the specimen DINO 2921 could falsify the hypothesis of Woodruff and Fowler (2012).

Yet we would propose that if Melstrom et al. (2016) are indeed correct about the spine morphology of DINO 2921, this could be an incredibly important key to understanding sauropod evolution within the Morrison Formation. If the stratigraphic resolution is correct (see the informative works of K. Truillo), then DINO 2921 comes from the lower portion of the Brushy Basin Member (Turner and Peterson, 1999; Carpenter, 2013), while other immature Barosaurus specimens (such as AMNH 7530 and 7535; note Tschopp et al. [2015] identify AMNH 7530 as Kaatedocus) are non-bifurcated and come from the upper portion of the Salt Wash Member (Turner and Peterson, 1999; Michelis, 2004; Tschopp and Mateus, 2013). If this resolution, taxonomy, and morphology is correct, then this has substantial implications for Barosaurus heterochrony. Again, all of this hinges on the correct initial identification, but if so, this means that Lower and Upper Morrison specimens have differing vertebral biomechanics (i.e., distinct differing morphologies), and given the stratigraphic and calculated temporal range, this may be initial grounds to begin the examination and inquire into heterochrony, and therefore the possibility of two Morrison Barosaurus taxa.

Camarasaurus. A minor point we would like to bring to light is the recognition of neural spine bifurcation within the basal macronarian Camarasaurus. Woodruff and Fowler (2012) note that in specimens from the Kenton Quarry, the neural spines of immature Camarasaurus sp. are morphologically similar to immature diplodocids in that the neural spines are short and non-bifurcated. No in depth examination on Camarasaurus sp. was carried out in the preliminary analysis (but is currently underway by DCW), but the possibility of Camarasaurus spine ontogeny was favored by Wedel and Taylor (2013). While display features can be modified throughout ontogeny (i.e., male peacock plumage and cassowary and helmeted guinea fowl casques), it would seem unusual that a biomechanical feature could be ontogenetic in one clade, yet static in a closely related clade. In examination of the cervical and dorsal series from the presumed immature Camarasaurus lentus (CM 11338), the neural spines are bifurcated, but the depth of bifurcation is shallow and the neural spine apices are much closer together than in a fully mature animal. A relatively small Camarasaurus sp. specimen at the Great Plains Dinosaur Museum (GPDM 220) has cervical and dorsal neural spines that likewise exhibit shallow bifurcation and narrow neural spines (Woodruff and Foster, 2017). These features alone have been previously thought to be valid autapomorphies of a new genus (N. Murphy and K. Carpenter, personal commun., 2012). However, analysis by Woodruff and Foster (2017) has contrarily demonstrated that GPDM 220 is a maturationally old, small statured individual. Thus GPDM 220 would further verify the complex relationship between vertebral mechanics and ontogeny within sauropods (Woodruff and Foster, 2017).

Haplocanthosaurus. Wedel and Taylor (2013) perform a laudable job verifying that the genus Haplocanthosaurus is not a juvenile Apatosaurus or Diplodocus (the specific lines of reasoning will not be addressed here but we recommend referral to their text; Wedel and Taylor [2013] p. 23-27). Being one of the rarest of Morrison taxa, Haplocanthosaurus is known from three species: H. delfsi, H. priscus, and “H. utterbacki”. Collected from the lower Brushy Basin Member of the Morrison Formation, CM 572 (H. priscus) and CM 879 (“H. utterbacki”) both were found meters away from each other in the Marsh-Felch Quarry 1. While CM 879 has not been histologically sampled to assess maturity, the general consensus (largely based on the overall morphology) is that it represents an immature animal (the differences and validity between H. priscus and H. delfsi shall not be addressed here; McIntosh and Williams [1988], Wedel and Taylor [2013]). As Wedel (2009) illustrates, CM 572 exhibits widespread neurocentral synostosis, whereas CM 879 exhibits primarily completely unfused neural arches (remember from the manuscript that vertebral fusion is not conclusively indicative of maturity within dinosaurs). Though very weakly expressed and exceedingly rare, incipient neural spine bifurcation has been observed within some specimens of Haplocanthosaurus (Appendix 7). Neural spine bifurcation is observed in a posterior dorsal of CM 879, and within an anterior dorsal of CM 572. We are well aware of the importance of serial position and the morphological differences between such vertebrae, however due to the rarity of this feature and the relative proximity a comparison shall still be made. In a posterior cervical of CM 879, the bifurcation is formed by a connection of two closely spaced “humps” (reminiscent of bifurcation observed in CM 555). Within an anterior dorsal of CM 572 these “humps” are spaced and the bifurcation trough is a shallow “V”-shape. Based on the spinal morphology, it would appear that, unlike other members of Diplodocidae, Haplocanthosaurus did not biomechanically require neural spine bifurcation. In conjunction with McIntosh and Williams (1988) and Wedel and Taylor (2013), we would consider “H. utterbacki” as a nomen dubium, and further agree that it represents an immature form of H. priscus. In addition to this, the variation between the two specimens would suggest that while incipient, the neural spine bifurcation observed in Haplocanthosaurus may also be ontogenetic. Verification of this point requires further specimens and overlapping material.

In regards to the phylogenetic assignment of Haplocanthosaurus, we would again stress the need for the recognition of ontogenetic stages. In his analysis of Diplodocoidea, J. Whitlock (2011a) shows that the character matrix for Haplocanthosaurus is a combination of several specimens including CM 879 (“H. utterbacki”), CM 572 (H. priscus), and CMNH 10380 (H. delfsi). While these individuals were included to complete otherwise missing characters from other incomplete specimens, the taxonomic uncertainty of Haplocanthosaurus could be due to the fact that the characters states representing it are from a combination of varying ontogenetic stages and potentially separate species (sensu Mannion et al., 2012).

APPENDIX 6.

Neural spine bifurcation in diplodocid anterior most cervical vertebrae. 1-2, Apatosaurus CM 555; 3,Barosaurus AMNH 7530; 4,Diplodocus sp. MOR 592. B.N.S = Bifurcated Neural Spine. All cervical vertebrae in anterior view. Not to scale. Scale bars equal 10 cm.

appendix6

 

APPENDIX 7.

Haplocanthosaurus incipient neural spine bifurcation. 1, Haplocanthosaurusutterbacki ” (CM 879) anterior dorsal in anterior view; 2,Hapalocanthosauruspriscus (CM 529) anterior dorsal in posterior view; 3,H. “utterbacki ” (CM 879) posterior dorsal in anterior view. I.B. = Incipient Bifurcation. Not to scale. Scale bar equals 10 cm.

appendix7

APPENDIX 8.

Elongation Index of diplodocid cervical vertebrae (EI = centrum length divided by cotyle diameter).

Diplodocus    
HQ 1 SMA 0003    
  Centrum Length (mm) Cotyle Diameter (mm) EI
C2 84 37 2.27027
C3 105.5 31 3.40323
C4 131 46.5 2.8172
C5 184 30.5 6.03279
C6 228 46 4.95652
C7 280 44 6.36364
C8 376 55.5 6.77477
C9 375 71 5.28169
C10 385 49 7.85714
C11 431 75 5.74667
C12 458 74 6.18919
C13 454 58.5 7.76068
C14 463 77 6.01299
C15 474 77 6.15584
HQ 2 SMA 0004    
  Centrum Length (mm) Cotyle Diameter (mm) EI
C2 83.5 35 2.38571
C3 107 27 3.96296
C4 132 - -
C5 158 28 5.64286
C6 192 26.5 7.24528
C7 218 37 5.89189
C8 247 34.5 7.15942
C9 264 38 6.94737
C10 296 53 5.58491
C11 295 51 5.78431
C12 316 56 5.64286
C13 326 66 4.93939
C14 314 61 5.14754
MOR 592    
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C2 114.4 41.2 2.7767
C3 128.3 41.9 3.06205
C4 - - -
C5 163.6 49 3.33878
C6 - - -
C7 247 37.5 6.58667
C8 256.5 47.4 5.41139
C9 271.6 67.8 4.0059
C10 291 62.4 4.66346
C11 279.4 103.5 2.69952
C12 266.7 112.5 2.37067
C13 239.2 180.8 1.32301
C14 349.3 73.2 4.77186
C15 304.8 96.2 3.1684
       
CM 84      
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C2 165 54 3.05556
C3 243 69 3.52174
C4 289 81 3.5679
C5 372 94 3.95745
C6 442 99 4.46465
C7 485 114 4.25439
C8 512 120 4.26667
C9 525 159 3.30189
C10 595 175 3.4
C11 605 210 2.88095
C12 627 225 2.78667
C13 638 231 2.7619
C14 642 295 2.17627
C15 595 245 2.42857
Apatosaurus    
CM 555      
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C2 140 63 2.22222
C3 221 84 2.63095
C4 274 96 2.85417
C5 270 60 4.5
C6 295 114 2.58772
C7 316 - -
C8 344 113 3.04425
C9 380 - -
C10 - - -
C11 480 193 2.48705
C12 460 245 1.87755
C13 - - -
C14 378 260 1.45385
C15 - - -
CM 563      
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C3 250 80 3.125
C4 300 125 2.4
C5 342 134 2.55224
C6 - - -
C7 415 170 2.44118
C8 415 205 2.02439
C9 445 215 2.06977
C10 475 250 1.9
NSMT-PV 20375    
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C3 352 97 3.62887
C4 - - -
C5 375 155 2.41935
C6 395 195 2.02564
C7 420 220 1.90909
C8 395 195 2.02564
C9 380 235 1.61702
C10 390 - -
C11 - - -
C12 475 240 1.97917
C13 - - -
C14 450 305 1.47541
CM 3018    
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C2 190 85 2.23529
C3 280 100 2.8
C4 370 100 3.7
C5 - - -
C6 440 150 2.93333
C7 450 190 2.36842
C8 485 225 2.15556
C9 510 230 2.21739
C10 530 250 2.12
C11 550 240 2.29167
C12 490 265 1.84906
C13 480 - -
Barosaurus    
AMNH 7530    
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C2 80 14 5.71429
C3 75 26 2.88462
C4 97 37 2.62162
C5 123 62 1.98387
       
C6-C12 are on display and could not be measured first hand
AMNH 7535    
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C2 87 15 5.8
C3 - - -
C4 102.5 23 4.45652
C5 - - -
C6 135 25 5.4
C7 140 21 6.66667
C8 166 36 4.61111
C9 - - -
C10 - - -
C11 234 32 7.3125
C12 - - -
C13 281 30 9.36667
C14 323 47 6.87234
AMNH 6341 (from McIntosh, 2005)  
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C8 618 130 4.75385
C9 685 123 5.56911
C10 737 168 4.3869
C11 775 145 5.34483
C12 813 155 5.24516
C13 850 180 4.72222
C14 865 155 5.58065
C15 840 160 5.25
C16 750 250 3
YPM 429 (from McIntosh, 2005)    
  Centrum Length
(mm)
Cotyle Diameter
(mm)
EI
C13 930 220 4.22727
C14 890 345 2.57971
C15 - 300 -
C16 720 365 1.9726