Zheng et al. On the absence of sternal elements in (: ) and (Aves: ) and the complex early evolution of the avian sternum

Supplementary Information

1. Table S1-S4

2. Histology description

3. Figures S1-11

4. References

Tables

Table 1. List of Anchiornis specimens from the Shandong Tianyu Museum of Nature as well as published material (n = 229) sorted by size. The number of gastralia is only listed when the preserved basket is relatively complete. Brackets indicate incomplete elements. Parentheses indicate number of preserved pairs of gastralia.

Collection No. Complete Articulated Soft-tissue Gastralia (pairs) Femur length (>90%) (mm) STM 0-32 x x x (>10) 40

STM 0-39 x x x x 40

STM 0-132 x x — x (13) 40

STM 0-134 x — x — 40

STM 0-148 x x — x 40

STM 0-5 x x x — 41

STM 0-116 — — — — 41

STM 0-142 x x x x 41

STM 0-56 x x x x 42

STM 0-187 x x x x (>8) 42

STM 0-104 x x x x 43 STM 0-140 x x x x 43

STM 0-182 x x x x 43

STM 0-195 — — — — 43

IVPP V14378 - x - x 43.2 holotype STM 0-1 x x x — 44

STM 0-57 x x x — 44

STM 0-85 x x — x (>7) 44

STM 0-121 x x — x (>11) 44

STM 0-153 — — x x 44

STM 0-189 x x — x (>8) 44

STM 0-38 x x x — 45

STM 0-60 — — x x 45

STM 0-167 x x — x (>5) 45

STM 0-212 x x x x (>5) 45

STM 0-65 x x x x 46

STM 0-186 x x x x 46

STM 0-194 x x x x 46

STM 0-223 — x x x 46

STM 0-10 x x — x 47

STM 0-59 x x x x 47

STM 0-82 x x x x 47

STM 0-136 — x x x 47

STM 0-164 x x x x 47

STM 0-113 — — — x 48

STM 0-120 — x x x (14-15) 48

STM 0-163 — — x — 48 STM 0-58 x x x — 49

STM 0-62 x x x — 49

STM 0-77 x x — x 49

STM 0-79 x x x x 49

STM 0-54 x x x — 49.5

STM 0-12 x x x x 50

STM 0-16 x x x x 50

STM 0-24 — x — x 50

STM 0-30 x x x x (10) 50

STM 0-33 — x x — 50

STM 0-41 x x x x 50

STM 0-74 x x — x (>7) 50

STM 0-75 x — — x 50

STM 0-114 — x x — 50

STM 0-141 x — x x 50

STM 0-145 x x — x (>5) 50

STM 0-150 x x x x (>8) 50

STM 0-154 x x x x 50

STM 0-156 x x — x 50

STM 0-174 x x — x (>5) 50

STM 0-183 x x — x (11) 50

STM 0-209 x x x x 50

STM 0-222 — — — — 50

STM 0-78 x x — x 51

STM 0-207 x — x x 51

STM 0-20 x x — x 52 STM 0-21 x — x — 52

STM 0-27 x x x x 52

STM 0-84 — — — x 52

STM 0-171 x x x x 53

STM 0-126 x — x x 54

STM 0-157 x x x x 54

STM 0-190 x x — x 54

STM 0-76 x x x x 55

STM 0-53 x x x x 56

STM 0-168 x x — x (>8) 56

STM 0-52 x x x x (>11) 57

STM 0-110 x x x x 57

STM 0-97 x x — x 58

STM 0-159 — — x x 58

STM 0-211 x x x — 58

STM 0-125 — — x — 59

STM 0-17 x x x x 60

STM 0-42 x x — x 60

STM 0-44 x x — x (>12) 60

STM 0-46 — — — x 60

STM 0-55 x x x x 60

STM 0-64 x x x x 60

STM 0-66 — — x — 60

STM 0-69 x x x x (>5) 60

STM 0-86 — — x x (>6) 60

STM 0-89 x x x x 60 STM 0-95 x — — x (>7) 60

STM 0-101 x x x x 60

STM 0-119 x x x x (>10) 60

STM 0-123 — — x x (>8) 60

STM 0-130 — x — x (>7) 60

STM 0-135 x — — x (>4) 60

STM 0-173 x x x x (>8) 60

STM 0-202 x x x x 60

STM 0-205 — x — — 60

STM 0-225 x x — x (>11) 60

STM 0-25 — — — x (6) 62

STM 0-73 x x — x 62

STM 0-87 — — x x (>8) 62

STM 0-108 — — x x (>5) 62

STM 0-118 x x x x 62

STM 0-203 x x — x 62

STM 0-165 x x x x (~12-14) 63

STM 0-178 x — x x (>7) 63

STM 0-7 x — x — 64

STM 0-71 x x x x (>5) 64

STM 0-92 x x — x (>10) 64

STM 0-170 x x x x 64

STM 0-204 x x x x 64

STM 0-4 x x x — 65

STM 0-9 x x x — 65

STM 0-40 x — x x 65 STM 0-50 x x x x 65

STM 0-70 — — x — 65

STM 0-111 x x x x 65

STM 0-138 x x x x (>10) 65

STM 0-179 x x x — 65

STM 0-185 x x — — 65

STM 0-201 — x x x (>8) 65

STM 0-61 x x — — 66

STM 0-102 x x — x 66

STM 0-218 — — — x 66

LPM-B00169 x x x — 66.2

STM 0-14 x x — x (13) 67

STM 0-37 x — x x (>10) 67

STM 0-49 — — — x 67

STM 0-216 x — x — 67

STM 0-19 x x x x (7) 68

STM 0-47 x x x x (>10) 68

STM 0-143 x x x x 68

STM 0-152 x — x x 68

STM 0-176 x x — x (>6) 68

STM 0-191 x x — x 68

STM 0-215 x x x — 68

STM 0-93 x — x x 69

STM 0-151 x x x — 69

STM 0-177 x x x x 69

STM 0-2 x x — x 70 STM 0-31 x x x x 70

STM 0-34 x x — x 70

STM 0-72 x — — x 70

STM 0-91 — — — x 70

STM 0-127 — — — — 70

STM 0-158 x x x x 70

STM 0-166 x x x x 70

STM 0-172 — x x — 70

STM 0-181 x x x x 70

STM 0-193 x x x x 70

STM 0-198 x — x x 70

STM 0-206 x — x x 70

STM 0-208 x x x — 70

STM 0-217 x x x x 70

STM 0-36 x — x — 71

STM 0-48 x x x x (>10) 71

STM 0-213 x x x x (>8) 71

STM 0-43 — — — — 72

STM 0-105 x x x x 72

STM 0-137 x x x x (>8) 72

STM 0-169 x x x x (>10) 72

STM 0-29 x x x x 73

STM 0-155 x x x x 73

STM 0-221 x x x — 73

STM 0-13 x x — — 73.5

STM 0-18 x x x x (7) 74 STM 0-100 x x — — 74

STM 0-3 x x x x 75

STM 0-23 x — — x 75

STM 0-28 x x x — 75

STM 0-106 x x x x (>6) 75

STM 0-161 x x — x 75

STM 0-175 x x x x (>5) 75

STM 0-188 x x x x 75

STM 0-192 x x — x 75

STM 0-99 x x — x (10-13) 76

STM 0-90 — x x x 79

STM 0-35 x x x x (>9) 80

STM 0-224 x x x x 80

STM 0-8 x x x x 86

STM 0-26 x x — x (11) 93

STM 0-6 — x — x —

STM 0-11 — — x x —

STM 0-15 — x — x —

STM 0-45 — — — — —

STM 0-68 x — x x (>11) —

STM 0-94 — — — x —

STM 0-98 x x x — —

STM 0-103 — — — — —

STM 0-107 — — x — —

STM 0-109 — x x — —

STM 0-115 — — x x — STM 0-117 — — x — —

STM 0-122 — — x x —

STM 0-124 — — x — —

STM 0-131 — — x x (>6) —

STM 0-133 — — — — —

STM 0-139 x x x — —

STM 0-146 — — — x —

STM 0-149 — — x — —

STM 0-160 x x — x —

STM 0-180 — — x — —

STM 0-184 x x — x —

STM 0-196 — — x x (>6) —

STM 0-197 — — x x —

STM 0-199 — — x x —

STM 0-200 x x — x (>7) —

STM 0-210 x x x x —

STM 0-214 x x x x (>5) —

STM 0-226 — — — — —

STM 0-22 x x — x —

BNHMC Ph828 — x x — —

STM 0-81 — x — x [ 32 ]

STM 0-80 x x x x [ 38 ]

STM 0-83 — — — x [ 39 ]

STM 0-67 — — x x (>5) [ 43 ]

STM 0-219 — — x x [ 46 ]

STM 0-51 — — — x [ 50 ] STM 0-144 x x x x (>8) [ 50 ]

STM 0-220 — x x x [ 50 ]

STM 0-128 — — — x(>10) [ 50]

STM 0-162 x x x x [ 53 ]

STM 0-63 — — x x (>8) [ 60 ]

STM 0-112 x x x x (>10) [ 63 ]

STM 0-147 x — — x (>12) [35]

STM 0-129 — — x x (>6) [46]

STM 0-96 x — x x [48]

STM 0-88 — x — x (>9) [66]

Table 2. List of Sapeornithiformes specimens from the Shandong Tianyu Museum of Nature as well as published sapeornithiform material (n = 106); all published specimens are considered assignable to Sapeornis chaoyangensis (O’Connor et al., 2011; Pu et al., 2013; Zhang et al., 2013). The number of gastralia is only listed when the preserved basket is relatively complete. Brackets refer to incomplete elements.

Collection No. Complete Articulated Soft—tissue Gastralia (pairs) Femur length (>90%) (mm) STM 15—18 x x x x (>9) 50

STM 15—66 x x x x (>12) 54

STM 16—18 — x x x (>7) 54

STM 15—30 x — x x 55

STM 15—65 x x — x 55

STM 15—43 x — — x 56

STM 15—41 x x — x (12) 56

DNHM D3078 x x x - 56.7

STM 15—6 x x — x 57

STM 15—58 — — — — 58 STM 16—15 — x — — 58

IVPP V13396 ‘S. x x - x (15-16) 58.3 angustis’ STM 15—59 x x x x (>10) 60

STM 16—1 — x — x 60

STM 15—50 x — — x 62

STM 15—8 x x — x 63

STM 15—42 x x — x 63

STM 15—44 x x x x (>7) 63

STM 15—33 — — — x (>5) 64

STM 15—72 x x x x (>10) 64

LPM B00018 - x - x(3) 64 ‘Shenshiornis’ STM 15—48 x x — x 65

STM 16—9 — x — x 65

41HIII0405 x x x x (11) 65

STM 15—19 x — — x 67

STM 15—17 x — — x 68

STM 15—36 x x x x 68

STM 15—60 x x — x 68

STM 16—16 — x — x 68

STM 16—21 — x — x 69

STM 15—5 x x x x 70

STM 15—7 x x — x 70

STM 15—12 x x — x 70

STM 15—22 x x — x 70

STM 15—24 x x — — 70

STM 15—35 x x x x (>8) 70 STM 15—51 x x — x 70

STM 15—64 x x — x (>10) 70

STM 15—68 x x — x 70

STM 16—5 — x x x ( >7) 70

STM 16—10 — x — x 70

STM 15—71 x x — x 71

STM 16—7 — x — x (>6) 72

STM 15—49 x x — x 73

STM 16—6 — x — x 73

IVPP V13275 x x - x 74

STM 15—2 — — — x 75

STM 15—21 x x — x 75

STM 15—23 x x — x 75

STM 15—26 x x — — 75

STM 15—27 x x — x (>12) 75

STM 15—28 x x x — 75

STM 15—32 — — — x (>8) 75

STM 15—38 x x — x 75

STM 15—46 x x — x 75

STM 15—55 x — — x 75

STM 15—56 x x — x 75

STM 15—69 x x — x 75

IVPP V13276 - x - x (15) 75

STM 15—15 x x — x 76

STM 15—40 x x x x (11) 76

STM 15—54 x — — x 76 STM 16—13 — x — — 77

STM 16—20 — x — x 77

STM 15—9 x x — x 78

STM 15—37 x x — x 78

STM 15—61 x — — — 78

STM 15—4 x x x x 80

STM 15—10 x x — x 80

STM 15—14 x x — — 80

STM 15—25 — x — — 80

STM 15—34 — — x — 80

STM 15—45 x x — x (>10) 80

STM 15—47 x x — x (>8) 80

IVPP V12698 - - - x 80.4 holotype STM 15—70 x x — x 81

STM 15—39 x x — x 82

STM 15—63 x x — — 83

STM 15—57 x x — — 85

STM 16—12 — x — x 85

STM 15—3 x — x — 90

STM 15—13 — — — x 90

STM 15—29 x x — x 93

STM 15—11 x x — x —

STM 15—16 — — — — —

STM 15—20 x x — — —

STM 15—52 — x — x —

STM 15—53 — — x x — STM 15—62 — — — x —

STM 15—67 — — — — —

STM 15—73 x — — — —

STM 15—74 — — — — —

STM 16—3 — — — x —

STM 16—8 — — — x —

STM 16—11 — x — x —

STM 16—17 — — — — —

STM 16—19 — — x — —

STM 16—22 — — — — —

STM 15—1 x x — x [ 45 ]

STM 16—4 — x — x [ 58 ]

STM 16—2 — x — x (>10) [ 70]

STM 16—14 — x — — [63]

STM 15—31 x x x x (>11)

CDPC-02- 08-001 x x - x ‘Didactylornis’ DNHM D1197 x x - x(11)

DNHM D2523 - - - x

Table S3. List of Jeholornithiformes specimens from the Shandong Tianyu Museum of Nature as well as published jeholornithiform material (n = 95). The number of gastralia is only listed when the preserved basket is relatively complete. Brackets refer to incomplete elements.

Collection number Complete (>90%) Articulated Soft-tissue Gastralia Sternum Femur length (mm) (pairs)

STM3-31 — x — x — 50

STM2-45 x x x — — 53

STM2-10 x x — x (5) x 54 STM3-19 — x — x — 54

STM3-26 — — — — — 54

STM2-1 x x — — — 55

STM2-18 x x x x (5) — 55

STM3-5 — x — — — 55

LPM0193 x x x x x 55.4 ‘Shenzhouraptor’ IVPP V13350 - x x - x 55.6

STM2-9 x x x — x 56

STM2-55 x x — — — 56

STM2-17 — x x x — 57

STM3-4 — x x x — 57

STM3-30 — — x — — 57

SDM2009.01 J. - X X x - 57.9 palmapenis STM2-14 x x — — x 58

STM2-48 — — — — — 58

STM3-13 — — — — — 58

STM3-21 — — x — — 58

STM2-34 x x x x x 59

STM2-52 — x — x — 59

STM2-32 x x — x — 60

STM2-41 x x — x x 60

STM3-9 — x — — — 60

STM3-22 — — — — — 61

STM2-42 — x — — x 63

IVPP V13533 x x x x ? ? 64

STM2-40 — — — x — 64 STM3-3 — x x x — 64

STM3-6 — x — x — 65

STM2-33 x x — — x 66

STM2-36 x x x x x 66

STM3-1 — x — — — 66

STM2-7 x x x x (5) x 67

YFGP-yb2 x x - - x 67.5

STM2-8 x x x x (8) x 68

STM2-39 x x — — x 68

STM3-11 — — — — — 68

STM3-32 — x — x — 68

STM2-5 x x x x x 70

STM2-12 x x — — x 70

STM2-13 — — x x x 70

STM2-25 x x x x x 70

STM2-26 x x — — x 70

STM2-29 x x x x x 70

STM2-51 x x x — — 70

STM3-16 x x — x (6) — 70

STM3-27 x x — — — 70

STM2-37 x x x x — 71

STM3-2 — x — — x 71

CDPC-02- 04-001 X x - x (>3) x 71.9 ‘Jixiangornis’

STM2-20 — — — x x 73 STM2-54 x x — x — 73

STM3-15 — — — — — 73

STM3-23 — — — x — 73

IVPP V13274 x x - x (6) x 75 holotype

STM2-6 x x x x x 75

STM2-30 x x — — — 75

STM2-47 x x — x (8) x 75

STM2-49 x — — — x 75

STM3-17 — x — — x 75

STM3-28 — x x — — 75

STM3-33 — — — x — 75

STM2-2 — — — — — 76

STM3-20 — x x x — 76

STM2-19 — — x x (6) x? 77

STM2-23 x x x — — 77

STM3-7 — — — x — 77

STM2-21 x x — x ( >5 ) — 78

STM2-35 x x — — x 79

STM2-46 x — x — x 79

STM2-15 x x x — — 80

STM2-38 x x x x x 80 STM3-25 — — x x — 80

STM2-4 x x x — — 85

STM2-24 x x x x x 100

STM2-22 — — — — — —

STM2-44 — — x — — —

STM3-8 — — — — — —

STM3-10 — — — — — —

STM3-12 — — — — — —

STM3-14 — — — — — —

STM3-18 — — — — — —

STM3-24 — — — — — —

STM3-29 — — — — — —

STM2-28 x — — — x —

STM2-31 x x x — x [35]

STM2-27 x x x — — [40]

STM2-53 — x — — — [40]

STM2-50 x x — — — [46]

STM2-11 x x x — — [50]

STM2-3 — x — — — [53]

STM2-16 x x — x (5) x [69]

Table S4. Comparative data regarding size, size of gastral basket, proportions of the coracoid, and presence of a sternum in derived maniraptorans. Femoral length (mm) is used as a proxy for size and was typically measured from the specimen preserving the most complete gastral basket for each taxon. We normalized the number of gastralia for size by dividing the minimum number of gastral pairs by the length of the femur in the associated specimen (when possible).

Femur Gastralia Coracoid Taxon Clade Sternum L Gastralia /Fem. L W:L Allosaurus Certatosauria absent 880 18 0.02 1.64 Sinocalliopteryx Compsognathidae unknown 236.1 12 0.051 0.99 unfused 195 17-18 0.087 0.82 Potentially fused 0.05 late in ontogeny 238 12 0.60 Sinornithoides Troodontidae absent 140 14 0.10 1.11 gui Dromaeosauridae fused 51 14-17 0.255 0.66 Anchiornis Troodontidae absent 73 13-14 0.178 1.01 Troodontidae absent 81 13 0.161 1.04 Aves absent 50.3 12-13 0.239 0.91 fused, additional 0.161 prima Aves: Jeholornithiformes ossifications 75 8-9 0.35 Aves: 0.267 sanctus Confuciusornithiformes fused 45 12-13 0.67 Sapeornis Aves: Sapeornithiformes absent 72 15-16 0.20 0.84 Eopengornis Aves: fused 27 10 0.37 0.58 Parabohaiornis Aves: Enantiornithes fused, keeled 37 6 0.147 0.35 Aves: Ornithuromorpha fused, keeled 34 5 0.077 0.71 *Measurements for Allosaurus were taken from Madsen (1976); measurements for Velociraptor were taken from Norell and Mackovicky (1999).

Histology

Methods

We selected specimens from the upper and lower size limit of our sample: STM0-

5, STM0-8 and STM0-93 were sampled for Anchiornis; STM15-6, STM15-32 and

STM15-70 were sampled for Sapeornis (Fig. S4). Two bone samples were taken from each of the specimens when possible, as close to midshaft as preservation allowed: the humerus and femur were sampled in all specimens except STM15-32, in which only the femur could be sampled, and STM0-8, in which the tibia was sampled and not the femur

(Fig. S4). The samples were taken using a micro-saw and were embedded in EXAKT Technovit 7200 one-component resin and allowed to dry for 24 hours. The samples were then cut and polished until the desired optical contrast was reached. The samples were viewed under normal and polarized light using a Leica DM-RX polarizing microscope.

Measurements were taken with the computer software ImageJ 1.43r. Histological terminology is mainly sensu de Ricqlès (1976) and Chinsamy-Turan (2005). We follow

Erickson et al. (2009) and consider channels as an indicator of the extent of vascularization.

Results

Anchiornis STM0-5 (small individual, femur length 41 mm): the humeral compacta is thin (3.2 mm thick) formed by a narrow (24 µm), even inner circumferential layer (ICL) of endosteally derived lamellated bone with well organized osteocyte lacunae and a thick outer layer of woven textured bone, densely packed with haphazardly arranged osteocyte lacunae. Vascularization is primarily longitudinal and some small primary osteons are present, located closer to the ICL than the periosteal surface. Areas of the compacta are a dark brown, indicating bacterial invasion. The femoral compacta varies in thickness (0.5–

0.8 mm). Like the humerus, there is a thin ICL and an outer layer with densely packed, haphazardly arranged osteocyte lacunae; the femur differs in that the upper layer is poorly vascularized with only a few longitudinal canals and small primary osteons and the haphazardly arranged osteocyte lacunae are relatively less densely packed. One area of the bone shows signs of heavy remodeling, with huge secondary osteons (diameter ranging from 80-140 µm) (Fig. S8a). In another region of the compacta the upper layer is formed of strongly parallel fibered bone, clearly identifiable under polarized light, with fat osteocyte lacunae and moderate vascularization (Fig. S8b).

Anchiornis STM0-93 (medium sized individual, femur length 69 mm): The humeral compacta possesses a thin, even ICL of endosteally derived lamellated bone tissue followed by a thick upper layer of woven bone with longitudinal and reticular vascularization. The osteocyte lacunae are plump and lack organization but lack the chaotic distribution observed in STM0-5. There are numerous primary osteons and no

LAGs are present. An isolated region with several secondary osteons is recognized. The secondary osteons are densely packed just outside the ICL. The femur also shows a thin

ICL and a thick upper layer of woven textured bone with primarily longitudinal vascularization. Closer to the ICL the woven textured layer is nearly haversian, with numerous large secondary osteons; this haversian layer varies in thickness, in some areas approaching the periosteal surface. Secondary osteons are partially eroded by the ICL in some places indicating that this remodeling occurred before the medullary expansion in which the current ICL was deposited. Compared with the humeral cortex, vascularization is lower in the femur, and there are fewer anastomosing circular and radial canals.

Anchiornis STM0-8 (large individual, femur length 86 mm): The humerus and tibia compacta are formed by three distinct layers: a think ICL of endosteally derived lamellated bone, a thick middle layer of strongly vascularized fibrolamellar bone, and a thin outer circumferential layer (OCL) of avascular parallel fibered bone marked by

LAGs. The osteocyte lacunae are plump and haphazardly arranged except in the OCL and

ICL where they are more organized and flattened. The humeral section (thickness varies from 0.71–0.86 mm) shows both longitudinal, plexiform, and reticular vascularization (Fig. S9). A few primary osteons are present, and all located in the inner half of the middle layer. The ICL preserves secondary osteons (Fig. S9a). The number of lines of arrested growth (LAGs) varies around the circumference of the compacta; there are anywhere from one to five LAGs present (Fig. 5a). The first LAG is a double LAG in most places; the outermost LAGs are closely spaced. The OCL, marked by the first LAG, is avascular, and the osteocyte lacunae become well increasingly organized approaching the periosteum. Sharp’s fibers, oriented almost perpendicular to the periosteum, cross the majority of the OCL (Fig. S9b). Vascularization is primarily longitudinal in the tibia compacta (thickness varies from 0.78–0.93 mm), which is nearly entirely formed of primary osteons. The number of LAGs is not uniform around the whole cortex, and the number varies from two to four; the first is followed by an annulus of lamellated bone.

The OCL bears several simple longitudinal canals, a few of which interrupt a LAG.

Sapeornis STM15-6 (smaller individual, femur length 57 mm): The compacta of the humerus (poorly preserved, thickness unknown) reveals a thin ICL, a middle layer of more woven textured bone, and an outer layer of more parallel fibered bone. The middle layer is highly vascularized with longitudinal and reticular canals, primary osteons, and densely packed, haphazardly arranged osteocyte lacunae. The upper layer, not a true

OCL, is parallel fibered, with osteocyte lacunae that are flatter, more organized, and arranged parallel to the bone surface. There are only longitudinal canals, some of which are located on the periosteal surface, indicating active bone deposition. The thickness of the middle and outer layer vary around the circumference of the bone, their relative thickness being inversely proportionate to each other. The femur is largely the same except with a thicker, more uneven ICL, and very little reticular vascularization. In one area the bone tissue just outside the ICL is avascular.

Sapeornis STM15-32 (medium individual, femur length 75 mm): Only the femur was sampled; the compacta is thick (0.51–0.64 mm) and displays a thick ICL of endosteally derived lamellated bone with plump but well organized osteocyte lacunae, a middle layer of more woven textured bone with primary and secondary osteons and longitudinal and reticular vascularization, and an OCL of poorly vascularized more parallel fibered bone with two to four LAGs near the outer surface. Only two LAGs are visible in some areas but the outer LAG splits into a double LAG in some places, and even appears to split a second time revealing four LAGs in others. The contact between the middle layer and

OCL is indistinct; the bone rather shows a gradual reduction in vascularization and increase in organization of the collagen fibers towards the periosteal surface.

Sapeornis STM15-70 (larger individual, femur length 81 mm): the compacta of the humerus shows a thick ICL (1/3 the width of the compacta) followed by an even thicker outer layer of woven-textured bone with longitudinal and reticular vascularization and numerous primary osteons. This layer is interrupted nearly half way (closer to medullary surface) by a single LAG. The bone tissue on either side of this LAG is largely the same, except that the circular anastomoses are more frequently observed in the outer cortex while reticular anastomoses are more common medial to the LAG. Secondary osteons are not present. The femoral section is poorly preserved and the histology can only be observed in one area; the bone is woven textured with plump disorganized osteocyte lacunae, numerous primary osteons, and primarily longitudinal vascularization.

Supplemental Figures

Figure S1. Close up of Sapeornis chaoyangensis preserving the complete or nearly complete gastral basket but no sternal ossifications. A, IVPP V13276; B, IVPP V13396

(subadult specimen, former holotype of ‘S. angustis’). Scale bars equal ten millimeters.

Anatomical abbreviations (not listed in Figure 1 caption): l, left; r, right.

Figure S2. Details of thoracic girdle in derived maniraptorans: A, coracoid and scapula in Jeholornis (Aves) STM2-49; B, coracoids in Caudipteryx STM4-3; C, scapulocoracoid in Anchiornis STM0-31; D, coracoid and sternal plate in Microraptor STM5-50 (young subadult specimen, sternal plates unfused); E, Sapeornis (Aves) STM15-45; F, sternum in

Microraptor IVPP V13352. Scale bars equal 10 mm. Anatomical abbreviations (not listed in Figure 1 caption): ct, coracoidal tubercle; f, coracoidal fenestra; pc, procoracoid process; sca, scapula; sn, supracoracoidal nerve foramen; stn, sternum.

Figure S3. Basal sterna. A1, Jeholornis STM2-46 (Jeholornithiformes), A2, IVPP

V13274; B, Eopengornis STM24-1 (Enantiornithes); C, Jinzhouornis IVPP V12352

(Confuciusornithiformes); D, juvenile IVPP V12552 (Enantiornithes); E,

Archaeorhynchus IVPP V17091 (Ornithuromorpha). Scale bars equal 10 mm.

Anatomical abbreviations (not listed in Figure 1, S2 captions): ao, accessory ossification; cl, craniolateral process; hyp, hypocleidium; it, intermediate trabecula; lp, lateral process; lt, lateral trabecula; xp, xiphoid process.

Figure S4. Mature specimens with arrows indicating where histological samples were collected. A, Anchiornis STM0-93; B, Sapeornis STM15-32.

Figure S5. Mature long bone histology of Anchiornis and Sapeornis under normal light.

A, transverse humeral section of a mature specimen of Anchiornis (STM 0-8) showing the well developed ICL and OCL with several LAGs (ranging from one to five throughout section); B, transverse femoral section of a mature specimen of Sapeornis

(STM15-32), showing the middle layer of woven textured bone sandwiched between the poorly vascularized ICL and OCL; note the first LAG is a double LAGs.

Figure S6. Complete transverse sections of femur and humerus of the smallest sampled specimen of Sapeornis (STM 15-6) under normal light. A, femoral compacta mainly composed by woven textured bone tissue highly vascularized by longitudinal canals; B, humeral histology is largely the same as the femur, except with less reticular vascularization.

Figure S7. Humeral and femoral histology of a large specimen of Sapeornis (STM 15-

70). A, complete transverse section of humerus under normal light; the medullary cavity is lined by a thick and poorly vascularized ICL, and the outer compacta is formed by woven bone tissue with longitudinal and reticular canals, interrupted by a LAG; B, transverse section of the femur under polarized light. The compacta is largely formed by woven bone tissue and the canals are mainly longitudinally oriented. No LAGs are visible in the poorly preserved femoral section.

Figure S8. Details of the transverse femoral section of the smallest sampled specimen of

Anchiornis (STM 0-5) under polarized light. A, region of compacta with large secondary osteons, indicating heavy remodeling of the bone tissue; B, peripheral region of the compacta showing the collagen fibers becoming increasingly parallel as they approach the outmost cortex.

Figure S9. Details of the transverse humeral section of the largest sampled specimen of

Anchiornis (STM 0-8). A, compacta near the ICL showing developed secondary osteons under normal light; B, compacta near the outermost cortex where Sharp’s fibers run almost perpendicular to the bone surface and nearly cross the entire OCL under normal light; C, cross section of the compacta, which is well vascularized by longitudinal canals with three LAGs present in the OCL where a single longitudinal canal is visible.

Figure S10. Fusion in Sapeornis STM15-32. A, left carpometacarpus, appears well fused with exception of suture between alular and major metacarpals, which is also present in other known specimens of Sapeornis and potentially represents the adult condition; B, left distal tibiotarsus and proximal tarsometatarsus, both fully fused with their respective tarsals. Anatomical abbreviations: al, alular metacarpal; c, proximal carpal; ma, major metacarpal; mi, minor metacarpal; tbt, tibiotarsus; tmt, tarsometatarsus.

Figure S11. Gastralia in basal : A, Jeholornis STM2-47; B, Confuciusornis STM13-

52; C, pengornithid enantiornithine Eopengornis STM24-1; D, bohaiornithid enantiornithine Parabohaiornis IVPP V18960, note cranial pair of gastralia articulates with the xiphoid process of the sternum; E, basal ornithuromorph Archaeorhynchus IVPP

V17075. Scale bars equal 10 mm. Anatomical abbreviations (not listed in Figure 1, S2-3 captions): cav, caudal vertebrae; ili, ilium. Supplemental References

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